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The midday depression of CO2 assimilation in leaves of Arbutus unedo L.: diurnal changes in photosynthetic capacity related to changes in temperature and humidity
Abstract
Parts of attached leaves of the sclerophyllous shrub Arbutus unedo were subjected to simulated mediterranean days. Gas exchange was recorded in order to recognize the causes of the midday depression in CO2 assimilation. Depressions could be induced in part of a leaf: they were local responses. The CO2-saturation curves of photosynthesis, determined during the morning and afternoon maxima of CO2 assimilation and during the minimum at midday, established that depressions in CO2 assimilation were in one-half of the investigated cases totally caused by reversible reductions in the photosynthetic capacity of the leaves, and in the other half almost totally caused by such reductions. An analysis of 37 daily courses showed that morning reductions and afternoon recoveries of stomatal conductance and rate of photosynthesis occurred simultaneously and in proportion to each other, with the result that the partial pressure of CO2 in the intercellular spaces remained more or less constant. Midday depressions occurred also in detached leaves standing in water. The initiation of a midday depression was not caused by a circadian rhythm, nor was high quantum flux or high temperature a requirement. There was no correlation between the rate of water loss from the leaves, or the amount of water lost, with the degree of reduction of the photosynthetic capacity. However, depressions occurred if an apparent threshold in the water-vapor pressure difference between leaf and air was exceeded. This critical value varied between about 20 and 30 mbar, depending on the leaf investigated. The dominating role of humidity in the induction of the midday depression was further demonstrated when leaf temperature was held constant and the vapor-pressure difference was made to follow the pattern of the mediterranean day: depressions occurred. Depressions however were hardly noticeable when the water-vapor pressure difference was held constant and leaf temperature was allowed to vary. In another set of experiments, leaves were subjected to variations in temperature and humidity independent of the time of the day, under otherwise constant conditions. Photosynthetic capacity and stomatal conductance proved to be almost insensitive to changes in temperature (in a range extending from 20 to 37° C) as long as the water vapor-pressure difference was held constant. If it was not, the rate of photosynthesis began to decline with increasing temperature after a threshold water-vapor pressure difference was exceeded. The position of the resulting apparent temperature optimum of photosynthesis depended on the humidity of the air. We suggest that the ability of A. unedo to respond to a dry atmosphere with a reversible reduction of its photosynthetic capacity (by a still unknown mechanism) is the result of a co-evolution with the development of a strong stomatal sensitivity to changes in humidity.
... Many studies measuring chlorophyll fluorescence at leaf level found midday reductions in fluorescence emissions (e.g. Martínez-Ferri et al., 2000;Pons and Welschen, 2003;Raschke and Resemann, 1986;Špunda et al., 2005;Xu and Shen, 2005;Zhang and Gao, 2000). This common phenomenon has been attributed to both stomatal and non-stomatal (i.e., photoinhibition of PS II) limitations under high light, high VPD, and/or high temperatures (e.g. ...
... Others have found that the increased oxygenation of RuBP was not compensated by increases in carboxylation activity, likely due to the negative effect of high temperatures on Rubisco activity (Pons and Welschen, 2003). Raschke and Resemann (1986) found that depressions occurred if a threshold between 2000 and 3000 Pa in water-vapour pressure difference between the air and the leaf was exceeded. Thus, the depression measured in the SIF emission for both ecosystems in this study is therefore not a surprise from an ecophysiological point of view, although to our knowledge the midday depression in the SIF signal has not yet been reported or actively considered in remotely sensed SIF studies. ...
... High light and high VPD have been shown to be critical factors for the midday depression (e.g. Pathre et al., 1998;Pons and Welschen, 2003;Raschke and Resemann, 1986), although varying in extent depending on species-acclimation to drought stress (e.g. Martínez-Ferri et al., 2000;Pathre et al., 1998), and such reactions to stress would gain from being added to model-based SIF. ...
Due to the large carbon dioxide (CO2) fluxes between terrestrial ecosystems and the atmosphere, dynamics of photosynthesis can have significant effects on atmospheric CO2 concentrations and lead to large uncertainties in ecosystem C budgets. Remote sensing approaches using sun-induced chlorophyll fluorescence (SIF) hold the potential to directly assess ecosystem photosynthesis. However, many challenges remain linked to using the SIF emission signal to estimate gross primary production (GPP). The goal of this study was to gain a better understanding of the relationships between GPP and SIF over different time scales (minutes to years) and under varying environmental conditions. Two different ecosystems were investigated, a cropland and a mixed forest, with continuous eddy covariance flux measurements. Continuous tower-based SIF retrievals were performed in 2015 and 2016 at both ecosystems.
In both ecosystems, SIF was found to be more affected by environmental conditions than GPP. Annual cycles for GPP and SIF differed at the mixed forest due in part to the influence of the different footprint size of the two independent measurements. Diurnal cycles in GPP and SIF corresponded well under unstressed conditions and followed the incoming photosynthetic photon flux density (PPFD). However, depressions in SIF were found at both sites either at midday or in the afternoon during the growing season. At the cropland site, reductions in SIF occurred at high PPFD (PPFD>1470 µmol m-2 s-1, R2=0.62) and high VPD (VPD>1590 Pa, R2=0.35). Whereas at the forest site, reductions in SIF were linked to high VPD (VPD>1250 Pa, R2=0.25), but not to high PPFD (R2=0.84). The depression in SIF was also associated with an increase in non-photochemical quenching, as indicated by the photochemical reflectance index (R2=0.78), showing the complementarity between SIF and non-photochemical quenching as different energy pathways. Our results show the importance of characterizing the influence of different environmental conditions on SIF-GPP relationships for specific ecosystems to reliably estimate GPP from remote sensing measurements.
... We considered two ways to explain this relationship as a conse quence of how water-limited plants respond to high temperatures. We first considered whether it could be the result of a midday depression (sometimes termed the diurnal depression), wherein plants in hot climates reduce stomatal conductance and assimilate less carbon during the hottest parts of the day (70)(71)(72). If this occurred in the trees studied, it would act to lower the assimilationweighted mean photosynthesis temperature (and thus the average V O /V C ) compared to the average daytime growing season temper ature we calculated using climate data alone. ...
... If this occurred in the trees studied, it would act to lower the assimilationweighted mean photosynthesis temperature (and thus the average V O /V C ) compared to the average daytime growing season temper ature we calculated using climate data alone. As this phenomenon is chiefly driven by vapor pressure deficit (71), the midday depres sion tends to increase with increasing water limitation (73) and would be more apparent in water-limited trees. ...
Photorespiration can limit gross primary productivity in terrestrial plants. The rate of photorespiration relative to carbon fixation increases with temperature and decreases with atmospheric [CO 2 ]. However, the extent to which this rate varies in the environment is unclear. Here, we introduce a proxy for relative photorespiration rate based on the clumped isotopic composition of methoxyl groups ( R –O–CH 3 ) in wood. Most methoxyl C–H bonds are formed either during photorespiration or the Calvin cycle and thus their isotopic composition may be sensitive to the mixing ratio of these pathways. In water-replete growing conditions, we find that the abundance of the clumped isotopologue ¹³ CH 2 D correlates with temperature (18–28 °C) and atmospheric [CO 2 ] (280–1000 ppm), consistent with a common dependence on relative photorespiration rate. When applied to a global dataset of wood, we observe global trends of isotopic clumping with climate and water availability. Clumped isotopic compositions are similar across environments with temperatures below ~18 °C. Above ~18 °C, clumped isotopic compositions in water-limited and water-replete trees increasingly diverge. We propose that trees from hotter climates photorespire substantially more than trees from cooler climates. How increased photorespiration is managed depends on water availability: water-replete trees export more photorespiratory metabolites to lignin whereas water-limited trees either export fewer overall or direct more to other sinks that mitigate water stress. These disparate trends indicate contrasting responses of photorespiration rate (and thus gross primary productivity) to a future high-[CO 2 ] world. This work enables reconstructing photorespiration rates in the geologic past using fossil wood.
... On sunny days, when there is a sufficient amount of solar radiation, photosynthetic activity often drops around midday. This phenomenon is known as the "midday depression of photosynthesis (hereafter: midday depression)", which is caused by various environmental factors such as water deficit, high temperature, and excessive sunlight (Roessler and Monson, 1985;Raschke and Resemann, 1986;Hirasawa et al., 1989). As photosynthesis is the fundamental physiological reaction for crop production, midday depression should be avoided to reduce yield loss. ...
... Diffusional limitation refers to CO 2 uptake, which is determined in part by the conductivities of stomata, the path through the mesophyll cells inside the leaf, and the leaf surface boundary layer. A low conductivity of stomata (decrease in stomatal conductance) was often observed during the midday depression (Roessler and Monson, 1985;Raschke and Resemann, 1986;Yokoyama et al., 2018), and therefore it should be regarded as one of the main factors behind midday depression. The other contributing limitations (e.g., photoinhibition and sugar accumulation) are collectively referred to as non-diffusional limitations. ...
... The translocation of photosynthetic products from source leaves was driven by the hydrostatic gradient between the source and sink [12]. The relationship of the source/sink is dynamic, changing with the diurnal variations of the environmental factors, such as lower humidity that reduces stomatal conductance and limits the photosynthetic rate in the source [13][14][15] and higher temperatures that increase the consumption of photosynthetic products in the sink [16][17][18]. Therefore, it is reasonable to assume that translocation of photosynthetic products was different at different time periods over a daily time scale. ...
... The decrease in stomatal conductance could explain 55% of the decrease in the photosynthetic rate [30]. Therefore, the decrease in Pn at midday was related to the decrease in stomatal conductance [15]. Under higher temperature conditions, rubisco activity decreased and respiration and photorespiration increased, which increased intercellular CO2 concentration, and reduced stomatal opening. ...
Photosynthesis and the allocation of photosynthetic products are the two main factors that determine plant growth. To understand the growth and productivity of Pinus massoniana Lamb., the diurnal changes in photosynthetic rate were continuously monitored. Furthermore, the translocation and allocation of the photosynthetic products synthesized in the morning and afternoon were explored using 13C pulse labeling. The results showed that: (1) on sunny days, the diurnal variation of the net photosynthetic rate showed a “double peak” curve, with an obvious “a depression” when temperatures were highest and humidity lowest. On cloudy days, it showed an irregular “jagged” curve, which was curve consistent with the variations in photosynthetically active radiation (PAR). Meanwhile, the photosynthetic rate changed with the transient changes in environmental factors such as PAR, temperature, and humidity. (2) The mean value of the net photosynthetic rate in the morning was higher than in the afternoon, and the response of the net photosynthetic rate to environmental change (PAR, temperature, humidity, and CO2 concentration) in the morning was greater than that in the afternoon. (3) The translocation of photosynthetic products synthesized in the afternoon was faster than that in the morning. Shortly after synthesis of photosynthetic products, the translocation of products synthesized in the morning tended toward upper organs (including current-year leaves and 1-year leaves), while the translocation of products synthesized in the afternoon decreased in the upper organs. However, after 15 days of 13C pulse labeling, the allocation of the photosynthetic products synthesized in the morning and afternoon tended to be the same. These results indicate that the differences in the photosynthetic products synthesized and the temporal differences in the translocation rates did not affect the final allocation of the photosynthetic products in the various organs of the P. massoniana. These results improve our knowledge of the functional phases of P. massoniana during the diurnal cycle.
