No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
High-Irradiance Stress in Higher Plants and Interaction with other Stress Factors
Abstract
Exposure of leaves to light levels in excess of what can be utilized in photosynthesis often results in a decline in photosynthetic activity (1). This high-light effect is especially evident after return of the leaf to a low light level as a reduction in the photon yield of photosynthetic O2 evolution or CO2 uptake. In common usage the term photoinhibition includes any sustained reduction in photosynthetic activity induced by excessive light, irrespective of mechanistic considerations, but does not include transient reductions that are rapidly reversible and likely to reflect short-term regulation. In my talk today I will attempt to distinguish between two kinds of high-light-induced reduction in the efficiency of photochemistry of PSII: 1) an increase in the rate constant for dissipation of excitation energy in the antenna and 2) a decrease in the rate constant for the photochemistry of PSII which is likely to be caused by damage to the PSII reaction centers.
... In particular, under high light stress conditions, the dissipation of the excess absorbed light energy occurs via the nonphotochemical quenching (NPQ) of chlorophyll fluorescence, a harmless nonradiative pathway of dissipation of energy. This defensive strategy involves the synthesis of antioxidant carotenoids, such as the secondary carotenoid astaxanthin, the pigment lutein, and the xanthophyll cycle pigments: violaxanthin, antheraxanhitn, and zeaxanthin [3][4][5][6][7]. Among the xanthophylls, also loroxanthin and fucoxanthin, mainly produced by marine strains such as Phaeodactylum and Isochrysis, have been found to be strong antioxidants. ...
... Indeed, under these conditions, the acidification of the thylakoid lumen occurs, and this can activate some enzymes involved in the carotenogenesis. For instance, the deepoxidation of violaxanthin to zeaxanthin, via antheraxanthin, is promoted by low pH in the thylakoid lumen [5,21,22]. ...
... This is supported further by the observation that whole chain electron transport rates are reduced proportionately at all temperatures in L. hirsutum exposed to chilling stress, whereas in L. esculentum, temperature sensitivity was altered dramatically after chilling, leading to increased electron transport at low temperatures (Figs. 2,3). Similarly, photosynthetic CO2 uptake (Table II) was reduced more in L. hirsutum compared with L. esculentum by the end of the chilling period, but recovery was more rapid. ...
... Bjorkman (2) has clearly shown that an increase in the rate constant for non-radiative energy dissipation, KD, will reduce the amount ofexcess excitation energy and, therefore, is likely to have a protective role. However, there is a cost once the stress is relieved, in that failure to return to a state of low nonradiative energy dissipation would result in a decreased efficiency of photochemistry and, hence, a reduced efficiency of overall photosynthesis (2). Perhaps this is the situation with L. esculentum, which had significantly less dry matter accumulation following the chilling stress (Table III). ...
Cell suspension cultures of alfalfa (Medicago sativa L. cv. Rangelander) maintained at 2°C for 14 days were much more resistant to freezing-induced damage than cultures maintained at 21 °C. While no gain in fresh weight occurred during acclimation, such cells had smaller vacuoles and appeared to have more membranous material in their cytoplasm. The latter observation was supported by cell fractionation studies which indicated that the acclimated cells contained more microsomal phospholipid and protein than control cells. In contrast, the total solute content was lower in acclimated compared to control cells. Acclimation increased the relative content of phospholipids and decreased the sterol content of membranes. Also the phospholipids of acclimated cells had a higher percentage of unsaturated fatty acyl chains than those of control cells. As a result, the fluidity of liposomes prepared from membrane lipids, measured by fluorescence depolarization, was higher for preparations from acclimated cells than the control cells. The results suggest that membrane modification, during low temperature culture, is a major component of the cold acclimation process in alfalfa cells and that the adjustments which occur in the solute and water content of the cells are minor components of the process.
... We also anticipated that the cooler temperatures combined with increased solar irradiance at high elevations would result in higher photoprotective stress responses through increases in xanthophyll cycle deepoxidation or increases in total xanthophyll pigment pool size, relative to the chlorophyll pool size, to avoid photodamage (Björkman, 1987;Demmig-Adams & Adams III, 2006;Savage et al., 2009;Streb & Cornic, 2012). Cold temperatures at very high elevations may limit inorganic soil nitrogen availability due to slower microbial nitrogen mineralization rates (Fisher et al., 2013;Körner et al., 1986;Nottingham et al., 2015;Pérez-Ramos et al., 2012). ...
High alpine regions are threatened but understudied ecosystems that harbor diverse endemic species, making them an important biome for testing the role of environmental factors in driving functional trait‐mediated community assembly processes. We tested the hypothesis that plant community assembly along a climatic and elevation gradient is influenced by shifts in habitat suitability, which drive plant functional, phylogenetic, and spectral diversity. In a high mountain system (2400–3500 m) Región Metropolitana in the central Chilean Andes (33°S, 70°W). We surveyed vegetation and spectroscopic reflectance (400–2400 nm) to quantify taxonomic, phylogenetic, functional, and spectral diversity at five sites from 2400 to 3500 m elevation. We characterized soil attributes and processes by measuring water content, carbon and nitrogen, and net nitrogen mineralization rates. At high elevation, colder temperatures reduced available soil nitrogen, while at warmer, lower elevations, soil moisture was lower. Metrics of taxonomic, functional, and spectral alpha diversity peaked at mid‐elevations, while phylogenetic species richness was highest at low elevation. Leaf nitrogen increased with elevation at the community level and within individual species, consistent with global patterns of increasing leaf nitrogen with colder temperatures. The increase in leaf nitrogen, coupled with shifts in taxonomic and functional diversity associated with turnover in lineages, indicate that the ability to acquire and retain nitrogen in colder temperatures may be important in plant community assembly in this range. Such environmental filters have important implications for forecasting shifts in alpine plant communities under a warming climate.
