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The control by atmospheric factors and water stress of midday stomatal closure in Arbutus unedo growing in a natural macchia
Abstract
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.
... Excess light, often occurring with high leaf temperature or water deficit, is a major environmental stress and causes the midday depression of photosynthesis (Schulze et al. 1980;Tenhunen, Lange & Braun 1981;Tenhunen, Lange & Jahner 1982;Tenhunen et al. 1984;Chaves et al. 1987;Downton, Grant & Loveys 1987;Demmig-Adams et al. 1989;Correia et al. 1990;Epron et al. 1992;Valladares & Pearcy 1997). Stomatal closure and photoinhibitory damage are the major physiological factors responsible for the midday depression. ...
... Several reports have emphasized the importance of stomatal closure in photosynthesis suppression in natural high-light environments (Tenhunen et al. 1981(Tenhunen et al. , 1982(Tenhunen et al. , 1984Roessler & Monson 1985;Raschke & Resemann 1986;Cheeseman et al. 1991). However, stomatal limitation of photosynthesis has rarely been quantitatively evaluated in situ. ...
Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured under full sunlight to reveal diffusional and non-diffusional limitations to diurnal assimilation in leaves of Arisaema heterophyllum Blume plants grown either in a riparian forest understorey (shade leaves) or in an adjacent deforested open site (sun leaves). Midday depressions of assimilation rate (A) and leaf conductance of water vapour were remarkably deeper in shade leaves than in sun leaves. To evaluate the diffusional (i.e. stomatal and leaf internal) limitation to assimilation, we used an index [1-A/A(350)], in which A(350) is A at a chloroplast CO2 concentration of 350 mu mol mol(-1). A(350) was estimated from the electron transport rate (J(T)), determined fluorometrically, and the specificity factor of Rubisco (S), determined by gas exchange techniques. In sun leaves under saturating light, the index obtained after the 'peak' of diurnal assimilation was 70% greater than that obtained before the 'peak', but in shade leaves, it was only 20% greater. The photochemical efficiency of photosystem II (Delta F/F-m') and thus J(T) was considerably lower in shade leaves than in sun leaves, especially after the 'peak'. In shade leaves but not in sun leaves, A at a photosynthetically active photon flux density (PPFD) > 500 mu mol m(-2) s(-1) depended positively on J(T) throughout the day. Electron flows used by the carboxylation and oxygenation (J(O)) of RuBP were estimated from A and J(T). In sun leaves, the J(O)/J(T) ratio was significantly higher after the 'peak', but little difference was found in shade leaves. Photorespiratory CO2 efflux in the absence of atmospheric CO2 was about three times higher in sun leaves than in shade leaves. We attribute the midday depression of assimilation in sun leaves to the increased rate of photorespiration caused by stomatal closure, and that in shade leaves to severe photoinhibition. Thus, for sun leaves, increased capacities for photorespiration and non-photochemical quenching are essential to avoid photoinhibitory damage and to tolerate high leaf temperatures and water stress under excess light. The increased Rubisco content in sun leaves, which has been recognized as raising photosynthetic assimilation capacity, also contributes to increase in the capacity for photorespiration.
... Soil water potential data indicate that the trees became drought stressed throughout the summer in mid-day, resulting in stomatal closure and the observed mid-day depression in E t. A mid-day depression in stomatal conductance has been observed by Pataki et al. (1998), Teixeira Filho et al. (1998, Tenhunen et al. (1981Tenhunen et al. ( , 1982, and Lange et al. (1982). Tenhunen et al. (1981Tenhunen et al. ( , 1982 and Lange et al. (1982) considered the mid-day depression to be a characteristic of Mediterranean species that allows them to reduce water losses when the evaporative demand is highest. ...
... A mid-day depression in stomatal conductance has been observed by Pataki et al. (1998), Teixeira Filho et al. (1998, Tenhunen et al. (1981Tenhunen et al. ( , 1982, and Lange et al. (1982). Tenhunen et al. (1981Tenhunen et al. ( , 1982 and Lange et al. (1982) considered the mid-day depression to be a characteristic of Mediterranean species that allows them to reduce water losses when the evaporative demand is highest. Further, they believe that it is probably controlled by the saturation vapor pressure deficit of the air. ...
The weather patterns of the west side of the Sierra Nevada Mountains (cold, wet winters and hot, dry summers) strongly influence how water is partitioned between transpiration and evaporation and result in a specific strategy of water use by ponderosa pine trees (Pinus ponderosa) in this region. To investigate how year-round water fluxes were partitioned in a young ponderosa pine ecosystem in the Sierra Nevada Mountains, water fluxes were continually measured from June 2000 to May 2001 using a combination of sap flow and eddy covariance techniques (above-and below-canopy). Water fluxes were modeled at our study site using a biophysical model, FORFLUX. During summer and fall water fluxes were equally partitioned between transpiration and soil evaporation while transpiration dominated the water fluxes in winter and spring. The trees had high rates of canopy conductance and transpiration in the early morning and mid-late afternoon and a mid-day depression during the dry season. We used a diurnal centroid analysis to show that the timing of high canopy conductance and transpiration relative to high vapor pressure deficit (D) shifted with soil moisture: during periods of low soil moisture canopy conductance and transpiration peaked early in the day when D was low. Conversely, during periods of high soil moisture canopy conductance and transpiration peaked at the same time or later in the day than D. Our observations suggest a general strategy by the pine trees in which they maximize stomatal conductance, and therefore carbon fixation, throughout the day on warm sunny days with high soil moisture (i.e. warm periods in winter and late spring) and maximize stomatal conductance and carbon fixation in the morning through the dry periods. FORFLUX model estimates of evaporation and transpiration were close to measured/calculated values during the dry period, including the drought, but underestimated transpiration and overestimated evaporation during the wet period.
... Θερμοκρασία εδάφους σε βάθος 50 cm (Osonubi and Davies 1980, Tenhunen et al. 1982, Jolly et al. 2005. ...
This study is a contribution to understanding the ways and mechanisms with which tree-stands interact with the environment, forming the micrometeorological regime of the soil-plant-atmosphere system, along with expanding knowledge on the mass and energy exchanges between vegetation and the atmosphere, under the Mediterranean climate conditions. The thesis is focused on the investigation of the micrometeorology of a natural deciduous ecosystem, compared to the weather conditions above, the specification of the interactions between the micrometeorological parameters on a diurnal, seasonal and annual basis (combined with the phenological phases of the vegetation), the differentiation of the micrometeorological regime following a forest fire or logging, the study of the weather effects on the vegetation water requirements and the relationship between water consumption and production, the evaluation of defense and adaptation mechanisms of Mediterranean ecosystems against extreme weather and climate conditions, the evaluation of the forest contribution on CO2 absorption, the study of radiation quantity, quality and distribution in and use by the canopy and finally the study of the ecosystem-environment energy exchanges.
Research was accomplished in a selected natural deciduous oak forest within the region of Corinth, S. Greece, where a well equipped micrometeorological station was placed. The data covers the period 1999-2006 with some missing gaps.
The results of the study show that the main portion of the global solar radiation, Rs and specifically the photosynthetically active radiation, PAR, is captured by the tree-tissues even when there is no foliage. Due to selective absorption of PAR, the light that finally reaches the forest floor is extremely reduced. Logging increases the radiation transmission and reduces reflection and absorption by the canopy, although radiation distributes more uniformly in the foliage. Crawling-fire effect on the optical properties of the ecosystem is rather periodic, because of changes in the properties of the ground only. The seasonal optical properties variation can be used for the accurate identification of the phenological stages of the trees. The exponential radiation model for the radiant energy distribution in the canopy gives satisfactory results, although the deciduous canopy architecture is non uniform.
The temperature and relative humidity profiles inside and over the canopy show an almost mirror effect to each other. During the day, maximum temperature and minimum relative humidity are observed at the top of the forest. Vapor pressure deficit profile is almost identical to that of temperature and vapor pressure, being maximum at the canopy top, reduces with height especially in summer. At night, the temperature and vapor pressure deficit profiles are positive, whereas those of relative humidity and vapor pressure are negative (in all seasons except in winter). Vapor pressure values increase with height in the forest and stabilize above canopy top. The daily temperature range over the ecosystem is relatively small, becoming maximum near the ground, especially at full foliage. The layer of dead leaves covering the forest floor protects roots from frost damage in winter and high temperature and evaporation loss in summer. The period favoring vegetation growth is quite long, but a significant part of it, is characterized by thermal stress.
Logging reduces drought conditions in foliage - by allowing better air mixture - and daily relative humidity range. In extremely low soil moisture conditions (mainly in August) oak trees inhibit their growth. Trees have been adapted to reduced water availability during summer, by developing an extended root system, which allows the exploitation of water in deeper layers. Development over a clay soil layer and formation of a relatively small leaf area increase their ability to survive under the intensive drought Mediterranean conditions. The soil-covering quite thick layer of slowly decomposing dead leaves reduces water losses through evaporation and strengthens vegetation to cope with dry periods.