... Midday depression in SIF was observed from our measurements, and this phenomenon was found to be more obvious in summer and autumn (Figure 4). Although few remotely sensed SIF studies have reported midday depression in SIF likely due to the lack of continuous, high temporal-resolution SIF measurements, the midday reduction in chlorophyll fluorescence emissions has been widely reported in many studies at leaf level (Martínez-Ferri et al., 2000;Raschke & Resemann, 1986;Špunda et al., 2005), which attributed this phenomenon to both stomatal and non-stomatal limitations from environmental stresses such as excess light and high VPD. Excess absorbed solar energy at midday increases energy allocation to non-radiative heat dissipation, which leads to reduced energy allocation to photochemistry and fluorescence, resulting in the midday depression in fluorescence (Demmig-Adams, 1990;Martínez-Ferri et al., 2000). ...
... Excess absorbed solar energy at midday increases energy allocation to non-radiative heat dissipation, which leads to reduced energy allocation to photochemistry and fluorescence, resulting in the midday depression in fluorescence (Demmig-Adams, 1990;Martínez-Ferri et al., 2000). The depression also occurs when the plant suffers from moisture stress with VPD exceeding the threshold (Raschke & Resemann, 1986). The midday depression in SIF from our measurements in mangroves corresponded to midday environmental stresses including excess light (Figure 2e) and high VPD (Figure 2g). ...
Accurate characterization of gross primary productivity (GPP) is critically important in assessing mangrove carbon budgets, but the current knowledge of the temporal variations of GPP in evergreen mangroves is very limited. Remote sensing of sun‐induced chlorophyll fluorescence (SIF) has emerged as a promising approach to approximating GPP across ecosystems, but its capability for tracking GPP in evergreen mangroves has not been assessed. The SIF‐GPP link at a subtropical mangrove and its environmental controls are explored using 1‐year time‐series measurements from tower‐based hyperspectral and eddy covariance systems. Both the relationship between SIF and GPP as well as that between SIFy (SIF yield: the ratio of SIF over absorbed photosynthetically active radiation [APAR]) and LUE (light use efficiency: the ratio of GPP over APAR) at diurnal and seasonal time scales are analyzed. The temporal variations of SIF and GPP shared overall similar changing patterns, but their functional relationship tended to be time scale‐dependent. Midday depressions in SIF were observed when environmental stresses occurred around noon (including excess light and high VPD), and the strength of the SIF‐GPP link was affected by changing environmental conditions. The SIFy‐LUE relationship was temporally more dynamic, tending to match during midday hours but diverge from each other during morning and afternoon hours. These findings confirm SIF can serve as a potential remotely sensed indicator of mangrove canopy photosynthesis. This paper provides the first, high temporal‐resolution, continuous SIF measurements in mangroves, and highlights the importance of the impacts of environmental conditions on the SIF‐GPP relationship.
... This daily g s -VPD pattern is characterized by high g s in the early morning when VPD is low and a decrease in g s as VPD increases over the course of the morning, as the temperature rises and the relative humidity drops (Fig. 4, Suppl. Fig. 3BD [34,38,[45][46][47]). Obviously, this daily g s -VPD pattern is influenced by seasonal conditions, for example, higher steady-state aperture and a slower decrease in g s are observed during rainy seasons, when VPD is relatively low [47]. ...
... The main role of ABA in evolution may have been in the adjustment of stomatal opening, as opposed to the common understanding of ABA as the stomatal-closing phytohormone. This would also explain angiosperms' relatively long phase of steady-state g s (guard cells of both WT and GCabi maintained some turgor) during late morning and midday ( Fig. 4B; [34,38,[45][46][47]) and the lag in the rate at which stomata open following a sharp increase in VPD, as compared to the rate at which they close [4,9,14]. ...
Abscisic acid (ABA) is known to be involved in stomatal closure. However, its role in stomatal response to rapid increases in the vapor pressure deficit (VPD) is unclear. To study this issue, we generated guard cell-specific ABA-insensitive Arabidopsis plants (guard-cell specific abi1-1; GCabi). Under non-stressed conditions, the stomatal conductance (gs) and apertures of GCabi plants were greater than those of control plants. This supports guard-cell ABA role as limiting steady-state stomatal aperture under non-stressful conditions. When there was a rapid increase in VPD (0.15 to 1 kPa), the gs and stomatal apertures of GCabi decreased in a manner similar that observed in the WT control, but different from that observed in WT plants treated with fusicoccin. Low VPD increased the size of the stomatal apertures of the WT, but not of GCabi. We conclude that guard-cell ABA does not play a significant role in the initial, rapid stomatal closure that occurs in response to an increase in VPD, but is important for stomatal adaptation to ambient VPD. We propose a biphasic angiosperm VPD-sensing model that includes an initial ABA-independent phase and a subsequent ABA-dependent steady-state phase in which stomatal behavior is optimized for ambient VPD conditions.
... On sunny days, plants actively engage in photosynthesis; however, despite sufficient solar radiation, the photosynthetic rate often decreases around midday. Under strong solar radiation, a decrease in photosynthetic rate around midday, known as midday depression of photosynthesis (hereafter midday depression), is frequently observed (Roessler and Monson 1985, Raschke and Resemann 1986, Ayari et al. 2000, Muraoka et al. 2000, Pelletier et al. 2016. Notably, identifying the environmental factors limiting the photosynthetic rate is essential for preventing crop yield loss caused by a decrease in photosynthesis. ...
The diurnal variations in the factors of photosynthesis reduction under well-watered greenhouse conditions remain poorly understood. We conducted diurnal measurements of gas exchange and chlorophyll fluorescence in strawberries (Fragaria × ananassa Duch.) for three sunny days. Quantitative limitation analysis was also conducted to investigate the diurnal variations of photosynthetic limitations [stomatal (SL), mesophyll (MCL), and biochemical limitation (BL)]. Under well-watered greenhouse conditions, a photosynthesis reduction was observed, and the respective limitations exhibited different diurnal changes based on the environmental stress severity. The main limitation was SL, varying between 11.3 and 27.1% around midday, whereas MCL and BL were in 4.3-14.2% and 1.7-8.5%, respectively, under relatively moderate conditions. However, both SL (11.2-34.2%) and MCL (4.8-26.4%) predominantly limited photosynthesis under relatively severe conditions, suggesting that stomatal closure was the main limitation and that the decline in mesophyll conductance was not negligible under strong environmental stress, even under well-watered greenhouse conditions.
... Stomatal limitation The decreasing range of C i is much lower than that of P n [53] Non-stomatal limitation C i remains almost constant [54,55] Stomatal and non-stomatal limitation ...
The Ejin Oasis is located in the lower reaches of the Heihe River Basin of northwestern China. It is one of the most arid regions in the world, and Populus euphratica Oliv. is the foundation species of the desert riparian forests there. The photosynthesis of P. euphratica is one of the first physiological processes that is most likely to be affected by the extremely arid climate conditions. The factors impacting photosynthesis can be divided into stomatal and non-stomatal limitations. In order to investigate whether the photosynthesis of P. euphratica was limited and, if so, whether this limitation was caused by drought stress in the P. euphratica Forest Reserve on the Ejin River, we analyzed stomatal, non-stomatal, and relative stomatal limitations (reflecting the relative importance of the stoma in controlling the processes of photosynthesis) of photosynthesis. The results show that, at the beginning of the midday depression of photosynthesis, the values of stomatal limitation of photosynthesis (Ls) peaked, with its predominance being supported by sub-stomatal CO2 concentrations (Ci) being at a minimum. Thereafter, Ls decreased and non-stomatal limitation (Ci/stomatal conductance (gs)) increased sharply, indicating that the non-stomatal limitation of photosynthesis was predominant. Both Ls and relative stomatal limitation of photosynthesis increased in the morning, and then decreased, whereas Ci/gs showed the opposite trend. We concluded that P. euphratica did not experience drought stress by analyzing leaf water potential, groundwater table, and the decoupling coefficient (a parameter characterizing the coupling degree between vegetation canopy and atmospheric water vapor flux); however, the Ls values of P. euphratica were much greater than those of other species. This was likely because P. euphratica has a relatively conservative water use strategy even when growing under favorable water conditions. Extremely high temperatures caused the closure of the stoma to reduce transpiration, resulting in more intense stomatal limitations of photosynthesis.
... According to Cowan (1982), plants adapt to their environment in such a way that water use efficiency is maximized, i.e., stomatal conductance is consistent with the principle of optimal control of plants in the regulation of CO 2 gain and water loss. From a plant physiological point of view, changes in plant WUE under drought conditions are mainly caused by stomatal limiting factors and non-stomatal limiting factors (Raschke et al. 1986;Chaves et al. 2002). Plants have developed relatively stable physiological and community structural characteristics and corresponding water use strategies during long-term adaptation to water, and stomata are more sensitive to drought and precipitation when plants are under mild and moderate water stress (Martínez-Vilalta et al. 2014). ...
Water use efficiency (WUE) is an effective indicator to study the coupling of terrestrial carbon and water cycles. The Tibetan Plateau (TP) is the most important ecological security barrier in China, and it is important to understand the characteristics of WUE and the change mechanism to study the carbon and water cycles of plateau ecosystems and the rational use of water resources. This study analyzes the spatial and temporal characteristics of WUE on the TP and the influence of climate factors on WUE based on the gross primary productivity (GPP) and evapotranspiration (ET) data from GLASS. The results show that from 1985 to 2018, the WUE of the TP is on the rise under the combined effect of GPP and ET; the regions with higher mean WUE values are the southeastern and eastern parts of the plateau, and the low value areas are the central and northwestern parts of the plateau. Compared with precipitation, WUE is influenced by temperature over a larger area. The correlations between precipitation and temperature and WUE in different eco-geographic regions are complex, and there is a threshold effect on the correlation between WUE and temperature and precipitation. Temperature is the main driver of WUE changes in HIIA and HIB1 regions, while precipitation has a greater impact on WUE changes in HIIC2, HIIC2, HIC2, HIID3, and HIIC regions. Precipitation, temperature, and elevation are the main factors explaining the variation of WUE in the TP. According to the risk detector, it can be determined that grassland vegetation in warm and humid steep areas of low and medium elevations is more able to maintain efficient use of water. Meanwhile, grasslands located in the shade of northern slopes have weaker transpiration, which is conducive to vegetation accumulation of growth water, and thus can ensure higher WUE. The related study can provide a reference for the response of vegetation WUE to global changes in key climatic regions.
... Gas exchange under high midday VPD showed that assimilation decreased more than expected based on the Gaastra-inferred c i , and the A/c i curve generated by manipulating external CO 2 (e.g. Fig. 6A, B; Farquhar and Raschke, 1978;Raschke and Resemann, 1988), motivating a hypothesis that high VPD somehow reduced g m . However, an alternative explanation subsequently appeared in the form of 'patchy' stomatal behaviour, which can lead to a bimodal distribution in apertures whose ensemble behaviour deviates from that of a unimodal distribution even when it retains the same overall mean (Downton et al., 1988;Beyschlag et al., 1992). ...
Background
Recent reports of extreme levels of undersaturation in the internal leaf air spaces have called into question one of the foundational assumptions of leaf gas exchange analysis, that leaf air spaces are effectively saturated with water vapor at leaf surface temperature. Historically, inferring the biophysical states controlling assimilation and transpiration from the fluxes directly measured by gas exchange systems has presented a number of challenges, including: 1) a mismatch in scales between the area of flux measurement, the biochemical cellular scale, and the meso-scale introduced by the localization of the fluxes to stomatal pores; 2) the inaccessibility of the internal states of CO2 and water vapor required to define conductances; and 3) uncertainties about the pathways these internal fluxes travel. In response, plant physiologists have adopted a set of simplifying assumptions that define phenomenological concepts such as stomatal and mesophyll conductances.