... In the presence of several abiotic stressors, sustained yield reductions were associated with both sustained increases in thermal energy dissipation (associated with zeaxanthin) and zeaxanthin contents [10]. Under water stress when watering was terminated, Nerium oleander leaves, which do not perform osmotic adjustment, exhibited sustained decreases in Chl fluorescence yields, indicating increased thermal energy dissipation and sustained increases of zeaxanthin levels [80]. Under heat stress, the rate of de-epoxidation of violaxanthin to zeaxanthin increased as leaf temperature increased, which is expected of an enzymatic reaction [81]. ...
Soybean (Glycine max (L.) Merr.) is an important crop that serves as a source of edible oil and protein. However, little is known about its molecular mechanism of adaptation to extreme environmental conditions. Based on the Arabidopsis thaliana sequence database and Phytozome, a soybean gene that had a highly similar sequence to the reduced induction of the non-photochemical quenching2 (AtRIQ2) gene, GmRIQ2-like (accession NO.: Glyma.04G174400), was identified in this study. The gene structure analysis revealed that GmRIQ2-like encoded a transmembrane protein. Elements of the promoter analysis indicated that GmRIQ2-like participated in the photosynthesis and abiotic stress pathways. The subcellular localization results revealed that the protein encoded by GmRIQ2-like was located in chloroplasts. The quantitative real-time (qRT)-PCR results revealed that GmRIQ2-like-overexpression (OE) and -knock-out (KO) transgenic soybean seedlings were cultivated successfully. The relative chlorophyll (Chl) and zeaxanthin contents and Chl fluorescence kinetic parameters demonstrated that GmRIQ2-like dissipated excess light energy by enhancing the non-photochemical quenching (NPQ) and reduced plant photoinhibition. These results suggested that GmRIQ2-like was induced in response to strong light and depressed Chl production involved in soybean stress tolerance. These findings indicate that the transgenic seedlings of GmRIQ2-like could be used to enhance strong light stress tolerance and protect soybean plants from photoinhibition damage. This study will serve as a reference for studying crop photoprotection regulation mechanisms and benefits the research and development of new cultivars.
... En este manuscrito presentamos las medidas que proponemos aplicar en al ámbito de interacción de las plantas de cultivo, el suelo y la atmósfera, es decir, en la escala de un ecosistema agrícola. En este nivel (particularmente en las especies C3) el factor ambiental irradiancia parece ser un punto común de confluencia para el estrés originado por factores múltiples (Alexieva et al. 2003;Björkman 1987). Como consecuencia, si se consigue mitigar el estrés causado por alto nivel de PAR en los campos de cultivo, pudiera disminuirse el impacto de otros factores ambientales inductores de estrés como el déficit de agua, la salinidad y la alta temperatura. ...
From an evolutionary point of view environmental stress has regulated the function, morphology, and diversity of cells, organs, individuals and plant communities. The stress-inducing factors are multiple, but the high irradiance, high and low temperatures, salinity, water deficit and nutrient deficiency are mentioned more frequently. The interaction of plants with the stress-inducing environments has produced in the plants a set of adaptive responses that can be studied in different description scopes: from organelles and subcellular structures to the level of plant communities. When it occurs for short time or low intensity, environmental stress can induce hardening, followed by induction of tolerance; on the other hand, when the plants´reactionplants´reaction is for a long time or for a significant stress intensity, the response of plants includes decreased growth, depletion of metabolic reserves and loss of productivity and yield, even reaching the death of plants. Current knowledge about these crop responses can be translated into agronomic practices aimed at mitigating the adverse effects of environmental stress. 1. Introducción El cambio climático es una realidad a la debemos enfrentarnos utilizando la tecnología, los conocimientos científicos y las políticas económicas y sociales que modifiquen la relación entre la sociedad humana y su entorno. El cambio climático representa ya en este momento un reto multifacético para la producción sustentable de alimentos, para la salud y en general para la cultura y los patrones actuales de nivel y calidad de vida de los humanos (Adger et al. 2009). En el caso particular de la producción de alimentos por medio de cultivos en campo (cereales, oleaginosas, hortalizas, etc.), los escenarios esperados señalan la ocurrencia cada vez más frecuente de eventos climáticos desfavorables para la producción agrícola. Ello obliga a modificar y ajustar a una nueva realidad los procesos productivos agrícolas (Olesen and Bindi 2002). Diferentes técnicas de producción agrícola, como el uso de espacios protegidos (invernaderos, malla sombra, túneles y acolchados) (Clark and Tilman 2017), las modernas técnicas de modificación genética (Brookes and Barfoot 2014), la ejecución de translational processes based on systems biology (Fukushima et al. 2009), y la implementación a gran escala de granjas verticales y plant factories (Despommier 2013) pueden aportar parte de los alimentos necesarios para la creciente población humana. Sin embargo, en este momento el obtener las calorías, minerales, fibra, etc. necesarios para la alimentación de los humanos y sus animales domésticos es todavía una empresa realizada casi en su totalidad sobre el suelo en campo abierto (Clark and Tilman 2017). El cambio a un sistema en donde el 100 % del alimento para una población sea producido en granjas verticales, plant factories u otros sistemas de alta tecnología implica un cambio profundo en la cultura y procesos de alimentación, como por ejemplo disminuir o eliminar el consumo de carne y el desperdicio de alimentos, entre otros (Clark and Tilman 2017). Considerando lo anterior parece que la producción de cultivos aún ocurrirá mayormente utilizando los suelos en sistemas de producción a campo abierto, por lo que la esperada mayor magnitud del estrés asociada al cambio climático no parece tener una solución que dependa totalmente del cultivo bajo condiciones controladas. En todo caso, incluso esperando disponer de sistemas robotizados, automatización y fuentes abundantes de energía, ya sea que la producción de alimentos se lleve a cabo en el campo, en el laboratorio o en una granja vertical o plant factory, en todas las mencionadas situaciones deberán aplicarse los conceptos de producción sustentable, cuidado de los recursos naturales, mitigación del impacto ambiental y la contaminación, ya que por definición cualquier proceso industrial tendrá un impacto sobre el entorno (Zhongyue Xu et al. 2015). Por otra parte, incluso los sistemas industriales avanzados para la producción de
... In this chapter, we present the measures that we propose to apply to the interaction domain of crop plants, soil, and atmosphere, that is, on the scale of an agricultural ecosystem. At this level (particularly in C3 species) the environmental factor irradiance seems to be a common confluence point for stress caused by multiple factors [20,21]. As a consequence, mitigating the stress resulting from high levels of PAR in crop fields could reduce the impact of other stress-inducing environmental factors such as water deficit, salinity, and heat. ...