The wind velocity profile over the canopy follows the typical logarithmic form, day and night. The wind speed values inside the forest (at and below the top) are very small. The canopy roughness parameters, d and zo, take the values 9.6±1.3 m and 1.2±0.9 m, respectively, at full leaf development and 11.2 m and 1.2 m at senescence. Forest surface seems to be very rough, absorbing momentum effectively. Limited tree logging has a negligible effect on wind profile, as vegetation gaps created seem to encourage vertical rather than horizontal air flow.
Net radiation fluxes, Rn, are almost 75% of Rs. Solar radiation is used more effectively during summer than in winter and at midday than at noon or morning. The thermal stress reduces the effective use of radiation but logging seems to increase effectiveness. On a 24 h basis, during the fully leafed period, Rn is composed by 53-56% latent heat, LE, and by 27-30% sensible heat, H. In August, the percentages become 16% and 31%, respectively, under rather extreme dry atmospheric conditions and reduced soil water availability.
The energy absorbed by the canopy (Rn-G) is “consumed” by 81% for H and LE (in June and July), whereas the rest is stored in plant tissues as heat, ΔΗ. ΔH becomes even greater in August, due to water shortage not allowing the productive use of the absorbed solar radiation.
Windspeeds greater than 0.6 m/s favor faster air mixing, reducing LE and H fluxes. However, a speed increase up to 0.6 m/s linearly increases heat fluxes. When trees are fully leafed and during daytime, the Bowen ratio, b, takes values between 0.70-1.67 and most of the time is greater than unit (with smaller values at midday). About 37-67% of the sum (LE+H) is devoted to LE (with lower percentages at midday). Soil heat flux, G, is quite small and gets positive values only a few hours during midday. The evapotranspiration efficiency of PAR absorbed by the ecosystem (ε=LE/PARabs) during the fully leafed period is about 0.69, a relatively small value (compared to similar ecosystems in northern Europe), due to reduced water availability in summer.
The annual water requirements of the ecosystem are about 440 mm, when the annual precipitation in the region is much greater. However, because of its imbalanced time distribution, the trees face water stress in summer, which becomes more intensive in August. The daily mean evapotranspiration rate is maximum in June and July (3.5 mm d-1), with an even greater value at midday (0.43 mm h-1). Under reduced soil water availability (as in August), stomatal resistance increases and water loss reduces rapidly. Stomatal closure is, thus, a defense mechanism that the vegetation calls up under strong drying atmospheric forces. Then, even at midday, the mean evapotranspiration rate is smaller than 0.16 mm h-1.
CO2 absorbance is maximum when the canopy has leaves. In other seasons, the ecosystem is functioning as a source rather than as a sink of carbon. The rates of CO2 absorption are increasing with absorbed radiation fluxes, becoming almost zero when PARabs is lower than 220 W m-2. The annual net carbon uptake by the ecosystem is 6.37 t C ha-1 y-1, when the greater monthly CO2 absorbance is achieved during June and July (mean value 752 g CO2 m-2 month-1) and especially at midday (0.82 g C m-2 h-1). On an annual basis, about 13,500 t CO2 are absorbed by the whole ecosystem (580 ha). Every tree absorbs about 7.3 kg CO2 per year. In August, CO2 absorbance reduces rapidly because of low photosynthetic rates and becomes about half of the maximum (July) values. Water use efficiency (WUE) is about 0.20 mmol H2O/μmol CO2 (water loss of 1 kg produces 3.3 g C). Maximum productive rates are achieved when soil moisture is about 0.35. At lower values CO2 and H2O fluxes reduce almost linearly. Mean daily values of VPD greater than 2 kPa lead to about 50% reduction in productivity.
Although the deciduous ecosystem develops under rather adverse (xerothermic) climatic conditions seems to be quite productive. It has relatively short water needs and developed adaptation (survival) mechanisms, such as the use of morning dew as an additional water source in summer, the root system development for deeper soil water use, the biomass development over a water holding clay soil layer, the evaptoranspiration and photosynthesis reduction under extremely dry conditions via stomatal closure, the development of a relatively small leaf area, the relatively inefficient use of the absorbed by leaves radiation and finally the existence of a thick layer of dead leaves covering the forest floor and protecting the root system against thermal and water stresses.
... At high values, photosynthesis and growth are significantly restricted. Previous studies recommend that VPD less than 900 Pa should exert little effect on stomata whereas values greater than 4100 Pa generally are sufficient to force complete stomatal closure, even when the soils are moist [45,46]. Although these restrictions are various by locations and species, we select a widely used set of parameters (VPD Min = 900 Pa and VPD Max = 4100 Pa) in this research [41]. ...
The fluctuation of streamflow in snowmelt-dominated watersheds may be an indicator of climate change. However, the relationship between the start of growing season (SOS) and the streamflow in snowmelt-dominated watersheds is not clear. In this study, we update the Coupled Hydro-Ecological Simulation System (CHESS) model by incorporating the Growing Season Index (GSI) module to estimate the start of the growing season. The updated CHESS model is then used to calculate the streamflow in the Cleve Creek, Incline Creek and Twin River watersheds located in Nevada in the United States from 1981 to 2017. This updated CHESS can be applied in any regions that are suitable for deciduous vegetation. The streamflow in the static and dynamic scheme in the three watersheds have been simulated between 1981 and 2017 with the NS of 0.52 and 0.80 in the Cleve Creek, 0.46 and 0.75 in the Incline Creek, and 0.42 and 0.70 in the Twin River watersheds, respectively. The results illustrate that the SOS have come around 3–5 weeks earlier during the last 37 years. The results illustrate a high correlation between the temperature and the timing of the SOS. Early SOS leads to a substantial increase in total annual transpiration. An increase in annual transpiration can reduce aquifer recharge and increase cumulative growing season soil moisture deficit. Comparing to the streamflow without vegetation, the streamflow with vegetation is smaller due to transpiration. As the SOS comes earlier, the peaks of the streamflow with vegetation also come earlier. If the shifts in SOS continue, the effects on annual rates of transpiration can be significant, which may reduce the risk of flooding during snowmelt. On the other hand, earlier SOS may cause soil moisture to decline during summer, which would increase the drought stress in trees and the risk of wildfires and insect infestation.
... This can also happen due to the stomatal closure that takes place during the middle of the day to reduce the water loss by the plants and simultaneously brings the CO 2 capture by the plants down [103]. Midday stomatal closure is also reported to happen during the well-watered (or no water-stress) conditions [104]. Similar observations have been reported earlier over different plant canopies [48,87]. ...
The Indian summer monsoon is one of the most important yet less understood synoptic processes on the Earth, characterized by an increased amount of rainfall over the entire Indian landmass. The different types of forest ecosystems existing over the Indian region offer a tremendous carbon sequestration potential useful for the global mitigation of climate change as predicted by the modelling studies. The monsoon results in a strong seasonality of the ecosystem-atmosphere carbon exchange due to the differential availability of two key controlling parameters of photosynthesis namely radiation and water. However, due to the sparsity of surface observations neither the carbon sequestration potential of these ecosystems nor its relation with the monsoon has been analysed comprehensively so far. This paper studies the ecosystem-atmosphere CO2 exchange at a tropical semi-evergreen moist deciduous forest and its relation with the monsoon over north-east India using the eddy covariance and associated meteorological measurements. In 2016, this ecosystem acts as a net source of atmospheric CO2 with net ecosystem exchange of 207.51 ± 157.37 gC m⁻² year⁻¹ and gross photosynthesis and ecosystem respiration of 2604.88 ± 179.43 and 2812.38 ± 22.05 gC m⁻² year⁻¹, respectively. The monsoon clouds are seen to introduce a bimodal pattern in the annual GPP record. The pre-monsoon and winter are the most and least favourable seasons for the photosynthetic CO2 uptake by this forest canopy. Additionally, the rate of increase of photosynthesis with evapotranspiration is maximum and minimum during the pre-monsoon and winter, respectively.
... Because T* a was not only influenced by evaporative cooling but also by other factors. Besides, several former studies have indeed observed afternoon stomatal closure in different vegetation types (Sayre,1926;Tenhunen et al., 1982;Correia et al., 1990). Urban vegetation in the warm seasons contributed considerable cooling effects (daytime: ...