Scope
Investigators have long been concerned that a failure of basic assumptions could be distorting our understanding of these phenomenological conductances, and the biophysical states inside leaves. Here we review these assumptions and historical efforts to test them. We then explore whether artifacts in analysis arising from the averaging of fluxes over macroscopic leaf areas could provide alternative explanations for some part, if not all, of reported extreme states of undersaturation.
Conclusions
Spatial heterogeneities can, in some cases, create the appearance of undersaturation in the internal air spaces of leaves. Further refinement of experimental approaches will be required to separate undersaturation from the effects of spatial variations in fluxes or conductances. Novel combinations of current and emerging technologies hold promise for meeting this challenge.
... This is the case where malate plays an important role in the behaviour of the stomatal cells, and consequently in the plant water balance. Based on literature and the work of Raschke and Resemann [20], Polevoi [9] gives the following schema (Fig. 1). ...
... This is the case where malate plays an important role in the behaviour of the stomatal cells, and consequently in the plant water balance. Based on literature and the work of Raschke and Resemann [20], Polevoi [9] gives the following schema (Fig. 1). ...
We applied in vivo impedance measurement in the entire plant. We aim to emphasize the value and the importance of applying the electrical bio-impedance measurement of the root and the first leaf of a wheat plant aged 14 days as a sample, compared to classical biophysical methods in plants, for instance electrical action potential. We conclude that bio-impedance measurement is a non-destructive biophysical technique. And it can be useful in plant organ diagnosis and their physiological states. There is a significant difference between electrical parameters of different organs that can be used to characterize each one of them.
... During summer middays, high VPD often reduces stomatal closure and hinders CO 2 assimilation in tropical (Ishida et al., 1999), Mediterranean (Raschke and Resemann, 1986) and even cool temperate regions (Kamakura et al., 2012). As long as the yearly mean duration of the midday depression is short and varies little over time, cellulose is a trustable archive for isotope-based climate reconstruction. ...
Climatic reconstructions based on tree-ring isotopic series convey substantial information about past conditions prevailing in forested regions of the globe. However, in some cases, the relations between isotopic and climatic records appear unstable over time, generating the “isotopic divergences”. Former reviews have thoroughly discussed the divergence concept for tree-ring physical properties but not for isotopes. Here we want to take stock of the isotopic divergence problem, express concerns and stimulate collaborative work for improving paleoclimatic reconstructions. There are five main causes for divergent parts in isotopic and climatic series: (1) artefacts due to sampling and data treatment, relevant for dealing with long series using sub-fossil stems; (2) stand dynamics, including juvenile effects mostly occurring in the early part of tree-ring series; (3) rise in atmospheric pCO2, which can directly influence the foliar behaviour; (4) change in climate, which may modify the isotope–climate causal links; and finally (5) atmospheric pollution, which may alter leaf and root functions. Future paleoclimate research would benefit from interdisciplinary efforts designed to develop further process-based models integrating multi-proxy inputs so as to help identify causes of isotopic divergences and circumvent some of them in inverse applications.
... The average values of P n over 0 − 6 h treatment were 13.2% lower than those during 6 − 12 h treatment. It has been reported that high light intensity, high leaf temperature and high ALVPD are important factors contributing to midday depression of photosynthesis (Chaves, Harley, Tenhunen & Lange, 1987;Correia, Chaves & Pereira, 1990;Kuppers, Wheeler, Kuppers, Kirschbaum & Farquhar, 1986;Raschke & Resemann, 1986;Roessler & Monson, 1985;Tenhunen et al., 1987). In our study, leaf temperature and ALVPD remained relatively constant while incident PPFD dynamically changed over the diurnal course of the treatment (Figure 2(a,d,e)). ...
Plants are exposed to high light intensity, high leaf temperatures and high air-to-leaf water vapor pressure deficit (ALVPD) during the day. These environmental stresses cause stomatal closure and photoinhibitory damage, leading to midday depression of photosynthesis. Chloroplast positioning is essential for the efficient operation of photosynthesis. However, chloroplast behavior before, during, and even after the midday depression of photosynthesis remains unknown. We investigated changes in the intracellular positioning of chloroplasts and photosynthetic traits under a diurnal pattern of light. Sorghum leaves were exposed to a 12-h regime of light mimicking the natural light environment, with constant leaf temperature and ALVPD. Net photosynthetic rate (Pn) showed a diurnal pattern, and midday depression in Pn was observed at 3.8 h of irradiation. Depression in Pn was attributed to stomatal limitation because the decrease in Pn was in accordance with the decrease in stomatal conductance. The maximum efficiency of photosystem II decreased with the increase in light intensity and remained low after 12 h of irradiation. Bundle sheath chloroplasts swelled after 8 h of irradiation, representing the accumulation of starch. Conversely, mesophyll chloroplasts exhibited avoidance response after 4 h of irradiation, and the avoidance position was maintained during the remainder of the daytime. These data suggest that chloroplasts are subject to light stress during and after the midday depression of photosynthesis. The intensity of natural light is excessive for most of the day and this light stress induces chloroplast avoidance response and depression of photosynthesis.
... Acclimation of T equ to growth temperature (and air humidity) or declining soil water availability could provide new information about coordination of A and G (Quick et al., 1992;Lawlor, 2002;Medrano et al., 2002). For example, midday depression of CO 2 assimilation on a clear, sunny day has often been ascribed to stomatal closure, causing a drop in C i that limits light-saturated photosynthesis (Raschke & Reeseman, 1986;Macfarlane et al., 2004). However, it remains difficult to distinguish between cause and effect, giving rise to covariation between C i , G and A (Lawlor & Cornic, 2002). ...
Date palms are remarkably tolerant to environmental stresses, but the mechanisms involved remain poorly characterised. Leaf metabolome profiling was therefore performed on mature (ML) and young (YL) leaves of two-year-old date palm seedlings that had been grown in climate chambers that simulate summer and winter conditions in eastern Saudi Arabia. Cultivation under high temperature (summer climate) resulted in higher YL H2O2 leaf levels despite increases in dehydroascorbate reductase (DHAR) activities. The levels of raffinose and galactinol abundances, tricarboxylic acid cycle intermediates, and total amino acid were higher under these conditions, particularly in YL. The accumulation of unsaturated fatty acids, 9,12-octadecadienoic acid and 9,12,15-octadecatrienoic acid was lower in ML. In contrast, the amounts of saturated tetradecanoic acid and heptadecanoic acid were increased in YL under summer climate conditions. The accumulation of phenolic compounds was favored under summer conditions, while flavonoids accumulated under lower temperature (winter climate) conditions. YL displayed stronger hydration, lower H2O2 levels and more negative δ13C values indicating effective reactive oxygen species scavenging. These findings, which demonstrate the substantial metabolic adjustments that facilitate tolerance to the high temperatures in YL and ML, suggest that YL may be more responsive to climate change.
... VPD serves to decrease water loss and prevent desiccation under high evaporative demand. As an example, changes in VPD during the day result in transient stomatal closure and, accordingly, in reduced net photosynthetic rate around midday, when the VPD values are largest (Raschke and Resemann, 1986;Brodribb and Holbrook, 2004). Furthermore, the combination of increased temperature and larger variability of water stress predicted for the future results in a VPDmodulated increase in transpiration and higher mortality rate during terminal drought in some species/ areas . ...
Guard cells shrink and close stomatal pores when air humidity decreases (i.e. when the difference between the vapor pressures of leaf and atmosphere [VPD] increases). The role of abscisic acid (ABA) in VPD-induced stomatal closure has been studied using ABA-related mutants that respond to VPD in some studies and not in others. The importance of ABA biosynthesis in guard cells versus vasculature for whole-plant stomatal regulation is unclear as well. Here, we show that Arabidopsis (Arabidopsis thaliana) lines carrying mutations in different steps of ABA biosynthesis as well as pea (Pisum sativum) wilty and tomato (Solanum lycopersicum) flacca ABA-deficient mutants had higher stomatal conductance compared with wild-type plants. To characterize the role of ABA production in different cells, we generated transgenic plants where ABA biosynthesis was rescued in guard cells or phloem companion cells of an ABA-deficient mutant. In both cases, the whole-plant stomatal conductance, stunted growth phenotype, and leaf ABA level were restored to wild-type values, pointing to the redundancy of ABA sources and to the effectiveness of leaf ABA transport. All ABA-deficient lines closed their stomata rapidly and extensively in response to high VPD, whereas plants with mutated protein kinase OST1 showed stunted VPD-induced responses. Another strongly ABA-insensitive mutant, defective in the six ABA PYR/RCAR receptors, responded to changes in VPD in both directions strongly and symmetrically, indicating that its VPD-induced closure could be passive hydraulic. We discuss that both the VPD-induced passive hydraulic stomatal closure and the stomatal VPD regulation of ABA-deficient mutants may be conditional on the initial pretreatment stomatal conductance.
... Acclimation of T equ to growth temperature (and air humidity) or declining soil water availability could provide new information about coordination of A and G (Quick et al., 1992;Lawlor, 2002;Medrano et al., 2002). For example, midday depression of CO 2 assimilation on a clear, sunny day has often been ascribed to stomatal closure, causing a drop in C i that limits light-saturated photosynthesis (Raschke & Reeseman, 1986;Macfarlane et al., 2004). However, it remains difficult to distinguish between cause and effect, giving rise to covariation between C i , G and A (Lawlor & Cornic, 2002). ...
We studied acclimation of leaf gas exchange to differing seasonal climate and soil water availability in slow‐growing date palm (Phoenix dactylifera) seedlings. We used an extended Arrhenius equation to describe instantaneous temperature responses of leaf net photosynthesis (A) and stomatal conductance (G), and derived physiological parameters suitable for characterization of acclimation (Topt, Aopt and Tequ).
Optimum temperature of A (Topt) ranged between 20–33°C in winter and 28–45°C in summer. Growth temperature (Tgrowth) explained c. 50% of the variation in Topt, which additionally depended on leaf water status at the time of measurement. During water stress, light‐saturated rates of A at Topt (i.e. Aopt) were reduced to 30–80% of control levels, albeit not limited by CO2 supply per se.
Equilibrium temperature (Tequ), around which A/G and substomatal [CO2] are constant, remained tightly coupled with Topt. Our results suggest that acclimatory shifts in Topt and Aopt reflect a balance between maximization of photosynthesis and minimization of the risk of metabolic perturbations caused by imbalances in cellular [CO2].
This novel perspective on acclimation of leaf gas exchange is compatible with optimization theory, and might help to elucidate other acclimation and growth strategies in species adapted to differing climates.
... Ozone treatment triggered elevation of Ca 2+ signals in arabidopsis seedlings within 30 s ( Figure 3B [60]), which would support the importance of Ca 2+ -dependent activation of RBOH and SLAC1 as the mechanism of rapid SLAC1 activation by ozone and/or ROS [38,52,[60][61][62][63][64]; however, time-resolved ozone-induced Ca 2+ elevation studies in guard cells remain to be addressed. [82] Air humidity and temperature can drastically change during the day resulting in transient stomatal closure, (e.g., around midday) to prevent failures within the plant hydraulic system [65,66]. Water content within the intercellular air spaces is close to saturation and a dry atmosphere could give rise to steep water vapor gradients across the stomatal pore. ...