... In this chapter, we present the measures that we propose to apply to the interaction domain of crop plants, soil, and atmosphere, that is, on the scale of an agricultural ecosystem. At this level (particularly in C3 species) the environmental factor irradiance seems to be a common confluence point for stress caused by multiple factors [20,21]. As a consequence, mitigating the stress resulting from high levels of PAR in crop fields could reduce the impact of other stress-inducing environmental factors such as water deficit, salinity, and heat. ...
... 光抑制 主要发生在光系统Ⅱ, 但其产生的机制现在还不是很 清楚. 光系统Ⅱ的天线色素分子吸收的光能以多种 形式耗散, 如以荧光的形式耗散、以非辐射的热能形 式耗散、传递给光系统Ⅰ或通过光系统Ⅱ的光化学 反应散失 [18] . 有证据表明, 叶绿体类囊体膜上天线或 反应中心激发态叶绿素的热耗散增加, 可能会导致光 抑制 [19] . ...
叶绿素a和β-胡萝卜素对植物叶片捕获光能有重要作用,
捕获的光能被用于进行光合作用.
在本研究中, 探究了叶绿素a和β-胡萝卜素的太赫兹光谱和可见光谱以及它们在光胁迫下的变化.
结果表明, 在光胁迫下叶绿素a和β-胡萝卜素的透射光谱和吸收光谱都发生了变化,
并且在光照15 min时变化最大,
这意味着此时的集体振动模变化最大.
在光胁迫下, 叶绿素a在可见区的吸收强度也下降,
这意味着叶绿素a分子发生了降解.
从上述结果可以看出,
太赫兹光谱对生物分子集体振动模的变化非常敏感,
尽管这些分子的构型没有发生变化.
太赫兹光谱可用来检测生物大分子的分解过程,
而可见光谱只能用来检测生物大分子的分解程度.
... Why photoinhibition happens is not very clear so far, but it occurs mainly in PSII. The light energy absorbed by the PSII antenna chlorophyll molecules can be dissipated as fluorescence, as heat by non-radiative dissipation, by transfer to photosystem I (PSI), and in photochemical activity by PSII (Björkman, 1987). There is evidence that photoinhibition of photosynthesis in vivo may be caused by increased thermal dissipation of excited chlorophylls at the antenna or the reaction center levels (Öquist, 1988). ...
Chlorophyll a and β-carotene play an important role in harvesting light energy, which is used to drive photosynthesis in plants. In this study, terahertz (THz) and visible range spectra of chlorophyll a and β-carotene and their changes under light treatment were investigated. The results show that the all THz transmission and absorption spectra of chlorophyll a and β-carotene changed upon light treatment, with the maximum changes at 15 min of illumination indicating the greatest changes of the collective vibrational mode of chlorophyll a and β-carotene. The absorption spectra of chlorophyll a in the visible light region decreased upon light treatment, signifying the degradation of chlorophyll a molecules. It can be inferred from these results that the THz spectra are very sensitive in monitoring the changes of the collective vibrational mode, despite the absence of changes in molecular configuration. The THz spectra can therefore be used to monitor the decomposing process of biological macromolecules; however, visible absorption spectra can only be used to monitor the breakdown extent of biological macromolecules.
... 1988, Horton and Hague 1988, Oxborough and Horton 1988 and it may possibly involve cyt bSS9 (Horton and Cramer 1975, Whitmarsh and Cramer 1977, Nanba and Satoh 1987, Thompson and Brudwig 1988. Nevertheless, a diversion of massive amounts of excitation energy, away from the photochemical reaction centres, has been suggested to occur within the pigment complexes associated with PS2 (i.~. in the ChI pigment bed), in the fonn2 58 of a radiationless dissipation of excitation energy (Bjorkman 1987, Demmig-Adams 1990, Horton 1990) involving all the carotenoids or at least zeaxanthin in a specific mechanism (Fig. 6). Demmig-Adams 1990.) ...