Urban vegetation can influence local air temperatures through its biophysical effects on surface energy balance. These effects produce gradients (ΔTa) between air temperature of vegetation spaces (Tveg) and air temperature of open spaces (Topen) (ΔTa=Tveg−Topen), hereafter referred to as vegetation cooling (negative values of ΔTa) and warming (positive values of ΔTa), respectively. But vegetation cooling or warming highly depends on background climate of urban areas as well as on vegetation states. Field observations are usually restricted to one or few cities, setting limitations to a general understanding. In this study, a synthetic analysis of 3634 point-scale in-situ observations from 77 global sites in 35 cites was conducted using the bootstrap sampling and hierarchical partitioning methods. Results show that vegetation cooling is generally stronger during the daytime periods, in warm seasons, at low latitude zones, for forest lands and at leaf growth stage, while vegetation warming usually occurs in the opposite contexts. Urban vegetation begins to exert considerable cooling effects when the daily mean background air temperature (BAT) is >10.0 °C, but on average has a slight warming effect when BAT is <10.0°C. Besides, vegetation cooling increases sharply when evapotranspiration is >61.7 mm/month or when area of urban vegetation is >35.2 ha. Plant growth stages (i.e., canopy leaf growth, senescence and dormancy stages) (37.6 ± 0.11%), a vegetation phenology proxy, acts as the primary biotic factor, while seasonality (23.0 ± 0.11%) and latitude (11.4 ± 0.07%) that control the background climate are two most important abiotic contributors. Our findings suggest approximate thresholds for distinguishing vegetation cooling/warming effects and provide helpful information for future urban greenspace planning aimed at mitigating local climate warming.
... Para numerosas especies este valor de Q es aproximadamente de~400 J.1mol m-2 S-l (Squire & Black 1981, Jones 1992). Tenhunen et al. (1981Tenhunen et al. ( Y 1982, Lange et al. (1982), Oliveira et al. (1992) observaron Ulla depresi6n de la conductancia estomatica al mediodia solar en el caso de numerosas especies mediterraneas. Consideraron que esta depresi6n es caracteristica de las especies mediterrâneas y les permite limitar las pérdidas de agua en el momento deI dia en el que la demanda climatica es maxima. ...
... Such overcompensating stomatal behavior during dry antecedent conditions may explain how Ψ D can recover to, and sometimes even exceed, Ψ B (Syvertsen et al., 1975; Fig. 1). While concurrent environmental conditions are known to influence midday stomatal closure and recovery of Ψ D (Schulze et al., 1974;Tenhunen et al., 1982), we demonstrate the importance of antecedent moisture status in regulating Ψ D patterns. Given that Ψ D lags daily-scale D (max D) by 2-4 d and lags W 30 by 1 and 7 d, shifts in plant Ψ regulation may occur in under 1 wk, which may be advantageous in a mostly dry ecosystem with pulsed moisture inputs (Noy-Meir, 1973;Loik et al., 2004). ...
Plant water potential Ψ is regulated by stomatal responses to atmospheric moisture demand D and soil water availability W, but the timescales of influence and interactions between these drivers of plant Ψ are poorly understood.
Here, we quantify the effects of antecedent D and W on plant Ψ in the desert shrub Larrea tridentata. Repeated measurements of plant baseline water potential ΨB and diurnal water potential ΨD were analyzed in a Bayesian framework to evaluate the influence of antecedent D and W at daily and subdaily timescales.
Both ΨB and ΨD exhibited negative, 2‐ to 4‐d lagged responses to daily‐scale D; conversely, plant ΨD responded almost instantaneously to subdaily D, though the direction of this response depended on antecedent moisture conditions. Plant ΨB and ΨD responded positively and immediately (no lag) to shallow W, which contrasts the negative, lagged (6–7 d) response to deep W.
The changing sensitivity of ΨD to subdaily D highlights shifting modes of plant Ψ regulation: D effects on ΨD range from negative to neutral to positive depending on past conditions and time of day. Explicit consideration of antecedent conditions across multiple timescales can reveal important complexities in plant responses.
... Tree functioning and gas exchange attributes were affected by water deprivation, as lower P n and g s values were observed under RDI treatments, mainly in the postharvest period (Fig. 6). This indicates that peach trees regulated their transpiration when subjected to water constraints (Girona et al. 1993;Ruiz-Sánchez et al. 2010), which is a common response of cultivated plants grown in Mediterranean climates (Schulze et al. 1972;Tenhunen et al. 1982;Pereira et al. 1986). A delay in the recovery of these functions after the stage II of fruit development was observed when full irrigation was restored, as previously reported for other species (Torrecillas et al. 1999;Romero et al. 2004). ...
Irrigation techniques that reduce water applications are increasingly applied in areas with scarce water resources. In this study, the effect of two regulated deficit irrigation (RDI) strategies on peach [Prunus persica (L.) Batsch cv. “Catherine”] performance was studied over three growing seasons. The experimental site was located in Murcia (SE Spain), a Mediterranean region. Two RDI strategies (restricting water applications at stage II of fruit development and postharvest) based on stem water potential (Ψs) thresholds (−1.5 and −1.8 MPa during fruit growth and −1.5 and −2.0 MPa during postharvest) were compared to a fully irrigated control. Soil water content (θv), Ψs, gas exchange parameters, vegetative growth, crop load, yield and fruit quality were determined. RDI treatments showed significantly lower values of θv and Ψs than control trees when irrigation water was restricted, causing reductions in stomatal conductance and photosynthesis rates. Vegetative growth was reduced by RDI, as lower shoot lengths and pruning weights were observed under those treatments when compared to control. However, fruit size and yield were unaffected, and fruit quality was slightly improved by RDI. Water savings from 43 to 65 % were achieved depending on the year and the RDI strategy, and no negative carryover effect was detected during the study period. In conclusion, RDI strategies using Ψs thresholds for scheduling irrigation in mid–late maturing peach trees under Mediterranean conditions are viable options to save water without compromising yield and even improving fruit quality.
... Integration of E l is also difficult as the saturation deficit changes diurnally. Tanner (1981) has approximated the daily mean of D a as 1.45 Â the mean of the saturation deficit calculated at minimum and maximum temperatures. ...
... Although all species demonstrated improved Ψ s in response to fog, the two cloud forest conifers had unique patterns with a maintenance or rise of leaf Ψ from 08 : 00 to 11 : 00 h measurements during fog periods. This shift during peak photosynthetic periods (prior to 'midday depression' e.g., Schulze et al. 1980, Tenhunen et al. 1982, Chaves et al. 1987 most likely is driven by the utilization of fog water through foliar water uptake and the simultaneous suppression of transpiration due to low leaf to air VPDs ). This could act as a mechanism that maximizes leaf water content during periods of greatest photosynthetic fluxes during the day, enhancing water-use efficiency (photosynthesis/ transpiration). ...
Many studies have demonstrated linkages between the occurrence of fog and ecophysiological functioning in cloud forests, but few have investigated hydraulic functioning as a determining factor that explains sharp changes in vegetation. The objective of this study was to compare the plant water status during cloud-immersed and non-immersed conditions and hydraulic vulnerability in branches and roots of species across a temperate, mountain fog ecotone. Because cloud forests are often dark, cool and very moist, we expected cloud forest species to have less drought-tolerant characteristics (i.e., lower Pe and P50-the pressures required to induce a 12 and 50% loss in hydraulic conductivity, respectively) relative to non-cloud forest species in adjacent (lower elevation) forests. Additionally, due to the ability of cloud forest species to absorb cloud-fog water, we predicted greater improvements in hydraulic functioning during fog in cloud forest species relative to non-cloud forest species. Across the cloud forest ecotone, most species measured were very resistant to losses in conductivity with branch P50 values from -4.5 to -6.0 MPa, hydraulic safety margins (Ψmin - P50) >1.5 MPa and low calculated hydraulic conductivity losses. Roots had greater vulnerabilities, with P50 values ranging from -1.4 to -2.5 MPa, leading to greater predicted losses in conductivity (∼20%). Calculated values suggested strong losses of midday leaf hydraulic conductance in three of the four species, supporting the hydraulic segmentation hypothesis. In both cloud forest and hardwood species, Ψs were greater on foggy days than sunny days, demonstrating the importance of fog periods to plant water balance across fog regimes. Thus, frequent fog did not result in systemic changes in hydraulic functioning or vulnerability to embolism across our temperate cloud forest ecotone. Finally, roots functioned with lower hydraulic conductivity than branches, suggesting that they may serve as more sensitive indicators of hydraulic functioning in these mesic, foggy ecosystems.
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... Diurnal courses of leaf water potential (Ψ l ) showed a circadian rhythm, typical of cultivated plants (Sánchez-Blanco et al., 1990;Tenhunen et al., 1982;Torrecillas et al., 1988), with maximum values at predawn and minimum at midday (Fig. 1). In well irrigated plants, Ψ l decreased during midday with a more pronounced decrease in July, which indicate the close dependence of evaporative demand of the atmosphere of this parameter, as showed the high correlation coefficient values between Ψ l and vapour pressure deficit (VPD) (Rudich et al., 1981;Torrecillas et al., 1988). ...