Plants grow and reproduce within a highly dynamic environment that can see abrupt changes in conditions, such as light intensity, temperature, humidity, or interactions with biotic agents. Recent studies revealed that plants can respond within seconds to some of these conditions, engaging many different metabolic and molecular networks, as well as rapidly altering their stomatal aperture. Some of these rapid responses were further shown to propagate throughout the entire plant via waves of reactive oxygen species (ROS) and Ca²⁺ that are possibly mediated through the plant vascular system. Here, we propose that the integration of these signals is mediated through pulses of gene expression that are coordinated throughout the plant in a systemic manner by the ROS/Ca⁺² waves.
... Direct interactions between the supply of water (blue arrows) and demand for water (red arrows) are shown by unbroken arrows, while indirect interactions are shown by dashed arrows plant species to changes in the evaporative gradient between the leaf and the atmo- sphere. Stomata typically close when the evaporative demand increases (Darwin 1898) allowing plants to maintain an optimal ratio of water loss to carbon gain (Cowan and Farquhar 1977), but the trigger for this clo- sure is likely to be transient leaf desiccation caused by insufficient hydraulic supply (Raschke and Resemann 1986;Saliendra et al. 1995). Analyses of the xylem sap flow of many species demonstrate a general pat- tern of stomatal closure during high midday evaporative demand due to limitations in the supply of liquid water from the xylem ( Oren et al. 1999). ...
In most terrestrial ecosystems, water availability is the principal governor of primary productivity. Vascular plants can only sustain high rates of photosynthetic activity by transporting enormous quantities of water from reserves in the soil to the sites of gas exchange in leaves to prevent desiccation of photosynthetic tissues. This demand for water requires plants to invest in a vascular system that begins as a simple pipe system in roots and branches and terminates in a sophisticated network of veins in the leaf. This chapter will examine the tight linkage between photosynthesis and the efficiency of water transport in leaves, explaining how plants use a non-living network of xylem to deliver water under high tension to evaporating cells. We explore how plants achieve high efficiency in water delivery by developing an intricately branched system of leaf veins as a means of piping water close to the stomatal layer, and how evolution has shaped the venation of higher plant species as densely reticulated networks.
... On sunny days, plants actively perform photosynthesis using the light energy, however, photosynthetic rate often drops around midday. This reduction in photosynthesis, known as midday depression of photosynthesis, has been frequently observed in plants grown in arid or semi-arid environments (Schulze, 1982;Roessler and Monson, 1985;Raschke and Resemann, 1986). It has also been observed in plants grown in temperate climates with well-watered rhizospheres in both field (Hirasawa et al., 1989;Muraoka et al., 2000) and greenhouse environments (Ayari et al., 2000;He et al., 2007;Allen and Vu, 2009). ...
To evaluate the effects of leaf wetting on midday depression of photosynthesis in horticultural crops, we analyzed leaf gas exchange (transpiration rate, stomatal conductance, photosynthetic rate) of tomato plants under two different treatments (the Wet treatment and No-wet treatment as control). The gas exchange was measured at 11:00 (the time at which leaf gas exchange was assumed to be active) and 14:00 (the time at which leaf gas exchange was assumed to be inactive due to midday depression) on the clear days of November 2 to 7, 2016. The gas exchange measurements in the Wet treatment were conducted just after droplets on the leaf surface had evaporated. In the No-wet treatment, transpiration rate, stomatal conductance, and photosynthetic rate at 14:00 were decreased compared to those at 11:00. This suggests that midday depression occurred due to stomatal closure induced by excessive transpirational water loss. In contrast, in the Wet treatment, there was no such depression of leaf gas exchange, suggesting that leaf wetting might contribute to maintaining stomatal aperture through improving leaf water status. Thus, leaf wetting could avoid midday depression of photosynthesis in tomato plants.
... VPD serves to decrease water loss and prevent desiccation under high evaporative demand. As an example, changes in VPD during the day result in transient stomatal closure and, accordingly, in reduced net photosynthetic rate around midday, when the VPD values are largest (Raschke and Resemann, 1986;Brodribb and Holbrook, 2004). Furthermore, the combination of increased temperature and larger variability of water stress predicted for the future results in a VPDmodulated increase in transpiration and higher mortality rate during terminal drought in some species/ areas . ...
Guard cells shrink and close stomatal pores when air humidity decreases, i.e. when the difference between the vapor pressures of leaf and atmosphere (VPD) increases. The role of abscisic acid (ABA) in VPD-induced stomatal closure has been studied using ABA-related mutants that respond to VPD in some studies and not in others. The importance of ABA biosynthesis in guard cells versus vasculature for the whole-plant stomatal regulation is also unclear. Here we show that Arabidopsis lines carrying mutations in different steps of ABA biosynthesis as well as pea wilty and tomato flacca ABA-deficient mutants had higher stomatal conductance compared to wildtype plants. To characterize the role of ABA production in different cells, we generated transgenic plants where ABA biosynthesis was rescued in guard cells or phloem companion cells of an ABA-deficient mutant. In both cases, the whole-plant stomatal conductance, stunted growth phenotype and leaf ABA level were restored to wildtype values, pointing to the redundancy of ABA sources and to the effectiveness of leaf ABA transport. All ABA-deficient lines closed their stomata rapidly and extensively in response to high VPD, whereas plants with mutated protein kinase OST1 showed stunted VPD-induced responses. Another strongly ABA-insensitive mutant defective in the six ABA PYR/RCAR receptors, responded to changes in VPD in both directions strongly and symmetrically, indicating that its VPD-induced closure could be passive hydraulic. We discuss that both the VPD-induced passive hydraulic stomatal closure and the stomatal VPD-regulation of ABA-deficient mutants may be conditional on the initial pretreatment stomatal conductance.
... The simplest explanation for the inhibition of photosynthesis during water stress would be that the stomata close and the internal CO 2 concentration decreases [54,55], since stomatal limitation is more severe when a plant is stressed than when it is not [54]. Therefore, it is rather surprising that photosynthesis often decreases in parallel with, or more than, stomatal conductance [56][57][58][59]. The photosynthetic rate in higher plants decreases more rapidly than respiration rate with increased water stress, since an early effect of water reduction in leaves is usually a partial or complete stomatal closure, markedly decreasing the movement of carbon dioxide into the assimilating leaves and reducing the photosynthetic rate up to ten times, according to the amount of water removal and the sensitivity of the plant [35]. ...
... There have been many reports that greater evaporative demand decreases photosynthesis and plant growth by decreasing stomatal conductance (gs) (Raschke and Resemann 1986, Dai et al. 1992, van de Sanden and Veen 1992, Wong 1993, Ottosen et al. 2002, Ben-Asher et al. 2013). On the other hand, several studies have suggested that high evaporative demand may not decrease photo-synthesis even if it reduces g s (Carins-Murphy et al. 2014, Shibuya et al. 2016b. ...
We studied growth and photosynthesis of cucumber (Cucumis sativus) seedlings under two vapor-pressure deficit levels (VPD; 0.4 and 3.0 kPa), two salinity levels (0 mM and 34 mM NaCl), and two CO2 concentrations ([CO2]; 400 and 1,000 μmol mol–1). Relative growth rate (RGR) decreased with increasing VPD, but the causal factor differed between salinity levels and CO2 concentrations. Under ambient [CO2], RGR decreased with increasing VPD at low salinity mainly due to decreased leaf area ratio (LAR), and decreased net assimilation rate (NAR) at high salinity. The decrease in intercellular [CO2] (Ci) with decreasing stomatal conductance caused by high VPD did not significantly limit net photosynthetic rate (PN) at low salinity, but PN was potentially limited by Ci at high salinity. At high [CO2], high VPD reduced LAR, but did not affect NAR. This is because the decrease in Ci occurred where slope of PN–Ci curve was almost flat.
... If the net photosynthetic rate (P n ) is not affected , acclimatization at high VPD can improve wateruse efficiency (WUE), which is an important criterion for transplant quality: carbohydrate accumulation combined with efficient water use is advantageous when transplanted seedlings experience a water deficit. However , high-VPD acclimatization may limit photosynthesis through both stomatal (diffusion) and non-stomatal (mesophyll) factors (Dai et al., 1992; Flexas and Medrano, 2002; Raschke and Resemann, 1986). In the This article is an Advance Online Publication of the authors' corrected proof. ...
Hardening with a high vapor-pressure-deficit (VPD) can decrease conductance (gs) and thereby enhance drought tolerance by reducing transpiration, which is particularly useful during transplant establishment. However, high-VPD hardening may decrease photosynthetic performance due to stomatal (diffusion) and non-stomatal (mesophyll) limitation. If gs can be lowered without significantly reducing photosynthesis, water-use efficiency (WUE = CO2 assimilation/transpiration), which is an important criterion for transplant quality, would improve. We investigated the photosynthetic properties of cucumber (Cucumis sativus L.) leaves acclimatized to different VPD levels (0.4 and 3.2 kPa at 28°C), and determined whether photosynthesis was limited by stomatal or non-stomatal factors at high VPD. The net photosynthetic rate (Pn) and gs were measured at a VPD of 0.8 kPa and a leaf temperature of 28°C under saturating light. The photosynthetic response to the intercellular CO2 concentration (Ci) was used to quantify the effects of VPD acclimatization on carboxylation efficiency. Pn did not differ significantly between acclimatization VPDs, but gs was much lower (×0.36) in high-VPD leaves at an ambient CO2 concentration of 400 μmol·mol⁻¹. Thus, the intrinsic WUE (= Pn/gs) of the high-VPD leaves was much higher (×2.85). The Ci of high-VPD seedlings was lower than that of low-VPD seedlings, but, this did not cause any significant reduction in Pn in the high-VPD treatments because the decrease in Ci occurred within a range in which photosynthesis was not limited by ribulose-1,5-bisphosphate carboxylation in this experiment. The Pn–Ci curve did not differ between the VPD treatments, indicating that carboxylation efficiency was not affected. When VPD-acclimatized seedlings experienced a limited water supply, the low-VPD leaves showed lower leaf water potential and more severe wilting than the high-VPD leaves 30 min after water limitation began. The gs of the high-VPD leaves was significantly lower (approximately ×0.5 in average) than that of the low-VPD leaves throughout the water-supply-limitation treatment. The lower gs maintained the water status of the high-VPD leaves at less-wilting levels by decreasing transpiration. These results indicate that high-VPD hardening could enhance the tolerance to short-term drought without stomatal or non-stomatal limitation of photosynthesis when controlling gs effectively.
... A similar pattern of declining g s and A early in the day has been reported in deserts, Mediterranean and tropical monsoon climates (Lüttge & Hertel 2009), and is known as a midday depression of photosynthesis. In these environments, assimilation rates decline during the hottest, driest part of the day because of stomatal closure in response to increasing air temperature and D (Raschke & Resemann 1986;Pons & Welschen 2003). Assimilation rate then recovers during the late afternoon (Tenhunen et al. 1984) or overnight (Lüttge & Hertel 2009) once D and temperature start to decline. ...