A synthesized version of the dioxygen chemistIy is reviewed. Since superoxide, hydrogen peroxide and hydroxyl radicals are products of the dioxygen reduction, a chemical characterization is advanced. In addition, Fe, Mn and Cu integrated in enzymes connected with reduction products of oxygen in plants are chemically characterized. An overview of the oxygen metabolism in photosynthesis follows. Accordingly, the production of triplet oxygen through the water-splitting complex as well as the production and control of singlet oxygen and hydrogen peroxide in the photo system 2 are evaluated. Furthermore, the superoxide and hydrogen peroxide production and control in photo system 1 are evaluated. The chloroplast oxy-radicals production under stress conditions, mainly an interaction between zeaxanthin and singlet oxygen under photoinhibitory conditions, as well as the interactions between sulphur dioxide and superoxide are given. Furthermore, the effects of low temperatures on the oxygen metabolism as well as the nutritional effects of Mn, Fe and Cu on the oxy-radicals production and control are evaluated.
... Response of photosynthesis to low irradiance is an indication of the functionality of the photochemical reactions (Björkman, 1987). Our measurements indicated that the photochemistry of photosynthesis was not impaired by drought stress. ...
Physiological characteristics, growth, and biomass production of rainfed and irrigated bell pepper [Capsicum annuum L. var. anuum (Grossum Group) 'Quadrato d'Asti'] plants were measured in the semiarid conditions of a Mediterranean summer to determine if drought stress effects are transient and do not affect plant growth and crop yield or are persistent and adversely affect plant growth and crop yield. A low midday leaf water potential indicated the occurrence of transient drought stress episodes in rainfed plants during the first 2 months of the study. Later on, predawn water potential also increased, indicating a persistent drought stress condition despite the occurrence of some rainfall. Photosynthesis was reduced when stress conditions developed, but the reduction was transient and limited to the central part of the day during the first 2 months. As plants aged, however, the impact of drought stress on photosynthesis was not relieved during the overnight recovery period. Stomatal conductance was reduced both during transient and permanent stress conditions while CO2 transfer conductance (i.e., conductance to CO2 inside the leaf) was only reduced when photosynthesis inhibition was unrecoverable. However, chloroplast CO2 concentration was higher in rainfed than in irrigated leaves indicating that CO2 availability was not limiting photosynthesis. Nonphotochemical quenching of fluorescence increased significantly in rainfed leaves exposed to permanent stress indicating the likely impairment of ATP synthesis. Transient inhibition of photosynthesis did not significantly affect leaf area index and biomass production, but growth was significantly reduced when photosynthesis was permanently inhibited. Fruit dry weight was even higher in rainfed plants compared to irrigated plants until drought stress and photosynthesis reduction became permanent. It is suggested that bell pepper growth without supplemental irrigation over the first part of the vegetative cycle does not impair plant growth and may even be useful to improve yield of early fruit.
... Quan s'inactiva el transport d'electrons en el PSII, s'incrementa la taxa de degradació de la proteïna Dl i aquest fet comporta un augment de F 0 (Rintamaki et al., 1994). D'altra banda, un descens de F m pot indicar un augment de l'extinció no fotoquímica ( (Björkman, 1986;Baker i Horton, 1987;Bolhàr-Nordenkampf et ai, 1989). Per altra banda, l'àrea sota la corba entre F 0 i F m és proporcional a la mida del pool d'acceptors d'electrons de la banda reduïda del PSII, una simple indicació que ens dóna el paràmetre tm, que és el temps que necessita la fluorescència per aconseguir la meitat de la distància entre F 0 i F m (Bolhàr-Nordenkampf et ai, 1989). ...
... It is important that vocabulary not impede the quest for knowledge, and for this reason it is necessary to reconsider definitions of photoinhibition based on reduced photosynthetic efficiency alone. Thus, it seems unreasonable to consider as photoinhibition those processes that promote the deflection of excess excitation from the reaction centre, and thereby lower quantum yield, but that at the same time protect the reaction centre from damage (Bjorkman 1987). Notions that one process of photoinhibition protects against other processes leading to photoinhibitory damage are not helpful. ...
... The utilisation of captured light energy, measured by chlorophyll fluorescence emitted from PS I1 either at room temperataure (Somersalo and Krause 1988) or 77 K (Bjorkman 1987), has been used to characterise the response of plants not only to photoinhibition under visible light, but also to UV-B radiation-induced injury (Tevini et al. 1988). Moreover, both light-saturated photosynthetic capacity and carboxylation efficiency, which provide information on the biochemical processes involved in the photosynthetic dark reactions, have also been reported to be inhibited by UV-B radiation (Vu et al. 1982a, 1982b, 1984Strid et al. 1990). ...
Responses to short-term supplementary ultraviolet-B (UV-B) radiation were studied in detached leaves of two indica rice cultivars (Er Bai Ai and Lemont) to evaluate whether this might be an initial method for screening for UV-B susceptibility. Leaf tissue from plants grown in a greenhouse (28ºC day/ 20ºC night, with a maximum irradiance of 800-1000 μmol photons m-2 s-1) was placed under moderate supplementary UV-B radiation for 20 h. The effects of this short-term treatment were measured by determining the ratio of variable to maximum chlorophyll fluorescence (Fv/Fm), quantum yield of photosynthetic O2 exchange, chlorophyll content, maximum Rubisco activity as well as the concentrations of total soluble protein and Rubisco protein. All the above parameters showed considerable declines, which were always greater in cv. Er Bai Ai than in cv. Lemont. The in vivo activation of Rubisco was markedly increased in detached leaves treated with supplementary UV-B compared with control leaves; the increase was greater in cv. Er Bai Ai than in cv. Lemont. The photosynthetic responses invoked in the detached rice leaves are remarkably similar to those observed previously [He et al. (1993). Aust. J. Plant Physiol. 20, 129-42] in intact rice plants which had a longer-term supplementary UV-B exposure of comparable cumulative biologically effective UV-B dosage. We conclude that rapid short-term responses of detached leaves allow early screening of relative sensitivity of rice cultivars to UV-B.