The aim of this paper was to study continuous and discrete parameters of plant-water relations in mature apricot trees, in order to analyse the physiological basis of these plant-based sensors as well to determine the most sensitive indicator of plant water deficits. The experiment was performed during 1998-2001 in twelve-year-old apricot trees (Prunus armeniaca L., cv. Búlida), growing under field conditions in Murcia, Spain (semi-arid climate, with 1500 mm average annual evaporation and 350 mm rainfall) in a clay loam texture soil (128 mm m-1 available soil water content and 1.45 Mg m-3 bulk density). Trees were submitted to two drip irrigated treatments: a control treatment irrigated at 100 % of seasonal ETc and a water deficit treatment (WD) irrigated at 50 % of the control treatment. The diurnal courses of leaf water potentials, leaf conductance, net photosynthesis, leaf temperature, sap flow and trunk diameter fluctuations were studied throughout one growing season. The deficit treatment showed stomatal regulation as an adaptive mechanisms to drought, which allows plants to regulate water loss more effectively. Leaf temperature values were higher in WD than in the control treatment, and acted as a good stress indicator. Net photosynthesis showed a good correlation with leaf conductance values. In deficit irrigation conditions, osmotic adjustment was not observed. Both leaf water potential and gas exchange parameters were well correlated with evaporative demand of the atmosphere. Trunk diameters oscillate over a 24-h cycle, reaching a maximum value just before sunrise and a minimum sometime in the afternoon. The difference between maximum and minimum values is termed maximum daily shrinkage (MDS). This parameter increased in WD plants during the first period of the growing season, however, when the water stress was accumulated (in the last period of the growing season), MDS shown an important decrease in the water stressed plants respect to the irrigated ones. The sap flow values were lower in the deficit than in the control treatment throughout the growing season, although the greatest differences were observed in spring. Trunk diameter and sap flow showed moderately good correlations with the mean daily ETo in adult apricot trees grown under non-limiting soil water conditions. This study was supported by a CICYT (HIDI 999-0951; AGL 2000-0387-C05-04) grants to the authors
... Both leaf and stem water potentials followed a circadian rhythm parallel to the evaporative demand of the atmosphere (VPD and ET 0 values, Fig. 2) on all measurement dates and independently of the irrigation treatment, which is typical of most cultivated plants (Tenhunen et al., 1982;Torrecillas et al., 1988;Sánchez-Blanco et al., 1990;Ruiz-Sánchez et al., 2007). The higher correlation coefficients found in the regression Ψ leaf vs. VPD compared with Ψ stem vs. VPD pointed to the greater dependence of Ψ leaf on the prevailing weather conditions. ...
Field-grown lemon trees [Citrus limon (L.) Burm. fil. cv. Fino] were subjected to different drip irrigation treatments: a control treatment, irrigated daily above crop water requirements in order to obtain non-limiting soil water conditions and two deficit irrigation treatments, reducing the water applied according to the maximum daily trunk shrinkage (MDS) signal intensity (actual MDS/control treatment MDS) threshold values of 1.25 (T1 treatment) and 1.35 (T2 treatment), which induced two different drought stress levels. Daily variations in leaf (Ψleaf) and stem (Ψstem) water potentials, leaf conductance, net photosynthesis, sap flow (SF) and trunk diameter fluctuations were studied on four occasions during the lemon fruit growth period. Ψstem and Ψleaf revealed a diurnal pattern in response to changes in evaporative demand of the atmosphere. Both water potentials decreased in response to water deficits, which were more pronounced in the T2 treatment. Ψstem was seen to be a better plant water status indicator than Ψleaf. The difference between the two values of Ψ (Ψstem-Ψleaf = ΔΨ) was closely correlated with sap flow, making it a suitable measure of leaf transpiration. Using the slope of this relationship, the canopy hydraulic conductance (KC) was estimated. When other continuously recorded plant-based indicators are not accessible, the concurrent measurement of leaf and stem water potentials at midday, which are relatively inexpensive to measure and user-friendly, act as sufficiently good indicators of the plant water status in field grown Fino lemon trees.
... High WUE t can be achieved by lowering stomatal conductance, g s (Leffler and Evans 2001) and/or increasing photosynthetic rates (Condon et al. 2002). Several studies have shown that WUE can be improved with stomatal closure at midday (Tenhunen et al. 1982) or through stomatal opening early in the morning (Bacon 2004). A positive correlation between WUE t and leaf carbon isotope composition, d 13 C is now established (Farquhar et al. 1989;Condon et al. 2002;Brendel et al. 2008), enabling a rapid screen for WUE in plants in many environments and from a large number of genotypes (Farquhar et al. 1989;Jones et al. 1993;Condon et al. 2002;Bacon 2004;Rajabi et al. 2009), such as those in a mapping population enabling the elucidation of Quantitative trait loci (QTL) for this trait. ...
Dedicated non-food bioenergy crops like poplar are needed as sustainable, low-input feedstocks for renewable energy in a future drier climate, where they can be grown on marginal soils. Such plants should have a low water, carbon, and chemical footprint. Capturing natural variation in traits associated with water use efficiency (WUE) is the first step to developing trees that require less water and may be adapted to drier environments. We have assessed stomatal conductance (gs) and leaf carbon isotope composition (δ13C, an indirect indicator of leaf WUE) in two Populus species, P. deltoides and P. trichocarpa and their F2 progeny, grown in the United Kingdom and in Italy. Populus deltoides leaves showed lower δ13C than P. trichocarpa, suggesting a higher WUE in P. trichocarpa, although without drought preconditioning, gs of P. trichocarpa was less responsive to dehydration and abscisic acid treatment than P. deltoides, suggesting that leaf anatomy may also contribute to δ13C in Populus. Quantitative trait loci (QTL) were identified for δ13C on eight linkage groups (LG) and two QTL for gs. From these. QTL and differential gene expression in response to drought from microarray data, we focused on three hotspots and identified 23 novel candidate genes on LG VI, X, and XVI. We have begun to unravel the genetic basis of WUE in bioenergy Populus revealing important underpinning data for breeding and improvement in poplar genotypes for a future drier climate.
... During the DI periods, g s was usually lower in the RDI treatment than in the control treatment, indicating that water loss was regulated via transpiration in response to water deWcit (Turner et al. 1985; Girona et al. 1993; Ruiz-Sánchez et al. 2000 ). This response has been recognised as a common feature in cultivated plants growing in Mediterranean and desert (semiarid or arid) climates (Schulze et al. 1972; Tenhunen et al. 1982; Pereira et al. 1986). After rewatering (full irrigation in RDI treatment) a delay in the recovery of g s with respect to that of l wasTable 3 Vegetative and yield eYciency parameters of 'Búlida' apricot trees in control, continuous deWcit irrigation (DI) and regulated deWcit irrigation (RDI) treatments during the experimental period 1996 1997 1998 1999 Increase in trunk cross sectional area (TCA, cm Means for each column followed by the same letter are not signiWcantly diVerent at P < 0.05, according to LSD test. ...
During four growing seasons, 10-year-old apricot trees (Prunus armeniaca L., cv. ‘Búlida’) were submitted to three different drip irrigation regimes: (1) a control treatment, irrigated at 100% of
seasonal crop evapotranspiration (ETc), (2) a continuous deficit irrigation (DI) treatment, irrigated at 50% of the control
treatment, and (3) a regulated deficit irrigation (RDI) treatment, irrigated at 100% of ETc during the critical periods, which
correspond to stage III of fruit growth and 2months after harvest (early postharvest), and at 25% of ETc during the rest
of the non-critical periods in the first two growing seasons and at 40% of ETc in the third and fourth. Soil–plant–water relation
parameters were sensitive to the water deficits applied, which caused reductions in leaf and soil water potentials. The longer
and severer deficits of the RDI treatment decreased fruit yield in the first two seasons. The RDI treatment pointed to two
threshold values that defined the level at which both plant growth and yield were negatively affected with respect to the
control treatment: (1) a predawn leaf water potential of around −0.5MPa during the critical periods, and (2) a 22% drop in
irrigation water. The total yield obtained in the DI treatment was significantly reduced in all the years studied due to the
lower number of fruits per tree. No changes in the physical characteristics of fruits were observed at harvest. RDI can be
considered a useful strategy in semiarid areas with limited water resources.
... described from natural Eucalyptus vegetation suffering severe drought (Davidson and Reid 1989), where low and constant L was accompanied by almost complete stomatal closure. Tenhunen et al. (1982) reported flat L diurnal curves in moderately watered and seasonally 265 Water Relations of Allosyncarpia ternata ...
Allosyncarpia ternata S.T.Blake (Myrtaceae) is an
evergreen tree, restricted largely to rocky habitats on the Arnhem Land
Plateau in the wet–dry tropics of northern Australia.