New Zealand kauri (Agathis australis) (D.Don) Lindl. is a large and long-lived tree species endemic to the species-rich forests of the north of the North Island. Agathis australis are culturally and ecologically significant, but little is known about their ecophysiology. In particular, environmental drivers of fluxes of carbon and water for A. australis trees have not been quantified. We measured leaf gas exchange to explore the effect of leaf age, tree size, foliar nitrogen concentration, photosynthetically active radiation (PAR) and vapour pressure deficit (D) on assimilation rates (A) and stomatal conductance (gs). We also measured carbon isotope discrimination of leaves and applied an optimal stomatal behaviour model. Both gs and A were highest for year one leaves (130 mmol m⁻² s⁻¹ and 5 μmol m⁻² s⁻¹, respectively) then declined with leaf age to < 80 mmol m⁻² s⁻¹ and < 3 μmol m⁻² s⁻¹, respectively, in 4–5-year-old leaves. Instantaneous water use efficiency (A/gs) was highly variable, but there was no leaf age-related pattern. Our diurnal results indicate that A. australis gs peaks early in the day (before 0900 h at 250 mmol m⁻² s⁻¹) and A is comparatively low, remaining below 9 μmol m⁻² s⁻¹ throughout the day. Overall, water use efficiency is low based on intrinsic water use efficiency and the stomatal model. Isotopic analysis indicated moderate water use efficiency over the life of leaves compared to other temperate conifers. This information is valuable for modelling carbon and water fluxes of A. australis and for improving our understanding of the threat of summer droughts to these forest giants.
... This is the case where malate plays an important role in the behaviour of the stomatal cells, and consequently in the plant water balance. Based on literature and the work of Raschke and Resemann [20], Polevoi [9] gives the following schema (Fig. 1). ...
... This is the case where malate plays an important role in the behaviour of the stomatal cells, and consequently in the plant water balance. Based on literature and the work of Raschke and Resemann [20], Polevoi [9] gives the following schema (Fig. 1). ...
We applied in vivo impedance measurement in the entire plant. We aim to emphasize the value and the
importance of applying the electrical bio-impedance measurement of the root and the first leaf of a wheat plant
aged 14 days as a sample, compared to classical biophysical methods in plants, for instance electrical action
potential. We conclude that bio-impedance measurement is a non-destructive biophysical technique. And it can
be useful in plant organ diagnosis and their physiological states. There is a significant difference between
electrical parameters of different organs that can be used to characterize each one of them.
... Los valores de EUA foliar son altos, característicos de especies de zonas áridas y similares a arbustos siempreverdes del género Calotropis en el litoral central de Venezuela en la temporada de lluvia (Tezara et al. 2011). La disminución de g s al mediodía, incluso en condiciones de buena disponibilidad hídrica, es una característica típica de especies de ecosistemas áridos, semiáridos y sabanas (Vareschi 1960;Raschke & Resemann 1986;Franco & Lüttge 2002). ...
... Our results are consistent with those obtained for many other crops. For example, midday depression occurred when a combina-tion of high temperature and low relative humidity led the VPD to exceed 2-3 kP a in strawberry trees (Arbutus unedo; Raschke and Resemann, 1986), 2 kP a in maize (Zea mays; Hirasawa and Hsiao, 1999) and 2 kP a in nectarine trees (Prunus persica var. nectarina; Osorio et al., 2006). ...
A cultural practice for cooling cranberry plants and avoiding yield losses due to overheating is to turn on sprinkler irrigation for a few minutes when a critical temperature threshold is reached. The purpose of this study was to determine: (i) the critical leaf temperature to start irrigation for evaporative cooling in cranberry production, (ii) the combined effects of heat and water stress, and (iii) the beneficial effects of evaporative cooling through irrigation on field microclimate and plant physiology. The optimum temperature range for carbon dioxide (CO2) assimilation was between 25 and 29 °C, with photosynthesis (An) declining by 11% at 33 °C and by 22% at 37 °C due in part to stomatal limitations. The reduction in An was greater under low soil moisture conditions. When applying sprinkler irrigation in the field, leaf temperature was reduced by 5–10 °C and the efficiency of evaporative cooling was greater, with greater vapor pressure deficit of the air within the crop canopy before the irrigation. Under controlled environmental conditions, one cooling event resulted in a carbon gain of 19% relative to untreated plants and was able to reduce midday depression. Additional cooling events during the day had no significant effect on carbon gain. Among the main cranberry-growing regions, New Jersey is an area where sprinkler irrigation for evaporative cooling could be beneficial for preventing yield limitations due to heat stress.
... In the control treatment, the NA was found to be lower during the peak PAR readings of the day (13:00) confirming that the "lunch break" phenomenon has a direct relationship with PAR [16]. However, Raschke and Resemann [17] studied the leaves of Arbutus undeo L. and showed that the photosynthetic "lunch break" phenomenon can occur at low levels of photosynthetically active radiation (about 500μmol m -2 s -1 ), and that it has a certain relationship with plant growth rhythm and genetic characteristics. Nevertheless, strong PAR is the basic driving force that causes a variety of changes in environmental factors such as reduced atmospheric humidity and increased atmospheric temperature, indirectly leading to the photosynthetic "lunch break" phenomenon [16]. ...
Photosynthetic Response of Soybean to
Microclimate in 26-Year-Old Tree-Based
Intercropping Systems in Southern Ontario,
Canada
... In the control treatment, the NA was found to be lower during the peak PAR readings of the day (13:00) confirming that the "lunch break" phenomenon has a direct relationship with PAR [16]. However, Raschke and Resemann [17] studied the leaves of Arbutus undeo L. and showed that the photosynthetic "lunch break" phenomenon can occur at low levels of photosynthetically active radiation (about 500μmol m -2 s -1 ), and that it has a certain relationship with plant growth rhythm and genetic characteristics. Nevertheless, strong PAR is the basic driving force that causes a variety of changes in environmental factors such as reduced atmospheric humidity and increased atmospheric temperature, indirectly leading to the photosynthetic "lunch break" phenomenon [16]. ...
In order to study the effect of light competition and microclimatic modifications on the net assimilation (NA), growth and yield of soybean (Glycine max L.) as an understory crop, three 26-year-old soybean-tree (Acer saccharinum Marsh., Populus deltoides X nigra, Juglans nigra L.) intercropping systems were examined. Tree competition reduced photosynthetically active radiation (PAR) incident on soybeans and reduced net assimilation, growth and yield of soybean. Soil moisture of 20 cm depth close (< 3 m) to the tree rows was also reduced. Correlation analysis showed that NA and soil water content were highly correlated with growth and yield of soybean. When compared with the monoculture soybean system, the relative humidity (RH) of the poplar-soybean, silver maple-soybean, and black walnut-soybean intercropped systems was increased by 7.1%, 8.0% and 5.9%, soil water content was reduced by 37.8%, 26.3% and 30.9%, ambient temperature was reduced by 1.3°C, 1.4°C and 1.0°C, PAR was reduced by 53.6%, 57.9% and 39.9%, and air CO2 concentration was reduced by 3.7μmol·mol-1, 4.2μmol·mol-1 and 2.8μmol·mol-1, respectively. Compared to the monoculture, the average NA of soybean in poplar, maple and walnut treatments was also reduced by 53.1%, 67.5% and 46.5%, respectively. Multivariate stepwise regression analysis showed that PAR, ambient temperature and CO2 concentration were the dominant factors influencing net photosynthetic rate.
Abstract. Climatic reconstructions based on tree-ring isotopic series convey substantial information about past conditions prevailing in forested regions of the globe. However, in some cases, the relations between isotopic and climatic records appear unstable over time, generating the ‘isotopic divergences’. Former reviews have thoroughly discussed the divergence concept for tree-ring physical properties, but not for isotopes. Here we want to take stock of the isotopic divergence problem, express concerns and stimulate collaborative work for improving paleoclimatic reconstructions.
There are five main causes for divergent parts in isotopic and climatic series. (1) Artefacts due to sampling and data treatment, relevant for dealing with long-series using sub-fossil stems. (2) Stand dynamics, including juvenile effects mostly occurring in the early part of tree-ring series. (3) Rise in atmospheric pCO<sub>2</sub>, which can directly influence the foliar behaviour. (4) Change of climate, which may modify the isotope-climate causal links. Finally (5), atmospheric pollution, which may alter leaf and root functions. Future paleoclimate research would benefit from interdisciplinary efforts designed to develop further process-based models integrating multi-proxy inputs, so to help identify causes of isotopic divergences and circumvent some of them in inverse applications.
This study investigates the effects of radiation heat-load reduction under different shading conditions on the growth of three-year-old bayberry (Myrica rubra) trees from 1 July through 31 October 2007 in the Zhejiang Province, a warm subtropical region of China. The trees were grown under direct sunlight (control) and under 25%, 50%, and 75% shading conditions using black plastic nets. Stomatal conductance and photosynthesis were greatest under 50% shading, as were plant height and leaf and root dry weights. Twenty-five percent shading did not significantly influence plant height or root and leaf dry weights, whereas 75% shading resulted in a decrease in root and leaf dry weights when compared with the controls. The photochemical efficiency and electron transport of PSII increased under shaded conditions due to an increase in D1 protein. The concentrations of chlorophyll a and b and the total chlorophyll content in leaves were increased in plants grown under 25% and 50% shading, but reduced in those grown under 75% shading. Under 50% shading, growth and biomass increased due to increased photosynthesis, which resulted from decreased photodamage and increased chlorophyll concentration. These data show that 50% shading promotes optimal growth in bayberry plants.
Steady state field measurements of the photosynthetic light-, temperature- and humidity-response of intact Arbutus unedo leaves were carried out parallel to the 1982/83 diurnal course measurements of CO2 and H2O-gas exchange described in parts I and II of the present series. The following physiologically meaningful parameters were calculated from these single factor response curves, and subsequently related to the annual course of microclimate conditions of the growing site and plant water status:
Diurnal courses of CO2 and H2O gas exchange of intact, attached leaves of the mediterranean evergreen sclerophyll Arbutus unedo were measured under natural ambient conditions during an entire vegetation period in a macchia in Portugal. The measurements were performed with a mobile field laboratory with two climatizised gas exchange cuvettes. During the entire season, measurements of leaf water potential (pressure chamber) were conducted in parallel with the gas exchange experiments. The gas exchange behaviour of well watered plants during the dry season was also studied. The primary period of photosynthetic carbon gain is between November and May. As soon as the first rains of the autumn begin, there is a pronounced increase in net photosynthesis. The maximum photosynthetic rates of the year are exhibited by the recently flushed young leaves in the spring. In contrast to other mediterranean evergreens net photosynthetic rates of Arbutus unedo leaves under normal atmospheric conditions do not decrease during the cold period in the winter. Therefore, as was also shown by factor-dependence measurements at the same time of year, the photosynthetic apparatus of these leaves seems to be relatively insensitive to low temperatures. Under water stress conditions in the summer, a marked decrease in net photosynthesis can be seen. Beginning at the end of May, a midday stomatal closure can be observed together with a simultaneous depression of net photosynthesis and transpiration. The water loss and the carbon gain of the leaves is drastically curtailed by these reactions. During the summer dry period, midday stomatal closure can also be observed with well watered plants. High CO2 loss which might be caused by a temperature-dependent increase of res piration in the summer is avoided by a corresponding temperature acclimation of dark respiration.
In the years 1982, 1983 and in spring 1984 the CO2 gas exchange of two spring geophytes (Allium ursinum, Arum maculatum), of a wintergreen herb (Asarum europaeum), of a summergreen herb (Mercurialis perennis) and of two summergreen grasses (Hordelymus europaeus, Mélica uniflora) was studied in the understorey layer of a beech forest on limestone.