... We focused on variable-tomaximum fluorescence ratio (Fv/Fm), which is considered a reliable measure of maximal (potential) efficiency of excitation capture by open PSII in dark-adapted conditions and is used as an estimate of the functional state of the photosynthetic apparatus in a given environmental situation. A decrease in Fv/Fm may be interpreted as a photoinhibition of PSII (Björkman, 1987). Photochemical efficiency in the light-adapted state of photosystem II (ΦPSII) also was calculated according to Genty et al. (1989). ...
... Although it may be convenient for the physiologist to consider the plant response to various stresses separately, most responses are neither independent nor specific. The facts that multiple stresses co-occur and that the response to several simultaneous stresses is usually not predictable by single-factor analyses make the study of the interactions both appropriate and complex (Björkman 1987; Gamon & Pearcy 1990b). A combination of different stress factors can result in intensification , overlapping or reversal of the stress effects (Osmond et al. 1986 ). Leaf photosynthesis is easily abolished by elevated temperatures (Berry & Björkman 1980), but high temperature and high PFD have a more profound effect on PSII photochemical activity together than they have separately (Ludlow 1987). ...
Gas exchange and chlorophyll fluorescence techniques were used to evaluate the acclimation capacity of the schlerophyll shrub Heteromeles arbutifolia M. Roem. to the multiple co-occurring summer stresses of the California chaparral. We examined the influence of water, heat and high light stresses on the carbon gain and sur- vival of sun and shade seedlings via a factorial experiment involving a slow drying cycle applied to plants grown out- doors during the summer. The photochemical efficiency of PSII exhibited a diurnal, transient decrease ( ΔF/Fm') and a chronic decrease or photoinhibition (Fv/Fm) in plants exposed to full sunlight. Water stress enhanced both tran- sient decreases of ΔF/Fm' and photoinhibition. Effects of decreased ΔF/Fm' and Fv/Fm on carbon gain were observed only in well-watered plants since in water- stressed plants they were overidden by stomatal closure. Reductions in photochemical efficiency and stomatal con- ductance were observed in all plants exposed to full sun- light, even in those that were well-watered. This suggested that H. arbutifolia sacrificed carbon gain for water conser- vation and photoprotection (both structurally via shoot architecture and physiologically via down-regulation) and that this response was triggered by a hot and dry atmo- sphere together with high PFD, before severe water, heat or high PFD stresses occur. We found fast adaptive adjust- ments of the thermal stability of PSII (diurnal changes) and a superimposed long-term acclimation (days to weeks) to high leaf temperatures. Water stress enhanced resis- tance of PSII to high temperatures both in the dark and over a wide range of PFD. Low PFD protected photochem- ical activity against inactivation by heat while high PFD exacerbated damage of PSII by heat. The greater intercep- tion of radiation by horizontally restrained leaves relative to the steep leaves of sun-acclimated plants caused pho- toinhibition and increased leaf temperature. When tran- spirational cooling was decreased by water stress, leaf temperature surpassed the limits of chloroplast ther- mostability. The remarkable acclimation of water-stressed plants to high leaf temperatures proved insufficient for the semi-natural environmental conditions of the experiment. Summer stresses characteristic of Mediterranean-type cli- mates (high leaf temperatures in particular) are a poten- tial limiting factor for seedling survival in H. arbutifolia, especially for shade seedlings lacking the crucial struc- tural photoprotection provided by steep leaf angles.
... The current version of the xanthophyll cycle was discovered by Yamamoto et al. (1962), and manipulation of the xanthophyll cycle by dithiothreitol was discovered by Yamamoto and Kamite (1972) (Fig. 1). One of the earliest and major contributions to the relationship between decreased Chl a fluorescence in high light and increased heat loss in whole plants was that by Björkman (1987) and Demmig et al. (1987). It was Demmig et al. (1987Demmig et al. ( , 1988 who first provided the connection between Chl a fluorescence quenching, heat loss and zeaxanthin. ...
This paper deals first with the early, although incomplete, history of photoinhibition, of 'non-QA-related chlorophyll (Chl) a fluorescence changes', and the xanthophyll cycle that preceded the discovery of the correlation between non-photochemical quenching of Chl a fluorescence (NPQ) and conversion of violaxanthin to zeaxanthin. It includes the crucial observation that the fluorescence intensity quenching, when plants are exposed to excess light, is indeed due to a change in the quantum yield of fluorescence. The history ends with a novel turn in the direction of research — isolation and characterization of NPQ xanthophyll-cycle mutants of Chlamydomonas reinhardtii Dangeard and Arabidopsis thaliana (L.) Heynh., blocked in conversion of violaxanthin to zeaxanthin, and zeaxanthin to violaxanthin, respectively. In the second part of the paper, we extend the characterization of two of these mutants (npq1, which accumulates violaxanthin, and npq2, which accumulates zeaxanthin) through parallel measurements on growth, and several assays of PSII function: oxygen evolution, Chl a fluorescence transient (the Kautsky effect), the two-electron gate function of PSII, the back reactions around PSII, and measurements of NPQ by pulse-amplitude modulation (PAM 2000) fluorimeter. We show that, in the npq2 mutant, Chl a fluorescence is quenched both in the absence and presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). However, no differences are observed in functioning of the electron-acceptor side of PSII — both the two-electron gate and the back reactions are unchanged. In addition, the role of protons in fluorescence quenching during the 'P-to-S' fluorescence transient was confirmed by the effect of nigericin in decreasing this quenching effect. Also, the absence of zeaxanthin in the npq1 mutant leads to reduced oxygen evolution at high light intensity, suggesting another protective role of this carotenoid. The available data not only support the current model of NPQ that includes roles for both pH and the xanthophylls, but also are consistent with additional protective roles of zeaxanthin. However, this paper emphasizes that we still lack sufficient understanding of the different parts of NPQ, and that the precise mechanisms of photoprotection in the alga Chlamydomonas may not be the same as those in higher plants.