Allosyncarpia ternata grows in a wide range of habitats,
including sites near permanent springs, where it forms a distinctive
closed-canopy forest with an understorey of rainforest plants, and sites on
exposed cliffs and hilltops, where it occurs in open forest and woodland. Leaf
water relations differ markedly between these contrasting sites. During the
dry season, trees at open sites show strong diurnal hysteresis in stomatal
conductance (gs); afternoon
depressions in gs coincide with
regular afternoon increases in vapour pressure deficit. Pressure–volume
analyses indicate that A. ternata maintains turgor down
to leaf water potential values of about –2.8 MPa, close to the minimum
experienced by hilltop leaves late in the dry season. By contrast, trees on
the ravine floor, with year-round access to water, exhibit much smaller
diurnal and seasonal variation in stomatal conductance and little seasonal
variation in leaf water potential. It is concluded that this flexible response
in leaf water relations to seasonally dry conditions is partly responsible for
the ability of A. ternata to occupy and dominate the
vegetation in such a wide variety of habitats. The near confinement of the
species to the Arnhem Land Plateau is in part due to the water-holding
capacity of the bedrock.
... The distribution of vegetation with different phenological patterns is thus, very sensitive to seasonal changes in VPD (Huffaker, 1942). From the literature, we find evidence that VPD less than 900 Pa should exert little effect on stomata whereas values greater than 4100 Pa generally are sufficient to force complete stomatal closure, even when the soils are moist (Osonubi & Davies, 1980;Tenhunen et al., 1982). Although these limits vary by locations and species (White et al., 2000), we chose a common set of para-meters for all sites. ...
The phenological state of vegetation significantly affects exchanges of heat, mass, and momentum between the Earth's surface and the atmosphere. Although current patterns can be estimated from satellites, we lack the ability to predict future trends in response to climate change. We searched the literature for a common set of variables that might be combined into an index to quantify the greenness of vegetation throughout the year. We selected as variables: daylength (photoperiod), evaporative demand (vapor pressure deficit), and suboptimal (minimum) temperatures. For each variable we set threshold limits, within which the relative phenological performance of the vegetation was assumed to vary from inactive (0) to unconstrained (1). A combined Growing Season Index (GSI) was derived as the product of the three indices. Ten-day mean GSI values for nine widely dispersed ecosystems showed good agreement (r>0.8) with the satellite-derived Normalized Difference Vegetation Index (NDVI). We also tested the model at a temperate deciduous forest by comparing model estimates with average field observations of leaf flush and leaf coloration. The mean absolute error of predictions at this site was 3 days for average leaf flush dates and 2 days for leaf coloration dates. Finally, we used this model to produce a global map that distinguishes major differences in regional phenological controls. The model appears sufficiently robust to reconstruct historical variation as well as to forecast future phenological responses to changing climatic conditions.
Olive tree is one of the most important crop at global scale. Apulia is the first olive-producing region in Italy, with a huge amount of farms that generate Integrated Pest Management (IPM) data. IPM requires the simultaneous use of different crop protection techniques to control pests through an ecological and economic approach. The crop protection strategies are correlated to the climate condition considering the very important relation among climate, crops and pests. Therefore, in this work is presented a new advanced On-Line Analytical Processing (OLAP) model integrating the Growing Season Index (GSI), a phenology model, to compare indirectly the farms by a climatic point of view. The proposed system allows analysing IPM data of different farms having the same phenological conditions over a year to understand some best practices and to highlight and explain different practices adopted by farms working in different climatic conditions.
Although most phenology models can predict vegetation response to climatic variations, these models often perform poorly in precipitation-limited regions. In this study, we modified a phenology model, called the Growing Season Index (GSI), to better quantify relationships between weather and vegetation canopy dynamics across various semi-arid regions of Iraq. A modified GSI was created by adding a cumulative precipitation control to the existing GSI framework. Both unmodified and modified GSI values were calculated daily from 2001 to 2010 for three locations in Eastern Iraq: Sulaymaniyah (north), Wasit (central) and Basrah (south) and a countrywide mean and compared to the Normalized Difference Vegetation Index (NDVI) from MODerate-resolution Imaging Spectroradiometer (MODIS) for the same time period. Countrywide median inter-annual correlations between GSI and NDVI more than doubled with the addition of the precipitation control and within-site correlations also show substantial improvements. The modified model has huge potential to be used to predict future phenological responses to changing climatic conditions, as well as to reconstruct historical vegetation conditions. This study improves our understanding of potential vegetation responses to climatic changes across Iraq, but it should improve phenological predictions across other semi-arid worldwide, particularly in the face of rapid climate change and environmental deterioration.
We present new findings on leaf and stomatal apparatus anatomy and ecophysiology of Welwitschia mirabilis Hook.f. that are relevant to survival in the desert. We show that the structure of the stomatal apparatus with thin areas in the guard cell walls is a key feature enabling an opportunistic survival strategy through reversible quick switch-over from water conservation to CO2 assimilation. Desert environment and greenhouse data demonstrated that stomatal conductance increased almost immediately after dawn to reach a maximum within approximately an hour, whereupon a steep decrease occurred. After discontinuation of induced drought in potted plants, fast recovery of stomatal conductance occurred while copious new root hairs developed within 50 h. Stomatal limitation proved to be the main photosynthetic constraint under induced drought. Under severe drought stress, biochemical limitation came into play. Chlorophyll fluorescence data of in-situ plants showed that the photosynthetic potential of leaf tissue is highest near the basal meristem, but although it decreases with age, it retains activity up to the leaf apex at the end of the green part. The photosynthetic potential of potted plants was optimal at 20 °C, analogous to mesophytic plants. Our data confirms our hypothesis that leaf surface and stomatal structure play a crucial role in moisture conservation and moderating leaf temperature for desert survival.
During the vegetation period of 1982 transpiration and leaf conductance of Mercurialis perennis as well as light intensity (PAR), air vapor pressure deficit and temperature of the air were measured continuously in the herb layer of a beech forest on limestone in the environs of Göttingen. The measurements were carried out with a climatized gas exchange chamber. In addition the course of leaf water potential was followed during the vegetation period in leaves of plants growing near the gas exchange chamber. The height of active controlled transpiration and leaf conductance was calculated.
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 conventional expression of experimental stomatal conductance, the assumption of separation of variables should be admitted without any validation of it. In this paper, we adopt the neural network for making the experimental expression. It becomes clear that the neural network has the capability in expressing the experimental data with excellent accuracy than in the conventional method.
The determination and interpretation of patterns in photosynthetic primary production which are characteristic of different lichen species within their respective habitats may lead to a better understanding of habitat preferences and species distributions. However, the necessary photosynthetic CO2 exchange measurements are difficult to accomplish in the field. Thus, only a few examples of diurnal courses of photosynthetic CO2 uptake by poikilohydric organisms may be found in the ecophysiological literature. Recently mathematical models have been developed which have been used to simulate the net photosynthetic performance of lichens under natural conditions. Because such models are usually based upon physiological parameters obtained from laboratory experiments, their validity can only be established by comparing simulated rates of photosynthesis with rates actually measured in the field. Thus, the development of instrumentation and methods which allow accurate determination of CO2 exchange of lichens under natural conditions is essential.
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.
Forests in the Fynbos biome of South Africa are mainly confined to riparian zones and sheltered ravines. A simple explanation for this restriction could be that the riparian trees are unable to survive the soil moisture deficits associated with the summer drought that is a feature of the mediterranean-type climate. A study was made of the water relations and photosynthetic characteristics of selected riparian trees (Brabejum stellatifolium, Brachylaena neriifolia and Cunonia capensis), and shrubs of the relatively xeric hillslopes adjoining such riparian habitats (Protea nitida and P. repens). Field measurements of xylem pressure potential, leaf conductance and net leaf photosynthesis were made in the wet winter of 1986 and during the spring and summer of 1986/7. Characteristics of microclimate and soil moisture, and leaf and canopy morphology were used to explain the observed stomatal behaviour. Despite marked differences in some leaf and canopy characteristics between hillslope and riparian species, stomatal behaviour, net photosynthesis and water-use efficiencies were very similar. Soil water potentials at 900mm depth at the riparian site remained above −0.1MPa throughout the study and only dropped below −0.05MPa during the dry period. Potentials at the hillslope site frequently dropped below −0.1MPa. The differences in soil moisture potentials at the two sites were reflected in the pre-dawn and midday plant xylem potentials but the magnitude of differences was much less than had been expected. Similarly, diurnal courses of leaf conductance and net leaf photosynthesis were very similar for hillslope and riparian species. We suggest the following reasons for the very similar patterns of physiological response: (a) soil moisture deficits at 900mm depth at the hillslope site were rapidly recharged following relatively light rainfall events and the periods between significant rainfall events were too short to induce drought; (b) the deep-rooted hillslope shrubs behave as phreatophytes and utilize water at depth; and (c) the hillslope communities have sparse canopies and evapotranspiration was too low to dry the soils to depth. Hillslope shrubs have low capacities for resource capture and use, and it appears that their sparse canopies have not equilibrated with available soil moisture as is the case with structurally similar sclerophyllous shrubs in other mediterranean-climate regions.