Low temperatures decrease the quality and reduce the yield of grapevine. Sucrose metabolism is among the key factors that explain the differences in cold resistance among different grapevines. We proposed that photosynthetic characteristics and carbohydrate metabolism are also associated with differences in cold resistance. To test this hypothesis, we measured the photosynthetic parameters of net photosynthesis, stomatal conductance, intercellular CO2 concentration, ΦPSII (Fv/Fm), and total chlorophyll concentration, as well as the variations of numerous major carbohydrates (starch, sucrose, glucose, fructose, mannose, maltose, and raffinose), under short-term exposure to low temperature. We also examined the gene expression of two sugar-associated metabolizing enzymes, neutral invertase and sucrose phosphate synthase, in different tissues at the transcriptional level under low temperature. The photosynthetic parameters, carbohydrate concentration, and sucrose metabolism–related enzyme activities of grape plantlets all exhibited significant changes under low temperature. Our results showed that these changes differed among cold-resistant varieties and were closely related with cold resistance. This study provides an important theoretical basis for research into the cold resistance of wine grapes.
In the following, 450 papers dealing with plant-water-relationships are mentioned out of a total of approximately 2300 publications on this topic from the years 1984/85 until 1987, which came to the notice of the author. For a more exhaustive coverage of titles and keywords reference is made to the annual volumes of “Waterin-Plants Bibliography” (e.g. Pospišilová and Solárová 1987).
Under conditions of light intensities exceeding the photosynthetic use carotenoids of the xanthophyll cycle have been shown to be involved in the dissipation of excess energy. When, after a period of darkness, low light-adapted leaves (400 mu mol m(-2) s(-1)) of cv. Orion vines were suddenly exposed to high light (800 mu mol m(-2) s(-1)) the zeaxanthin (Z) content of the leaves increased significantly within 3 min at the expense of violaxanthin (V); a steady state was reached after ca. 20 min. On the contrary, when high light-adapted leaves were abruptly exposed to darkness the Z content decreased and the V content increased to a steady state within 2.5 h. In both trials, the intermediate substance of the xanthophyll cycle, antheraxanthin (A), remained almost constant at a relative low level. In field experiments with cv. Gf.Ga-47-42 an increase of sunlight in the morning was accompanied by increases of A+Z and decreases of V while a decline of sunlight in the afternoon was associated with decreases of A+Z and an increase of V. In laboratory and field experiment the xanthophyll-irradiance relation showed hysteresis. The epoxidation state (EPS, V+0.5A / V+A+Z) decreased in the morning to a minimum at noon and then increased again in the afternoon reflecting a distinct depression of photosynthesis at midday.
High midday temperatures can depress net photosynthesis. We investigated possible mechanisms underlying this phenomenon in leaves of Eperua grandiflora (Aubl.) Benth. saplings. This tropical tree establishes in small gaps in the rainforest canopy where direct sunlight can raise midday temperatures markedly. We simulated this microclimate in a growth chamber by varying air temperature between 28 and 38 degreesC at constant vapor pressure. A decrease in stomatal conductance in response to an increase in leaf-to-air vapor pressure difference (DeltaW) caused by an increase in leaf temperature (T(leaf)) was the principal reason for the decrease in net photosynthesis between 28 and 33 degreesC. Net photosynthesis decreased further between 33 and 38 degreesC. Direct effects on mesophyll functioning and indirect effects through DeltaW were of similar magnitude in this temperature range. Mitochondrial respiration during photosynthesis was insensitive to T(leaf) over the investigated temperature range; it thus did not contribute to midday depression of net photosynthesis. Internal conductance for CO(2) diffusion in the leaf, estimated by combined gas exchange and chlorophyll fluorescence measurements, decreased slightly with increasing T(leaf). However, the decrease in photosynthetic rate with increasing T(leaf) was larger and thus the difference in CO(2) partial pressure between the substomatal cavity and chloroplast was smaller, leading to the conclusion that this factor was not causally involved in midday depression. Carboxylation capacity inferred from the CO(2) response of photosynthesis increased between 28 and 33 degreesC, but remained unchanged between 33 and 38 degreesC. Increased oxygenation of ribulose-1,5-bisphosphate relative to its carboxylation and the concomitant increase in photorespiration with increasing T(leaf) were thus not compensated by an increase in carboxylation capacity over the higher temperature range. This was the principal reason for the negative effect of high midday temperatures on mesophyll functioning.
During the flowering period, the diurnal changes of Pn of Scutellaria baicalensis and environmental factors in three different habitats (Linjiang, Changchun, Taonan) were measured, and the obtained data were analyzed to explore the diurnal changes of Pn and its relationship with environmental factors from three different ecological zones. The conclusion will provide the theoretical basis of physiology and ecology of photosynthesis for scale field cultivation of S. baicalensis in Jilin province. The results indicated that the diurnal changes of Pn of S. baicalensis in three different habitats showed a typical double-peak curve with a slight "midday depression of photosynthesis" , and the Pn reduction at noon originated from the co-effects of stomata factor. There was a significant (p <0. 01 ) positive relationship between the Pn and PAR in the three zones. The positive relationship was significant (p < 0. 01 ) between the Ca and the Pn in Changchun, and significant (p<0.05) between the relative humidity and the Pn in Taonan. The direct impact of environmental factors to Pn in three different habitats was the order of PAR > Ca > Ta > RH > T, in Linjiang, Ta > RH > PAR > Ca > TL in Changchun, PAR > RH > TL > Ca > Ta in Taonan. Low air humidity was an important ecological factor in determining the "midday depression of photosynthesis"- High temperature and humidity in Linjiang, low CO 2 concentration in Changchun, and low relative humidity in Taonan were the main environmental factors influencing the net photosynthetic rate of S. baicalensis , in which was affected by the integration and interactions of PAR, Ta, RH, and Ca. The dominant factor for the net photosynthetic rate of S. baicalensis was different in different habitats.
Diurnal courses in gas exchange, photochemical efficiency and water relations were monitored during two late summers in three groups of adult Quercus robur L. trees, planted along an urbanization gradient that correlated positively with the degree of die-back exhibited by the trees. Leaf carbon: nitrogen ratios, proline and polyphenol levels were monitored to explain why the intermediate group of trees were more severely infested (p ≤ 0.01) with Asterolecanium quercicola (Bouche). All three groups of trees showed a significant correlation of net photosynthesis (A) with photon flux density (PPFD), but A correlated more positively with the pre-dawn leaf water potential ψ(pd) of the moderately (trees of group b, i.e. at the edge of town) and severely (frees of group c, i.e. urban) water-stressed trees. A of the rural trees and stomatal conductance (g) of the three groups of trees showed little correlation ψ(pd) values. Possibly due to the long-term effect of stress, g, as reflected by changes in the transpiration rate (E), showed a significantly (p≤0.01) higher sensitivity to relative ambient humidity (RH) in the trees of groups b and c. Photochemically, a close coupling was found to exist between A, ψ(pd), RH, the time needed to reach the maximum fluorescence level, i.e. FTm, and S, i.e. the complementary area normalized to the variable fluorescence, indicating that the trees were also affected at this level of organization. Proline accumulation occurred in the trees of group c but not in the trees of group b, as opposed to the polyphenolic concentrations which were significantly (p ≤ 0.05) higher in the trees of group b than in the trees of group c. A possible explanation for the higher infestation of A. quercicola on the trees in group b is given in terms of their host specificity and changes in these trees' nitrogen levels.
As an essential parameter in carbon cycle models, the photosynthetic optimal tempera-ture plays a key role in determining the model accuracy. Thus, it is of great importance to study the response of photosynthetic optimal temperature to changes in environmental temperature. In this study, photosynthetic temperature responses in leaves of Cinnamomum camphora and Osman-thus fragrans were examined by using the Li-6400XT photosynthetic analyzer (Li-Cor, Inc. , Lin-coln, Nebraska, USA) at 10-20 days intervals. Photosynthetic temperature response curves were fitted with an unsymmetrical parabolic curve, and the photosynthetic optimal temperature and maximum photosynthetic rate were calculated from the parameters of the fitted curves. Results showed that photosynthetic temperature response curves of both C. camphora and O. fragrans ex-hibited a pattern of parabolic curve, and the breadth of the curves had positive correlations with the ambient temperature. For the two species, the relationships between optimal temperature and ambient temperature showed significant linear correlations within a certain temperature range and the optimal temperature increased with the increase of ambient temperature, with C. camphora being more sensitive to changes in the ambient temperature than O. fragrans. The maximum pho-tosynthetic rate was linearly correlated with optimal photosynthetic temperature. Similarly, the maximum photosynthesis rate of C. camphora was more sensitive to changes in the optimum pho-tosynthetic temperature than that of O. fragrans. We concluded that C. camphora had a greater photosynthetic capacity than O. fragrans in terms of their abilities in response to change in ambi-ent temperature. © 2014, Editorial Board of Chinese Journal of Ecology. All rights reserved.
Whether plants adapt the local condition limited or not mostly depend on whether they may coordinate the relationship between carbon assimilation and water dissipation or not, that is, plant water use efficiency is the key factor of their surviving. However, at the different scale, the meaning of plant water use efficiency is different, this paper summed up the studying progress of WUE at the leaf level, which it is mostly researched in the recent, and presented following points studying the leaf water use efficiency for the future; for the methods, Foliar carbon isotope is the optimal way measuring plant foliar long water use efficiency at present, and the substitution indices of δ C will continue to be a prospective direction; For studying contents, we should strengthen the research on δ13 C and WUE of the riparian trees in the extreme arid area; Combined the plant ecophysiology, biology with the stable isotope technique, exploring the mechanism of plant foliar water use efficiency, especially reinforcing the studying of laws and interior mechanism of plant foliar long water use efficiency variety by means of the model of double isotope; using multi-means, research on the WUEs through various temporal-spatial scales and conversion.
Photosynthetic characteristics of leaf and calyx of Pairs polyphylla var. yunnanensis in Kunming (2100 m a.s.l.) and Zelong (985 m a.s.l.) of Yunnan Province, Southwest China were investigated in July, 2013. The results showed that the diurnal variations of the net photosynthetic rates (Pn) of leaf and calyx at the two sites exhibited a doublespeak curve with a midday depression of photosynthesis at 14:00. The maximum Pn values of leaf and calyx were observed at 10:00 and 12:00, respectively. The relatively high temperature and low relative humidity attributed to the midday depression of photosynthesis of P. polyphylla var. yunnanensis in Kunming and Zelong, respectively. The daily average Pn, maximum Pn, light saturation point and initial quantum yield of either leaf or calyx in Kunming were higher than in Zelong, whereas the lower light compensation point and dark respiratory rates of leaf and calyx were found in Kunming, which implied a higher photosynthetic capacity of Pairs polyphylla var. yunnanensis in Kunming. The photosynthetic characteristics of calyx and leaf were similar in Kunming, which showed a wide adaptability of P. polyphylla var. yunnanensis to light. The maximum Pn and light saturation point of calyx were significantly lower (P<0.05) than those of leaf in Zelong, while the significantly higher initial quantum yield was observed for calyx, which revealed a narrow adaptability to light and a high utilization efficiency to weak light for calyx of P. polyphylla var. Yunnanensis at the low altitude site. Calyx removal resulted in the decrease of fruit dry mass, and the extent of decrease in Zhelong was significantly smaller than that in Kunming. © 2015, editorial Board of Chinese Journal of Ecology. All rights reserved.