Plants, algae, and photosynthetic bacteria all contain carotenoids, which are lipid-soluble natural compounds. They can act as both light-harvesting complex and photoprotectors. Due to their nature, they are able to neutralize the effect of the presence of singlet oxygen and free radicals, acting as quenchers; for this function, an important and crucial role as an antioxidant has been attributed to a large number of carotenoids. Their production has been studied in several microalgal species, which represent a natural source of these antioxidants. In particular, Haematococcus, Chlamydomonas, Chlorella, Dunaliella, diatoms such as Phaeodactylum and Isochrysis, and dinoflagellates are able to synthesize large amounts of carotenoids. Among the most powerful antioxidant carotenoids, the xanthophylls loroxanthin, neoxanthin, lutein, violaxanthin, antheraxanthin, zeaxanthin, and α-carotene and β-carotene are the ones most synthesized under photo-oxidative stress conditions. Under physiological stresses, such as high light exposure, nutrient limitation-starvation, excessive low-high temperatures, the photosynthetic activity decreases, and different metabolic pathways are activated. The study of the physiological response to different stresses helps to understand the mechanisms which regulate the accumulation of antioxidant compounds. This information can be useful for optimizing the growth conditions of microalgal strains, the high carotenoid producers, for increasing their productivity, in terms of both antioxidants and biomass, and for the scale-up of the process from laboratory to outdoor cultures.
The quenching of chlorophyll fluorescence caused by photodamage of Photosystem II (qI) is a well recognized phenomenon, where the nature and physiological role of which are still debatable. Paradoxically, photodamage to the reaction centre of Photosystem II is supposed to be alleviated by excitation quenching mechanisms which manifest as fluorescence quenchers. Here we investigated the time course of PSII photodamage in vivo and in vitro and that of picosecond time-resolved chlorophyll fluorescence (quencher formation). Two long-lived fluorescence quenching processes during photodamage were observed and were formed at different speeds. The slow-developing quenching process exhibited a time course similar to that of the accumulation of photodamaged PSII, while the fast-developing process took place faster than the light-induced PSII damage. We attribute the slow process to the accumulation of photodamaged PSII and the fast process to an independent quenching mechanism that precedes PSII photodamage and that alleviates the inactivation of the PSII reaction centre.
Given increasing water deficits across numerous ecosystems world‐wide, it is urgent to understand the sequence of failure of leaf function during dehydration.
We assessed dehydration‐induced losses of rehydration capacity and maximum quantum yield of the photosystem II (Fv/Fm) in the leaves of 10 diverse angiosperm species, and tested when these occurred relative to turgor loss, declines of stomatal conductance gs, and hydraulic conductance Kleaf, including xylem and outside xylem pathways for the same study plants. We resolved the sequences of relative water content and leaf water potential Ψleaf thresholds of functional impairment.
On average, losses of leaf rehydration capacity occurred at dehydration beyond 50% declines of gs, Kleaf and turgor loss point. Losses of Fv/Fm occurred after much stronger dehydration and were not recovered with leaf rehydration. Across species, tissue dehydration thresholds were intercorrelated, suggesting trait co‐selection. Thresholds for each type of functional decline were much less variable across species in terms of relative water content than Ψleaf.
The stomatal and leaf hydraulic systems show early functional declines before cell integrity is lost. Substantial damage to the photochemical apparatus occurs at extreme dehydration, after complete stomatal closure, and seems to be irreversible.
Water stress is one of the most important physiological stress factors that adversely affect soybeans in many critical aspects of their growth and metabolism. Soybean’s growth, development and productivity are severely diminished, when soil or cell water potential becomes inadequate to sustain metabolic functioning. However, little has been done to gather comprehensive information regarding the specific changes that occur in water-stressed plants at the anatomical and morphological level. In this study, deviations in root growth, shoot growth, stomatal conductance, yield components and anatomical features are reported. Treatments with two levels of water stress imposed by reducing irrigation (once in 7 days or once in 15 days) revealed that, all cultivars (Dundee, LS 677, LS 678, TGx 1740-2F, TGx 1835-10E and Peking) were highly susceptible to prolonged water stress, exhibiting severe dehydration and death. A 15.0 and 30.0% survival frequency was obtained in plants irrigated once in 7 days; LS 677 and Peking, respectively. Unlike many other stresses, water deficit did not only affect the density of stomata, but, photosynthesis was affected by the lower levels of tissue CO2. These results suggest that, balanced biochemical, physiological, anatomical and morphological regulations are necessary for increased growth and yields in soybean.