Photosynthetic characteristics and leaf-water relations of 16 plant species belonging to different growth forms of mountain fynbos vegetation (South Africa) were investigated. Measurements were mainly performed at the dry border of fynbos after abundant winter rain in October 1983. Only the C3-mode of photosynthesis was found among the investigated plants. Light saturation of photosynthesis did not occur up to a photon flux density of 2 000μmolm⁻² s⁻¹. Maximum rates of photosynthesis of all investigated species ranged from 2.8 to 12.7μmolm⁻² s⁻¹ with a mean of 5.6μmolm⁻² s⁻¹. For evergreen Proteaceae, leaf age markedly determined photosynthesis and water-use efficiency. Daily CO2 uptake per unit leaf area increased about 2.5-fold from young to mature leaves, while water-use efficiency increased maximally by 85%. Predawn water potential of all investigated plants ranged from 0.3 to 1.2MPa and decreased, except for Rhus dissecta (2.7MPa), until noon by only about 1MPa. In all plants, leaf water potential recovered until sunset, indicating the abundance of water in the soil. Due to the absence of a marked water deficit in most plants no midday depression of leaf conductance was found. With the exception of the geophyte Dilatris corymbosa, maximum leaf conductance was generally low, ranging from 61 to 176mmolm⁻² s⁻¹ and transpiration ranged from 2.2 to 9.2mmolm⁻² s⁻¹. Our results of either photosynthesis or leaf conductance do not show any difference corresponding with the clear classification into growth forms or families. Due to the low mineral and nutrient content of fynbos soils, ion content in the plants was low. Generally, potassium exceeded sodium content. Calcium occurred in traces only. For proteoid plants, inorganic ions account for 20–40% of the osmotic potential and for ericoid plants, 70–90%. The results are discussed in comparison with other mediterranean vegetation and show that fynbos seems to have no exceptional position.
Woody plants such as trees have a significant economic and climatic influence on global economies and ecologies. This completely revised classic book is an up-to-date synthesis of the intensive research devoted to woody plants published in the second edition, with additional important aspects from the authors' previous book, Growth Control in Woody Plants. Intended primarily as a reference for researchers, the interdisciplinary nature of the book makes it useful to a broad range of scientists and researchers from agroforesters, agronomists, and arborists to plant pathologists and soil scientists. This third edition provides crutial updates to many chapters, including: responses of plants to elevated CO2; the process and regulation of cambial growth; photoinhibition and photoprotection of photosynthesis; nitrogen metabolism and internal recycling, and more. Revised chapters focus on emerging discoveries of the patterns and processes of woody plant physiology. * The only book to provide recommendations for the use of specific management practices and experimental procedures and equipment * Interdisciplinary approach will appeal to a broad range of scientists, researchers, and growers * Thoroughly updated with the latest research devoted to woody plants.
Comparative field studies of cheatgrass (Bromus tectorum L.) with Sandberg's bluegrass (Poa sandbergii Vasey) were conducted to further our understanding of the plant characteristics that contribute to success in habitats where water is a limiting factor. To evaluate the effect of soil water on phenological development, stomatal conductance, and xylem pressure potential of these grasses, observations were made in the field for 2 growing seasons (1986 and 1987). Stomatal conductance, transpiration, and xylem pressure potential data, gathered as soils dried during 1986, indicated that water stress developed earlier and to a greater degree in Sandberg's bluegrass than in cheatgrass. Xylem pressure potential was lower in Sandberg's bluegrass than in cheatgrass, and the difference increased throughout the growing season. Stomatal conductance and transpiration were greater for cheatgrass than for Sandberg's bluegrass. Maintenance of high soil water potentials by irrigating through the 1987 growing season retarded phenological development and delayed senescence by about 10 days for both species. Predawn xylem pressure potential for irrigated plants remained higher than for nonirrigated plants; however, as the plants senesced, xylem pressure potential also decreased in the nonstressed plants.
Examined the daily activity patterns of the pollinators of Lavandula latifolia (Labiatae). Lavender open flowers and produce nectar uninterruptedly over daytime; both flower production and nectar secretion rates are highest early in the morning and late in the afternoon. Pollen availability in the flower population reaches a maximum in late afternoon, while nectar and sugar availability peak around the middle of the day. There was considerable variation both among major groups (Hymenoptera, Diptera, Lepidoptera), and among species within groups, in the timing of foraging at flowers. As a consequence of this, and of interspecific differences in pollen transfer effectiveness and average flight distance between consecutively visited flowers, the daytime period is not homogeneous with regard to the potential pollinating effectiveness of the pollinators active at a given time. There is not, however, a good correlation between the daily cycles of floral resource production and availability, on one hand, and of components of pollinating effectivness, on the other. Both the plant's and the pollinators' daily cycles probably largely represent independent responses to diel rhythmicity of the physical environment. Matches and mismatches found here between daily patterns of floral resources and aspects of pollinating effectiveness are epiphenomena lacking particular adaptive significance to the plant. -from Author
Plant growth and leaf photosynthesis were studied for saplings of two species in the Dipterocarpaceae family planted on open bare ground in the Philippines. The greatest rate of growth in height in the initial 14 months after planting was for Shorea contorta, followed by Dipterocarpus grandiflorus. The light-saturated photosynthetic rate (Pmax) for S. contorta and D. grandiflorus was 9.9 ± 0.29 μmol m-2S-1 and 5.5 ± 0.28 μmol m-2S-1, respectively. Dark respiration and apparent quantum yield efficiency did not differ between them. The daily course of photosynthesis showed the existence of a midday depression in net photosynthesis for S. contorta and D. grandiflorus on open bare ground. When the leaf-to-air vapor pressure difference (L-AirVPD) was abruptly raised to 30 hPa, the decrease in the rate of photosynthesis was the same for potted saplings of S. contorta and D. grandiflorus in the nursery. One reason for the difference in growth between species is thought to be the difference in carbon gain depending on a difference in Pmax between the species in the morning, before the leaves suffer from large L-AirVPD at midday.
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.
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.
During the "midday depression" of net CO2 exchange in the mediterranean sclerophyllous shrub Arbutus unedo, examined in the field in Portugal during August of 1987, several parameters indicative of photosynthetic competence were strongly and reversibly affected. These were the photochemical efficiency of photosystem (PS) II, measured as the ratio of variable to maximum chlorophyll fluorescence, as well as the photon yield and the capacity of photosynthetic O2 evolution at 10% CO2, of which the apparent photon yield of O2 evolution was most depressed. Furthermore, there was a strong and reversible increase in the content of the carotenoid zeaxanthin in the leaves that occurred at the expense of both violaxanthin and β-carotene. Diurnal changes in fluorescence characteristics were interpreted to indicate three concurrent effects on the photochemical system. First, an increase in the rate of radiationless energy dissipation in the antenna chlorophyll, reflected by changes in 77K fluorescence of PSII and PSI as well as in chlorophyll a fluorescence at ambient temperature. Second, a state shift characterized by an increase in the proportion of energy distributed to PSI as reflected by changes in PSI fluorescence. Third, an effect lowering the photon yield of O2 evolution and PSII fluorescence at ambient temperature without affecting PSII fluorescence at 77K which would be expected from a decrease in the activity of the water splitting enzyme system, i.e. a donor side limitation.
Photosynthetic carbon assimilation rates and leaf water use efficiency were highest (maximum instantaneous values, c12 mu mol CO2m-2s-1 and 7 mmol CO2mol H2O-1, respectively) in spring, somewhat reduced in winter, and strongly depressed by the middle of summer when severe drought conditions prevailed. With transition from winter to spring and summer, as environmental conditions became warmer and drier, there was an increasing tendency for a midday depression in gas exchange rate as well as a decreasing capacity in the afternoon for recovery to the same net photosynthesis rates measured in the morning. Midday depression in gas exchange and stomatal conductance occurred in leaves positioned both vertically and horizontally inside measurement cuvettes. Leaf dark respiratory rates also changed with season; the temperature necessary to evoke similar rates increased with transition from winter to summer.-from Authors
Although soil compaction in the field may benefit or inhibit the growth of plants, the harmful effects are much more common. This paper emphasizes the deleterious effects of predominantly high levels of soil compaction on plant growth and yield. High levels of soil compaction are common in heavily used recreation areas, construction sites, urban areas, timber harvesting sites, fruit orchards, agroforestry systems and tree nurseries. Compaction can occur naturally by settling or slumping of soil or may be induced by tillage tools, heavy machinery, pedestrian traffic, trampling by animals and fire. Compaction typically alters soil structure and hydrology by increasing soil bulk density; breaking down soil aggregates; decreasing soil porosity, aeration and infiltration capacity; and by increasing soil strength, water runoff and soil erosion. Appreciable compaction of soil leads to physiological dysfunctions in plants. Often, but not always, reduced water absorption and leaf water deficits develop. Soil compaction also induces changes in the amounts and balances of growth hormones in plants, especially increases in abscisic acid and ethylene. Absorption of the major mineral nutrients is reduced by compaction of both surface soils and subsoils. The rate of photosynthesis of plants growing in very compacted soil is decreased by both stomatal and non-stomatal inhibition. Total photosynthesis is reduced as a result of smaller leaf areas. As soils become increasingly compacted respiration of roots shifts toward an anaerobic state. Severe soil compaction adversely influences regeneration of forest stands by inhibiting seed germination and growth of seedlings, and by inducing seedling mortality. Growth of woody plants beyond the seedling stage and yields of harvestable plant products also are greatly decreased by soil compaction because of the combined effects of high soil strength, decreased infiltration of water and poor soil aeration, all of which lead to a decreased supply of physiological growth requirements at meristematic sites. Many protocols have been developed, with variable success, to alleviate the adverse effects of soil compaction on the growth and development of woody plants. These include planting of compaction-tolerant species, controlling vehicular and animal traffic, amending soils by adding coarse materials and/or organic matter, replacing compacted soils with uncompacted soils, loosening soils with aerating equipment, installing drainage systems and judiciously applying fertilizers. Prevention of soil compaction before planting is usually much preferred over post-planting treatments because the latter are expensive and difficult to apply, may not be adequately effective and may injure plant roots.