Understanding how photosynthesis responds to the environment is crucial for improving plant production and maintaining biodiversity in the context of global change. Covering all aspects of photosynthesis, from basic concepts to methodologies, from the organelle to whole ecosystem levels, this is an integrated guide to photosynthesis in an environmentally dynamic context. Focusing on the ecophysiology of photosynthesis how photosynthesis varies in time and space, responds and adapts to environmental conditions and differs among species within an evolutionary context the book features contributions from leaders in the field. The approach is interdisciplinary and the topics covered have applications for ecology, environmental sciences, agronomy, forestry and meteorology. It also addresses applied fields such as climate change, biomass and biofuel production and genetic engineering, making a valuable contribution to our understanding of the impacts of climate change on the primary productivity of the globe and on ecosystem stability.
Gas exchange characteristics were studied in two mangrove species, Aegiceras corniculatum (L.) Blanco and Avicennia marina (Forstk.) Vierh. var australasica (Walp.) Moldenke, grown under a variety of salinity and humidity conditions. The assimilation rate was measured as a function of the intercellular CO(2) concentration [A(c(i)) curve]. The photosynthetic capacity decreased with increase in salinity from 50 to 500 millimolar NaCl, as shown by decline in both the initial linear slope and the upper plateau of the A(c(i)) curve, with A. corniculatum being the more sensitive species. The decline in photosynthetic capacity was enhanced by increase in the leaf to air vapor pressure difference from 6 to 24 millibars, but this treatment caused a decrease in only the upper plateau of the A(c(i)) curve. Stomatal conductance was such that the intercellular CO(2) concentration obtaining under normal atmospheric conditions occurred near the transition between the lower linear and upper plateau portions of the A(c(i)) curves. Thus, stomatal conductance and photosynthetic capacity together co-limited the assimilation rate, which declined with increasing salinity and decreasing humidity. The marginal water cost of carbon assimilation was similar in most treatments, despite variation in the water loss/carbon gain ratio.
Well watered shrubs of ten Mediterranean sclerophyllous species were studied. under simulated habitat conditions in an environmental chamber. Temperature, air humidity and light intensity were altered stepwise to simulate a diurnal course of weather conditions similar to that measured in an evergreen macchia in Portugal on a typical cloudless summer day. Twigs with leaves were enclosed in cuvettes which reproduced the growth chamber climate and which allowed measurement of CO2 exchange and transpiration. The species investigated differed considerably in stomatal responses at midday to high air temperature and steep leaf to air vapor pressure gradient. Arbutus unedo, Laurus nobilis, Phillyrea angustifolia, Quercus coccifera, Q. ilex, and Q. suber exhibited pronounced midday closure of stomata, which resulted in a strong depression of net photosynthesis and transpiration rates, followed by reopening of stomata and higher rates of CO2 uptake and water loss. Arbutus andrachne, Nerium oleander, Pistacia lentiscus, and Smilax aspera, on the other hand, showed only a single maximum in stomatal conductance. We conclude, that the differences may reflect species-specific sensitivities of the stomatal apparatus to atmospheric stress in the form of high temperature and low air humidity. All of the species investigated exhibited a lower leaf conductance and a lower net photosynthesis in the afternoon when compared under the same environmental conditions as in the morning. This time dependent response was independent of the absolute degree of maximal conductance and independent of whether or not the diurnal course of stomatal conductance was two-peaked. These observations indicate that it is necessary in the future to consider more carefully the effects of time dependent processes on regulation of stomatal conductance.
Arbutus unedo, Laurus nobilis, Phillyrea angustifolia, Quercus coccifera, Q. ilex and Q. suber exhibited pronounced midday closure of stomata, which resulted in a strong depression of net photosynthesis and transpiration rates, followed by reopening of stomata and higher rates of CO2 uptake and water loss. Arbutus andrachne, Nerium oleander, Pistacia lentiscus and Smilax aspera, on the other hand, showed only a single maximum in stomatal conductance. Differences may reflect species-specific sensitivities of the stomatal apparatus to atmospheric stress in the form of high temperature and low air humidity. All of the species investigated exhibited a lower leaf conductance and a lower net photosynthesis in the afternoon when compared under the same environmental conditions as in the morning. -from Authors
Large areas of the lower epidermis of full-grown leaves of Polypodium vulgare (and Valerianella locusta) are normally separated from the mesophyll by an extensive subepidermal airspace. Epidermal stripes were prepared for experiments to simulate these conditions in order to investigate stomatal reactions. They were placed with their inner surface in contact with an airspace of uniformly high humidity. The outer surface was treated with air of varying degrees of humidity. The stomatal reactions were observed by microscope and the opening of the guard cells determined photographically.
Treatment of the outer side of the epidermis with dry air led to a rapid closing of the stomata, whilst moist air caused opening. This induction of opening and closing movements could be repeated up to 15 times with the same stoma by changing the degree of humidity. Neighbouring groups of stomata showed different apertures according to their individual humidity conditions. The degree of aperture of the stomata depended on the water potential of the ambient air and also on the humidity conditions in the subepidermal airspace.
The cause of this stomatal behaviour could lie in the “peristomatal transpiration”. In this way, the guard cells are able to function as “humidity sensors” which “measure” the difference in water potential inside and outside the leaf. Their aperture thus is controlled by their individual transpiration conditions. This controlling mechanism could be very important for the water economy of plants. They would appear to be able to reduce their transpiration through an increase in diffusion resistance of the stomata during decreasing humidity in the ambient air, without changing the water status of the whole leaf.
Measurements of transpiration, leaf water content, and flux of water in a cotton plant exhibiting sustained oscillations, in stomatal conductance are presented, and a model of the mechanism causing this behaviour is developed. The dynamic elements, of the model are capacitors-representing the change of water content with water potential in mesophyll, subsidiary and guard cells-interconnected by resistances representing flow paths in the plant. Increase of water potential in guard cells causes an increase in stomatal conductance. Increase of water potential in the subsidiary cells has the opposite effect and provides the positive feed-back which can cause stomatal conductance to oscillate. The oscillations are shown to have many of the characteristics of free-running oscillations in real plants. The behaviour of the model has been examined, using an analogue computer, with constraints and perturbations representing some of those which could be applied to real plants in physiological experiments. Aspects of behaviour which have been simulated are (a) opening and closing of stomata under the influence of changes in illumination, (b) transient responses due to step changes in potential transpiration, root permeability and potential of water surrounding the roots, (c) the influence of these factors on the occurrence and shape of spontaneous oscillations, and (d) modulation of sustained oscillations due to a circadian rhythm in the permeability of roots.
The stomata of plants growing in the Negev Desert, namely the stomata of the mesomorphic leaves of Prunus armeniaca, the xeromorphic stems of Hammada scoparia, and the succulent leaves of Zygophyllum dumosum, respond to changes in air humidity. Under dry air conditions diffusion resistance increases. Under moist air conditions diffusion resistance decreases. When the stomata close at low air humidity the water content of the apricot leaves increases. The stomata open at high air humidity in spite of a decrease in leaf water content. This excludes a reaction via the water potential in the leaf tissue and proves that the stomatal aperture has a direct response to the evaporative conditions in the atmosphere. In all species the response to air humidity is maintained over a period of many hours also when the soil is considerably dry. The response is higher in plants with poor water supply then in well watered plants. Thus for field conditions and for morphologically different types of photosynthesizing organs the results confirm former experiments carried out with isolated epidermal strips.
A series of experiments is presented investigating short term and long term changes of the nature of the response of rate of CO2 assimilation to intercellular p(CO2). The relationships between CO2 assimilation rate and biochemical components of leaf photosynthesis, such as ribulose-bisphosphate (RuP2) carboxylase-oxygenase activity and electron transport capacity are examined and related to current theory of CO2 assimilation in leaves of C3 species. It was found that the response of the rate of CO2 assimilation to irradiance, partial pressure of O2, p(O2), and temperature was different at low and high intercellular p(CO2), suggesting that CO2 assimilation rate is governed by different processes at low and high intercellular p(CO2). In longer term changes in CO2 assimilation rate, induced by different growth conditions, the initial slope of the response of CO2 assimilation rate to intercellular p(CO2) could be correlated to in vitro measurements of RuP2 carboxylase activity. Also, CO2 assimilation rate at high p(CO2) could be correlated to in vitro measurements of electron transport rate. These results are consistent with the hypothesis that CO2 assimilation rate is limited by the RuP2 saturated rate of the RuP2 carboxylase-oxygenase at low intercellular p(CO2) and by the rate allowed by RuP2 regeneration capacity at high intercellular p(CO2).
The effect of osmotic dehydration on metabolic reactions in three different subcellular compartments (chloroplast, cytoplasm and mitochondria) was studied in vacuum-infiltrated thin leaf slices from various plants, in the absence of stomatal control. The reactions tested were CO2 fixation in the light (chloroplast), CO2 fixation in the dark (cytoplasm), and O2 uptake in the dark (mitochondria). In most plants, the sensitivity of dark CO2 fixation to dehydration was similar to the sensitivity of photosynthesis. In leaf slices from a plant with Crassulacean acid metabolism (Kalanchoe pinnata), dark CO2 fixation (which reached similar rates as light fixation) was slightly more sensitive to osmotic stress than photosynthesis. Dark respiration (measured as O2 uptake) was significantly more resistant to hypertonic stress than both types of CO2 fixation. In crude leaf extracts from spinach, the response of soluble enzymes from the three different subcellular compartments to high concentrations of various electrolytes and neutral compounds was examined and compared with the in-vivo data.
High transpiration rates were found to affect the photosynthetic capacity of Xanthium strumarium L. leaves in a manner analagous to that of low soil water potential. The effect was also looked for and found in Gossypium hirsutum L., Agathis robusta (C. Moore ex Muell.) Bailey, Eucalyptus microcarpa Maiden, Larrea divaricata Cav., the wilty flacca tomato mutant (Lycopersicon esculentum (L.) Mill.) and Scrophularia desertorum (Munz) Shaw. Two methods were used to distinguish between effects on stomatal conductance, which can lower assimilation by reducing CO2 availability, and effects on the photosynthetic capacity of the mesophyll. First, the response of assimilation to intercellular CO2 pressure (C
i) was compared under conditions of high and low transpiration. Second, in addition to estimating C
i using the usual Ohm's law analogy, C
i was measured directly using the closed-loop technique of T.D. Sharkey, K. Imai, G.D. Farquhar and I.R. Cowan (1982, Plant Physiol, 60, 657–659). Transpiration stress responses of Xanthium strumarium were compared with soil drought effects. Both stresses reduced photosynthesis at high C
i but not at low C
i; transpiration stress increased the quantum requirement of photosynthesis. Transpiration stress could be induced in small sections of leaves. Total transpiration from the plant did not influence the photosynthetic capacity of a leaf kept under constant conditions, indicating that water deficits develop over small areas within the leaf. The effect of high transpiration on photosynthesis was reversed approximately half-way by returning the plants to low-transpiration conditions. This reversal occurred as fast as measurements could be made (5 min), but little further recovery was observed in subsequent hours.
The carbon-dioxide response of photosynthesis of leaves of Quercus suber, a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a "standard" time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO2 pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO2 uptake. Midday depression of CO2 uptake is the result of i) a decrease in CO2-saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO2 response curve (carboxylation efficiency), and iii) a substantial increase in the CO2 compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO2 pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO2 may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.