We studied the leaf structural, water status, and fast fluorescence responses of two palms, Socratea exorrhiza and Scheelea zonensis, under natural dry season conditions in a clearing (high light [HL] palms) and the forest understory (low light [LL] palms) on Barro Colorado Island, Panama. HL-Socratea leaves were more shade-adapted, less xeromorphic, and more strongly affected by drought than HL-Scheelea. Fv/Fm (the ratio of variable to maximum chlorophyll fluorescence) and t½ (the half-rise time of Fm) was lower in HL-leaves of both species, indicating photoinhibition. In HL-Scheelea, the light-induced reduction of Fv/Fm was much less than in HL-Socratea, and Fv/Fm recovered completely overnight. Patterns of relative water content, specific leaf dry weight, stable carbon isotope composition, and leaf conductance suggest that increased drought resistance in Scheelea reduces susceptibility to photoinhibition. An increase in Fo indicated the inactivation of PSII reaction centers in HL-Socratea. The very low chlorophyll a/b ratio and alterations in chloroplast ultrastructure in HL-Socratea are consistent with photoinhibition. Under LL, the species showed no appreciable interspecific differences in chlorophyll fluorescence. Excess light leads to low values of Fv/Fm in HL-plants relative to LL-plants on both leaf surfaces, particularly on the lower surface, due to a decrease of Fm in both surfaces and an increase in F., of lower surface. For both species, Fo for the lower surfaces of HL-plants was higher and t½ was markedly lower than for the upper surface, as is typical for shade-adapted leaves. Xeromorphic leaf structure may reduce susceptibility to photoinhibition during the dry season. Drought-enhanced photoinhibition could limit the ability of some species to exploit treefall gaps.
Recent achievements in the investigation of the properties of the carotenoids in relation to photoprotection are discussed. Focus is on the presence of carotenoids in the reaction centre and light harvesting complex. and their functions in photoprotection. Emphasis is given to the xanthophyll cycle and ß-carotene. which play a significant role in photoprotection by dissipating the excessive excitation energy.
Different sources of stress, when coupled to high light fluence, produce photoinhibitive damage on broadleaved evergreens at irradiations which are 4–5 times higher than reported on annual leaves. Quantum yield of PSII photochemistry Qy(p) as indicated by the ratio Fv/Fm at 692nm (Kitajima and Butler) was found to be quite a convenient probe of photoinhibitive damage; Bjorkman3 on Hedera canariensis, and same other authors on different species found highly signifiant relationship between quantum yield for oxygen evolution, Qy(ox) and Qy (p). The same procedure when applied to our plants resulted however in a trend lowered by a constant factor in respect to those experiences (fig.1).
The effect of salinity, photoinhibition and interaction of the two on various photosynthetic parameters was investigated in leaves and isolated chloroplasts of barley. Plants were grown in a controlled environment and irrigated with different concentrations (0–200 mM) of NaCl + CaCl2 salts. Photoinhibition treatment was given at 1600 UE/M2/S for four hour at 5 C and 20 C. The light saturated rate of CO2 uptake and maximum quantum yield decreased with increasing salt concentrations. Photoinhibition also decreased the CO2 assimilation rate and apparent quantum yield. Stomatal conductance was more sensitive to salt stress than photoinhibition. Salt stress and photoinhibition both modified the amplitude of leaf variable fluorescence (Fm) at 20 C in the presence of DCMU. Electron transport activity measured in isolated chloroplasts from plants grown at low salinity levels showed a slight increase in photosystem II (PS II) and photosystem I (PS I) activity in comparison with control; higher salt concentrations did not affect the electron transport activity. In control plants photoinhibition significantly decreased the PS II activity but no changes were observed in PS I activity. Interaction of photoinhibition and salt stress increases the inhibition of PS II activity in comparison with control photoinhibited plants.
Photosynthesis by plants in terrestrial or aquatic environments is a source of biomass and a sink for CO2 emissions. The rate of photosynthesis is limited by intrinsic thermodynamic and biological constraints, and by environmental factors. Increased [CO2] in the atmosphere and changes in climate will affect plants via photosynthesis and other aspects of plant metabolism, probably widening the gap between potential and actual global photosynthesis. Although plant metabolism can be changed.. by genetic means like plant breeding and genetic engineering, relative ignorance in some areas of plant sciences is limiting the application of these potentially powerful resources.
An overview is presented on the documented interactions that Ca has with salt stress effects on plants. Significant interactions at the cell level occur in the cell wall, at membranes and membrane transport processes, cytosolic Ca2+ activity, and intracellular solute regulation, all contributing to effects on cell elongation. Recent research links the dynamics of cell growth quantitatively to the spatial distribution of solutes, with Ca playing a major regulatory role.
Salt stress interfered with Ca allocation to the growth zone of a young sorghum leaf, but supplemental Ca partly mitigated this effect. Regulation of organic solutes was investigated by means of 1H-NMR spectroscopy and GLC. Proline and asparagine accumulated in the young sorghum leaf, more so for plants supplied with low Ca where growth was curtailed by salt. Salt stress also had major effects on photosynthetic and carotenoid pigments, as determined by HPLC. Chlorophyll a and b and violaxanthin levels decreased but zeaxanthin increased, and high Ca supply amplified these responses to salt. The depletion of violaxanthin may be related to enhanced ABA synthesis.
The response of photosynthetic organisms to high or excessive photon flux densities (PPFD) is a topic which interested several investigators in photosynthesis research during the first half of the twentieth century (e.g. Emerson 1936; Myers and Burr 1940; Kok 1956). Until recently, the phenomenon of photoinhibition, the inhibition of photosynthesis by excessive light, was equated with damage to the photosynthetic apparatus (Powles 1984; see also Demmig-Adams and Adams 1992a). In the majority of studies on photoinhibition, it is, in fact, likely that some form of damage was responsible for the majority of the decrease in photosynthetic competence observed, as most studies have involved the exposure of leaves, organisms, or even isolated chloroplasts or thylakoids to PPFD which was many times greater than that experienced during development.