The responses of CO2 exchange and overnight malate accumulation of leaf and stem succulent CAM-plants to water stress and the particular climatic conditiens of fog and föhn in the southern Namib desert have been investigated. In most of the investigated CAM plants a long term water stress gradually attenuated any uptake of external CO2 and led to CO2 release throughout day and night. No CAM-idling was observed. Rainfall or irrigation immediately restored daytime CO2 uptake while the recovery of the noctural CO2 uptake was delayed. Dawn peak of photosynthesis was only found in well watered plants but was markedly reduced by the short term water stress of a föhn-storm. Morning fog with its higher diffuse light intensity compared with clear days increased photosynthetic CO2 uptake considerably. Even in well watered plants noctural CO2 uptake and malate accumulation were strongly affected by föhn indicating that the water vapour pressure deficit during the night determines the degree of acidification.
Ecosystem simulation models use descriptive input parame- ters to establish the physiology, biochemistry, structure, and allocation patterns of vegetation functional types, or biomes. For single-stand simulations it is possible to measure required data, but as spatial resolution increases, so too does data unavailability. Generalized biome parameterizations are then re- quired. Undocumented parameter selection and unknown model sensitivity to parameter variation for larger-resolution simulations are currently the major limitations to global and regional modeling. The authors present documented input parameters for a process-based ecosystem simulation model, BIOME- BGC, for major natural temperate biomes. Parameter groups include the fol- lowing: turnover and mortality; allocation; carbon to nitrogen ratios (C:N); the percent of plant material in labile, cellulose, and lignin pools; leaf mor- phology; leaf conductance rates and limitations; canopy water interception and light extinction; and the percent of leaf nitrogen in Rubisco (ribulose bis- phosphate-1,5-carboxylase/oxygenase) (PLNR). Using climatic and site de-
Diurnal time courses of net CO2 assimilation rates, stomatal conductance and light-driven electron fluxes were measured in situ on attached leaves of 30-year-old Turkey oak trees (Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O2 concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O2 to compute total light-driven electron fluxes, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation. This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light-driven electron flow, net CO2 assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible diurnal decreases. The latter observation indicated the occurrence of a large non-photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to photorespiration. The rate of carbon loss by photorespiration (R1) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de-excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.
Arbutus unedo is a sclerophyllous evergreen, characteristic of Mediterranean coastal scrub vegetation. In Italy, trees of A. unedo have been found close to natural CO2 vents where the mean atmospheric carbon dioxide concentration is about 2200 μmol mol−1. Comparisons were made between trees growing in elevated and ambient CO2 concentrations to test for evidence of adaptation to long-term exposure to elevated CO2. Leaves formed at elevated CO2 have a lower stomatal density and stomatal index and higher specific leaf area than those formed at ambient CO2, but there was no change in carbon to nitrogen ratios of the leaf tissue. Stomatal conductance was lower at elevated CO2 during rapid growth in the spring. In mid-summer, under drought stress, stomatal closure of all leaves occurred and in the autumn, when stress was relieved, the conductance of leaves at both elevated and ambient CO2 increased. In the spring, the stomatal conductance of the new flush of leaves at ambient CO2 was higher than the leaves at elevated CO2, increasing instantaneous water use efficiency at elevated CO2. Chlorophyll fluorescence measurements suggested that elevated CO2 provided some protection against photoinhibition in mid-summer. Analysis of A/Ci curves showed that there was no evidence of either upward or downward regulation of photosynthesis at elevated CO2. It is therefore anticipated that A. unedo will have higher growth rates as the ambient CO2 concentrations increase.
Abstract Field measurements of the gas exchange of epiphytic bromeliads were made during the dry season in Trinidad in order to compare carbon assimilation with water use in CAM and C3 photosynthesis.
The expression of CAM was found to be directly influenced by habitat and microclimate. The timing of nocturnal CO2 uptake was restricted to the end of the dark period in plants found at drier habitats, and stomatal conductance in two CAM species was found to respond directly to humidity or temperature. Total night-time CO2 uptake, when compared with malic-acid formation (measured as the dawn-dusk difference in acidity, ΔH+), could only account for 10–40% of the total ΔH+ accumulated. The remaining malic acid must have been derived from the refixation of respired CO2 (recycling). Within the genus Aechmea (12 samples from four species), recycling was significantly correlated with night temperature at the six sample sites. Recycling was lowest in A. fendleri (54% of ΔH+ derived from respired CO2), a CAM bromeliad with little water-storage parenchyma that is restricted to wetter, cooler regions of Trinidad.
Gas-exchange rates of C3 bromeliads were found to be similar to those of the CAM bromeliads, with CO2 uptake from 1 to 3 μmol m−2 s−1 and stomatal conductances generally up to 100 mmol m−2 s−1. The midday depression of photosynthesis occurred in exposed habitats, although photosynthetically active radiation (PAR) limited photosynthesis in shaded habitats. CO2 uptake of the C3 bromeliad Guzmania lingulata was saturated at around 500 μmol m−2 s−1 PAR, suggesting that epiphytic plants found in the shaded forest understorey are shade-tolerant rather than shade-demanding.
Transpiration ratios (TR) during CO2 fixation in CAM (Phase I and IV) and C3 bromeliads were compared at different sites in order to assess the efficiency of water utilization. For the epiphytes displaying marked uptake of CO2, TR were found to be lower than many previously published values. In addition, the average TR values were very similar for dark CO2 uptake in CAM (42 ± 41, n= 12), Phase IV of CAM (69 ± 36, n= 3) and for C3 photosynthesis (99 ± 73, n= 4) in these plants. It appears that recycling of respired CO2 by CAM bromeliads and efficient use of water in all phases of CO2 uptake are physiological adaptations of bromeliads to arid microclimates in the humid tropics.
The response of three eucalypt species ( Eucalyptus pulchella, Eucalyptus coccifera and Eucalyptus delegatensis ) to a severe drought in the summer of 1982/83 was examined at Snug Plains, south‐eastern Tasmania. Few large differences in leaf water potential (Ψ l ) or stomatal conductance (gs) were apparent even at the height of the drought when both Ψ l and soil water potentials (Ψ s ) reached ca. — 4.5 MPa. However, E. pulchella maintained a higher relative water content (RWC) in its leaves than E. coccifera and E. delegatensis , and showed less severe crown damage. After the first light rains substantial interspecific differences in Ψ l and gs occurred. Eucalyptus pulchella restored normal Ψ l , gs and RWC more rapidly than the other two species and, even for severely droughted trees, crown growth commenced via epicormic buds near the ends of its branches while for E. delegatensis and E. coccifera crown regeneration was via epicormic buds arising from stems and larger branches. This resulted in a change in dominance in certain stands and showed that E. pulchella was more drought‐resistant than E. coccifera , which was in turn more resistant than E. delegatensis . This conclusion was confirmed during competition experiments using potted seedlings. However, potted seedlings differed from mature field trees by maintaining moderate gs at high vapour pressure deficits and closing stomata at Ψ l below ca. — 2.0 MPa.
Substantial variation in the severity of drought symptoms was observed over short distances. This variation appeared to be determined by the moisture‐holding capacity of the soil and the biomass of the stand. Although differences in the rooting patterns of seedlings were evident, field measurements of Ψ l and Ψ s suggested that all three species were exploiting the same water resource.
In contrast to previous studies, the results suggest that large interspecific differences in tissue hydration and crown damage may be present, even though differences in Ψ l , gs and characteristics of the root system may appear small.