(±)-Abscisic acid (ABA) at 10(-5) M was added to the transpiration stream of leaves of 16 species (C3 and C4, monocotyledons and dicotyledons). Stomatal responses followed one of three patterns: i) stomata that were wide and insensitive to CO2 initially, closed partially and became sensitive to CO2; ii) for stomata that were sensitive to CO2 before the application of ABA, the range of highest sensitivity to CO2 shifted from high to low intercellular partial pressures of CO2, for instance in leaves of Zea mays from 170-350 to 70-140 μbar; iii) when stomata responded strongly to ABA, their conductance was reduced to a small fraction of the initial conductance, and sensitivity to CO2 was lost. The photosynthetic apparatus was affected by applications of ABA to various degrees, from no response at all (in agreement with several previous reports on the absence of effects of ABA on photosynthesis) through a temporary decrease of its activity to a lasting reduction. Saturation curves of photosynthesis with respect to the partial pressure of CO2 in the intercellular spaces indicated that application of ABA could produce three phenomena: i) a reduction of the initial slope of the saturation curve (which indicates a diminished carboxylation efficiency); ii) a reduction of the level of the CO2-saturated rate of assimilation (which indicates a reduction of the ribulose-1,5-bisphosphate regeneration capacity); and iii) an increase of the CO2 compensation point. Photosynthesis of isolated mesophyll cells was not affected by ABA treatments. Responses of the stomatal and photosynthetic apparatus were usually synchronous and often proportional to each other, with the result that the partial pressure of CO2 in the intercellular spaces frequently remained constant in spite of large changes in conductance and assimilation rate. Guard cells and the photosynthetic apparatus were able to recover from effects of ABA applications while the ABA supply continued. Recovery was usually partial, in the case of the photosynthetic apparatus occasionally complete. Abscisic acid did not cause stomatal closure or decreases in the rate of photosynthesis when it was applied during a phase of stomatal opening and induction of photosynthesis that followed a transition from darkness to light.
(14C)abscisic acid release from preloaded leaf slices of various plants was stimulated by hypertonic treatment. Wilting-induced efflux of ABA from the mesophyll and a transient increase of (14C)ABA in the lower epidermis was also observed in preloaded leaf discs. The data indicate that water stress does not only affect ABA synthesis and metabolism, but also ABA distribution between symplast and apoplast and between various leaf tissues. External osmotic potentials (πo) needed for a given ABA efflux were closely related to the osmotic potentials of the cell sap (πcs) which was very different for various plants. The trigger for stress induced ABA efflux from the mesophyll appeared to be cell shrinkage rather than turgor changes.
The temperature dependence of net photosynthetic assimilation of CO2 by snowgum (Eucalyptus pauciflora Sieb. ex Spreng.) was investigated. CO2 assimilation was divided into its component processes, stomatal and biochemical. The biochemical limitation was investigated with gas-exchange techniques and found to conform well to a recent model of C3 photosynthesis. In line with the model, net assimilation was further divided into ribulose 1,5-bisphosphate (RuP2) regeneration or electron-transport/photophosphorylation limitation, limitation by RuP2 carboxylase/oxygenase (Rubisco; EC 4.1.1.39) activity, together with loss of CO2 in non-photorespiratory respiration. The estimated temperature dependence of electron transport agreed well with one published for uncoupled electron transport, while the estimated temperature dependence of the catalytic activity of Rubisco was slightly less than that reported from biochemical determinations. The estimated rate of non-photorespiratory respiration was about 0.6 times the rate of respiration at night and appeared to have the same temperature dependence. With this information the temperature dependence of the biochemical limitation was modelled. Stomatal conductance was assumed to follow the theory of constant marginal water cost of carbon assimilation (δE/δA) and net assimilation rate at ambient concentration of CO2 was predicted. It was concluded that, for a given Rubisco activity and RuP2-regeneration capacity, both temperature optimum and net assimilation rate at the optimum temperature increased with increasing stomatal conductance.
This review describes the current hypotheses of how humidity and plant and soil water status may interact and regulate stomatal conductance and photosynthesis. This review will focus on the effects of 1. humidity, 2. leaf water potential and leaf turgor, and of 3. soil water status on leaf conductance, transpiration, and CO/sub 2/ assimilation.
Scattering of green light and chlorophyll fluorescence by spinach leaves kept in a stream of air or nitrogen were compared with leaf adenylate levels during illumination with blue, red or far-red light. Energy charge and ATP-ADP ratios exhibited considerable variability in different leaves both in the dark and in the light. Variability is explained by different possible states of the reaction oxidizing triose phosphate or reducing 3-phosphoglycerate. Except when oxygen levels were low, there was an inverse relationship between light scattering and chlorophyll fluorescence during illumination with blue or red light. When CO2 was added to a stream of CO2-free air, chlorophyll fluorescence increased, sometimes after a transient decrease, and both light scattering and leaf ratios decreased. Similar observations were made when air was replaced by nitrogen under blue or high-intensity red light. Under these conditions, over-reduction caused inhibition of electron transport and phosphorylation in chloroplasts. However, when air was replaced by nitrogen during illumination with low-intensity red light or far-red light, light scattering increased instead of decreasing. Under these light conditions, ratios were maintained in the light. They decreased drastically only after darkening. Although ratios responded faster than light scattering or the slow secondary decline of chlorophyll fluorescence due to illumination, it appeared that in the steady state, light scattering and chlorophyll fluorescence are useful indicators of the phosphorylation state of the leaf adenylate system at least under aerobic conditions, when chloroplast and extrachloroplast adenylate systems can effectively communicate.
Midday closure of stomata of well-watered ( between-10 and-25 bar) or moderately stressed ( between-25 and-35 bar) Arbutus unedo plants occurs when midday leaf temperatures increase above 30C and vapor pressure difference between leaf air spaces and the external air increases above approximately 30 mbar/bar. Moderate water stress decreases maximum conductance and may result in greater sensitivity to high leaf temperature and vapor pressure dificit, which results in earlier closure and later reopening of stomata. Severe water stress ( of-50 bar) changes the form of the daily pattern observed for leaf conductance. A single morning peak in conductance occurs followed by decrease in conductance over the remainder of the day. Morning fog in Portugal during the dry season may facilitate stomatal opening and may allow improvement of carbon balances of the leaves for short periods, but contributes little to improvement of plant water balances over the longer term.
of secondary importance. We report here that the photosynthetic apparatus of Arbutus unedo is sensitive also to much lower degrees of air vapor deficit, responding to air humidity differences with changing sensitivity during the total course of the day. Plant performance in the field will be compared with measurements under simulated natural conditions in a growth chamber at low and high humidities, applying saturating ambient CO2 in order to exclude stomatal limitations. Materials and methods
Experiments with Prunus armeniaca were carried out under conditions of constant temperature but varying air humidity. Experiments were also contucted with a constant water vapor difference between the evaporating sites in a leaf and the air, but with varying leaf temperature. These served as a basis for predicting the daily course of total diffusion resistance under the natural climatic conditions of a desert. For the simulation, the rsults of the experiments at constant conditions with only one variable factor are fitted with empirical equations which serve as “calibration curves” to predict the change in diffusion resistance caused by a change in humidity and temperature calculated from the meteorological data of a desert day. The simulation shows that for P. armeniaca humidity and temperature are the dominating factors in controlling the daily course of diffusion resistance. For meteorologically very different days the simulation allows the increase in diffusion resistance in the morning to be predicted with an accuracy of 90%–105% as compared to directly observed measurements. In the afternoon, especially after extreme climatic conditions during the morning, the deviation between predicted and observed values of diffusion resistance may be greater, but not more than -20% to -30%. This possibly indicates the existence of an additional factor of significance which was not included in the simulation. The two peaked curves of net photosynthesis and transpiration characteristic of plants living under arid conditions can be explained in this species by the humidity-and temperature-controlled stomatal response. This stomatal regulation leads to a decreasing total daily transpirational water loss on a dry day as compared to a moist one. The significance of this controlling mechanism for the primary production and the water relations of P. armeniaca is discussed.
Endogenous abscisic acid content (ABA) of Arbutus unedo leaves growing under natural conditions in a macchia near Sobreda, Portugal, was very high (0.25 to 2.3 μg g1 fresh weight). Highest concentrations were found during the very early morning hours and at midday. During the late morning hours and in the late afternoon ABA concentrations decreased to between one-third and one-fourth of peak values. The samples for ABA content were obtained from both irrigated (Ψ between-10 and-25 bar) and non-irrigated plants experiencing natural water stress during the dry season (Ψ of-50 bar). During the course of the measurement day, stomatal conductance was relatively constant and conductance of watered plants was 50 to 100% greater than that of unwatered plants. No clear correlations between ABA content and stomatal conductance and/or xylem water potential were observed. Despite large differences in water potential and differences in degree of stomatal opening, absolute concentrations of ABA were not found to differ.
Small quantities (8–14 pmoles cm2 leaf area) of ABA were applied to leaves of irrigated and non-irrigated Arbutus unedo plants by injection into the petiole. These extremely small ABA doses resulted in transient reductions in stomatal conductance. The effectiveness with which injected ABA closed stomata was highest during the morning and decreased substantially at midday. Increased sensitivity to injected ABA may again occur in the late afternoon but recent measurements suggest that this may depend on long-term drought experience of the plants. The characteristics of the response to injected ABA were similar in irrigated and non-irrigated plants although irrigated plants responded in general more strongly.
Shrubs of the Mediterranean sclerophyllous species Arbutus unedo and Quercus ilex were studied under simulated habitat conditions in an environmental chamber. Temperature, humidity, and light intensity were altered stepwise to simulate diurnal changes in conditions similar to those measured in an evergreen macchia in Sobreda, Portugal. Leaves were enclosed in cuvettes which reproduced the growth chamber climate and which allowed measurement of gas exchange. Increasing atmospheric stress in the form of higher temperature and lower humidity on successive days gradually results in midday depression of transpiration rate and net photosynthesis rate of leaves due to midday stomatal closure.
It was found for two species that net carbon dioxide uptake rates were reduced at constant intercellular carbon dioxide partial
pressure when single attached leaves were exposed to large leaf to air water vapour pressure differences. Leaf temperature,
irradiance, and ambient carbon dioxide and oxygen partial pressures were kept constant. Net carbon dioxide uptake rates decreased
linearly with increasing vapour pressure difference, even in cases where transpiration rates were highest at intermediate
values of vapour pressure difference. Decreases in net carbon dioxide uptake rates were quickly reversible. Different wind
speeds across the measured leaf, different vapour pressure deficits around the rest of the shoot, and transient responses
of net carbon dioxide uptake rate to abrupt changes in vapour pressure difference all gave the same response of net carbon
dioxide uptake rate to vapour pressure difference. The data show that the inhibition of net carbon dioxide uptake rate at
a given vapour pressure difference was not simply related to whole leaf transpiration rate or stomatal conductance.
Stomata of Zea mays L. respond to changes in hydrostatic pressure in the water supply of the leaves almost instantaneously and in all leaf parts simultaneously. Therefore, the leaf is a hydraulic unit. The stomata are part of it and their aperture is controlled by the water potential in the water-conducting system. Stomatal aperture is not uniquely related to the relative water content of a leaf. The relation depends also on the humidity in the air and is different for the upper and the lower epidermis.