Adult trees of Quercus petraea were submitted to controlled water shortage in a natural stand near Nancy, France. Diurnal course of net CO2 assimilation rate (A) was measured in situ together with chlorophyll a fluorescence determined on dark adapted leaves. In 1990, trees experienced a strong water stress, with predawn and midday leaf water potentials below –2·0 and –3·0 MPa, respectively. Diurnal course of A of well-watered trees exhibited sometimes important midday decreases in A related to high temperature and vapour pressure deficit. Decreases in initial (Fo) and maximal (Fm) fluorescence and sometimes in photochemical efficiency of photosystem II (Fv/Fm) were observed and probably revealed the onset of mechanisms for thermal de-excitation. These mechanisms were shown to be sensitive to dithiothreitol. All these effects were reversible and vanished almost completely overnight. Therefore, they may be considered as protective mechanisms adjusting activity of photosystem II to the electron requirement for photosynthesis. Water stress amplified these reactions: A was strongly decreased, showing important midday depression; diurnal reductions in Fm and Fv/Fm were enhanced. The same trends were observed during summer 1991, despite a less marked drought. These protective mechanisms seemed very effective, as no photoinhibitory damage to PS II could be detected in either water stressed or control trees.
Cited By (since 1996): 132, Export Date: 5 July 2012, Source: Scopus
Female Osyris quadripartita plants exhibit uninterrupted reproductive activity throughout the year, due to the long duration of successive stages in the cycle and marked within-crop developmental asynchrony. Cycles corresponding to the flowering seasons of consecutive years overlap in each individual. Flowering takes place in spring, and fruits develop in the dry summer season and ripen at any time of the year. Variation in flowering time explains a negligible proportion of variation in ripening time. The greatest reproductive losses are incurred in the phase extending from closed flowers through unripe fruits, mostly due to ovary abortion. Only 30% of closed flowers eventually reach this latter stage. In contrast, 75% of unripe fruits complete their development, with subsequent dispersal of seeds. The probability of the setting of ripe fruit steadily decreases from early to late season flowers, due to increased ovary abortion rates. Resource limitation in the dry summer season seems responsible for this pattern of selective fruit maturation.
Faba beans are known to be susceptible to water stress. The aim of the present research was to find out, if this sensitivity
is related to an incapability of the plants to close the stomata effectively during times of water stress. For reasons of
comparison oats were included in the investigation, as oat plants are known to respond less sensitively to water shortage
than faba beans. The experiment was conducted on a loess-derived soil during a relatively dry vegetative season. Leaf area
development and soil water use of beans was later in the season as compared to oats. Maximum leaf area and water extraction
rates were attained end of June to beginning of July during pod development. Total leaf water potential ι of beans was always
higher than in oats. At a given ι the osmotic potential π was less and the pressure (turgor) potential P was higher as compared
to the cereal crop. To changing ι beans responded much more pronounced than oats in reducing adaxial and abaxial leaf conductance.
The sensitivity in stomata regulation of faba beans became also apparent by a distinct oscillation of ι and conductance during
the course of a day. It is concluded that the water-stress susceptibility of faba beans is brought about by a reduced CO2 diffusion into the leaves, thus lowering net assimilation rates.
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.
The fact that plants respond to dry and hot environmental oonditions with midday-depression of gas exchange has been known since the early 1920s. At first the increase in stomatal resistance at midday was explained as a response to internal water status of the leaves after high transpirational water loss during the morning where soil water supply was limiting. However, recent investigations with apricot trees cultivated in the Negev desert demonstrated that stomata respond to ambient air humidity which induces stomatal closure at midday independent of bulk leaf water conditions. Although these measurements were carried out with irrigated trees, the possibility that some water stress occurred could not be fully excluded. In the present investigations, therefore, stomatal behavior was examined in plants with definitely unlimited water supply in the root zone.
Daily time courses of leaf diffusion resistance, transpiration rate, for leaf environmental variables (measured with a steady-state porometer) and of xylem water potential (measured with a pressure chamber) were obtained for evergreen sclerophylls and cultivated Mediterranean plants growing at the Quinta São Pedro Research Station in Sobreda, south of Lisbon, Portugal. Leaves of the macchia plants Arbutus unedo, Myrtus communis, and Smilarx aspera as well as leaves of the cultivated tree Oastanea sativa exhibited a strong midday depression of gas exchange. Quercus suber and Quercus coccifera plants reduced their leaf water loss under hot and dry conditions less. Transpirational water loss was high from leaves of Ficus carica, well watered Nerium oleander, and Eriobotrya japonica.
The characteristic eco-physiological feature of sclerophyllous Mediterranean plants may be summarized as the ability to maintain sufficient metabolic activity and productivity in an environment which two unfavourable seasons: a period of drought during summer and early autumn, and short periods of low temperatures in winter. The ability of evergreen plants to tolerate dry seasons must include sufficient drought resistance and certain adaptive mechanisms, which enable the plants to remain productive when water resources are reduced. The resistance of Mediterranean sclerophylls to lethal desiccation has been studied by Oppenheimer (1932), Rouschal (1938), Oppenheimer, Leshem (1966), and responsive increase in desiccation resistance of olive leaves during summer drought was shown by Larcher (1963a). Concurrent measurements of water relations and CO2-gas exchange of sclerophyllous plants have been carried out in the Mediterranean region (France: Killian 1933, Eckardt et al. 1975, Italy: Weinmann, Kreeb 1975), with species native to California and Chile (Mooney et al. 1975, Dunn 1975, Gigon 1979), and in Australia (Hellmuth 1971). However, a thorough analysis of the components of drought adaptation in the metabolism and dry matter production of sclerophyllous Mediterranean trees has still to be done.
This chapter describes community water budgets, seasonal and annual patterns of plant water use, and the canopy and physiological controls on water use.
The seasonal change in diurnal patterns of net photosynthesis and daily carbon gain is studied in relation to the plant water status of the irrigated and non-irrigated naturally growing desert species Hammada scoparia, Zygophyllum dumosum, Artemisia herba-alba and Reaumuria negevensis. Comparison is made to cultivated Prunus armeniaca. Under non-irrigated natural conditions Hammada scoparia, a C4 plant, showed one-peaked flat diurnal courses of CO2 uptake which changed into a pattern of a high morning peak of CO2 uptake or slightly two-speaked curves in the late dry season. In contrast, the C3 species Zygophyllum dumosum, Artemisia herba-alba and Prunus armeniaca changed from one-peaked to distinct two-peaked patterns. At the end of the dry season, non-irrigated plants showed respiration only. Reaumuria negevensis had one-peaked curves with a low level of CO2 uptake. There is no general relation between day-time CO2 gain and pre-dawn water potential for the investigated species. In order to characterize the effect of soil drought, the CO2 gain during day-time of non-irrigated plants is expressed as a percentage of the CO2 gain of the irrigated counterparts. After an initial period of minimal drought effect, the relative day-time CO2 gain decreases almost linearly with cumulative water stress as determined by the daily addition of pre-dawn water potentials for the non-irrigated plants since the last rainfall. The slope of decrease differs from species to species. The relation of daily CO2 gain to maximal net photosynthesis is discussed. Initially, at a good plant water status, the daily CO2 gain does not decrease in proportion to the maximal photosynthetic rates as a result of stomatal control at high photosynthetic activity. At increasing water stress the daily CO2 gain decreases more than proportionally to the decrease of the maximal rates.
General types of diurnal leaf stomatal behavior were described by Stocker (1956) who suggested an increasing degree of midday stomatal closure depending on the degree of water stress experienced by a plant. Initially the closure may restrict water use in such a manner that a single peaked curve is obtained for transpiration rate plotted against time, the changes in conductance at midday only nearly compensating for changes in evaporative demand and resulting in an approximately constant transpiration rate. The daily pattern of CO2 exchange which is often directly affected by conductance changes may, under these conditions, reveal the stomatal closure response, and one observes two peaks in net photosynthesis rate. With further increase in plant water stress, Stocker suggested that extreme midday stomatal closure may affect the daily time course of water loss to such an extent that a two-peaked curve of transpiration rate versus time results, with a depression at midday.
The extensive early literature on foliar uptake of moisture, spanning nearly three centuries, has been reviewed by Stone (1957a, 1970) and Gessner (1956a). Early field studies by a number of workers demonstrated water uptake by leaves or stems of intact plants (Lloyd 1905; Wetzel 1924; Krause 1935). These and other laboratory experiments (see Stone 1957 a, 1970) led to a unanimity of opinion on the ability of leaves to absorb water, but a strong divergence of views on the ecological and physiological significance of this uptake. Remarkably, however, the flow of papers on the subject over the past 50 years has done little to resolve the questions of significance, and foliar water uptake remains as controversial a subject as ever.
Negative hydrostatic pressure can be measured in plants.
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
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.
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.
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Response and adap-tation to water stress in Nerium oleander Mittfiglicher Stomataschluss bei Apri-kose (Prunus armeniaca) und Wein (Vitis vinifera) im Freiland trotz guter Bodenwasserversorgung
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