During the past years chlorophyll fluorescence has developed as one of the most frequently used measuring tools in plant science. This somewhat unexpected and remarkable development was triggered by recent progress in instrumentation for measuring fluorescence yield under ambient light conditions and by the increased awareness among plant scientists, as in the general public, of aspects of environmental and stress physiology. Since the discovery of the “Kautsky effect” in 1931, fluorescence had always served as a pioneer tool. However, for more than 50 years it was mainly used by biophysically oriented scientists for basic photosynthesis research. The phenomenology of fluorescence changes in intact cells was considered far too complex to provide more than qualitative information. Because the fluorescence characteristics were known to be strongly affected by preillumination, it appeared necessary to thoroughly dark-adapt a sample before recording dark-light induction curves. Additionally, as the complexity of fluorescence changes increased with increasing illumination time, it was mostly the rapid initial induction kinetics which were analyzed to assess the functioning of the primary reactions. Within less than a decade, a completely different situation has evolved. Along with the availability of new instrumentation and analytical methods for fluorescence analysis under normal daylight conditions, the interest has shifted from the primary reactions to the level of overall electron transport efficiency and photosynthesis regulation, and from induction kinetics to investigations of steady-state reactions. Chlorophyll fluorescence, which used to be a tool preferentially applied in dark laboratories, has made the step into the full sunlight, where in situ photosynthesis takes place with all its intricate and still poorly understood regulatory mechanisms in response to environmental factors.
Two spieces of Acropora corals (A. pulchra and A. millepora) were used to study the effects of phosphate stress on the photosynthesis of symbiotic algae. The Fv/Fm of the symbiotic algae was stable at the control level of phosphate concentration, but was depressed at under 15μmol/L and 30μmol/L of phosphate during the initial culture state compared with the control, and then increased after 2 and 5 days respectively, but to lower levels than that of the control. The density of the symbiotic algae at 15μmol/L and 31μmol/L concentration 5.59% and 14.69%. The symbiotic algae of A. pulchra and A. millepora were able tolerate phosphate concentration < 30 μmol/L, but their Fv/Fm and density were reduced significantly.
To evaluate the potential impact of irrigation water contaminated with microcystins (MCs) on agricultural production, we studied the accumulation of MCs, and changes in chlorophyll content and chlorophyll fluorescence parameters in rice seedlings treated with MCs at different concentrations (1, 100, 1000 and 3000 μg·L-1). The results show that MCs accumulation increased with the increase of MCs concentration. MCs at 1 μg·L-1 increased the growth and chlorophyll content in rice leaves whereas ETR, Fv/Fm, qP and qN had no change. High concentrations of MCs (≥100 μg·L-1) decreased the growth, chlorophyll content, Fv/Fm, ETR, qP and qN while increased F0. After a 7-day recovery, MCs accumulation in rice leaves was lower than those measured during the stress period. For the group treated with 100 μg·L-1 MCs, F0 and qN had no obvious change whereas Fv/Fm, ETR and qP were higher than those measured during the stress period although they were still lower than those of the control. The results indicated that the damage caused by MCs on photosynthetic capacity was reversible. When rice seedlings were treated with higher concentrations MCs (1000 and 3000 μg·L-1), Fv/Fm, ETR, qP and qN were still lower than those of the control, even worse than those measured during the exposure period. It was indicated that high concentration MCs reduced the efficiency of primary light energy conversion and the potential activity of PSII. Furthermore, the damage caused by high concentration MCs on photosynthetic function in plants was irreversible.
Plants transferred to a different environment may become more susceptible to various stresses because they have not developed adequate patterns of resource allocation and evolved the morphological and physiological features required by the new ‘demands’. This is the case for micropropagated plants which often do not survive transfer from in vitro culture to the greenhouse or the field [56], where high irradiances and low air humidity become stressful to the young plants just starting to become auto trophic. Understanding mechanisms of stress physiology as well as the plant’s potential to acclimate to new environments is of particular importance if we are to predict and improve performance and survival of plants during the process of acclimation, or acclimatization. The latter is the horticultural terminology for acclimation, meaning the guided process of adjustment of the plants to a new environment [20].
The fluorescence yield at room temperature of the lichens Ramalina maciformis and Peltigera rufescens, containing either green or blue-green algae (Cyanobacteria) as phycobionts, has been investigated during rehydration of the dry lichens by water vapor uptake or by wetting with liquid water. In the dry state the fluorescence yield with all reaction centers open, Fo, was low and no variable fluorescence could be induced with both species. Whereas R. maciformis, containing green algae, regained normal fluorescence behavior during water vapor uptake, the photosynthetic apparatus of the blue-green algae-containing P. rufescens stayed inhibited and could be reactivated only by addition of liquid water. During stepwise rehydration at increasing air humidities, a pattern was established for the recovery of the different fluorescence parameters in R. maciformis. At a dry-weight related water content between 30 and 40%, Fo rose sharply. Maximal variable fluorescence yield expressed as (Fv)m/Fo, strongly increased in the same range of water content and remained constant above a water content of 50%. Non-photochemical fluorescence quenching, qNP, determined at the end of a period of actinic illumination, decreased with increasing water vapor uptake. While spraying the lichen with liquid water did not induce a further decrease of qNP, slow dehydration at lowered air humidity led to a minimal value of qNP at a water content of 65 % indicating optimal photosynthetic rate under these conditions. These results extend the conclusions drawn from earlier gas exchange experiments that blue-green algae-containing lichens are unable to reactivate photosynthesis by water vapor uptake. During a re- and de-hydration cycle, no hysteresis in the hydration dependence of the fluorescence parameters was found. From this and the presence of a stable and low Fo value at prolonged incubation in nearly water vapor saturated air, we conclude that the reactivation of photosynthesis in blue-green algae-containing lichens is not prevented through high diffusion resistances for water.
The relation between the quantum yield of oxygen evolution of open photosystem II reactions centers (Φp), calculated according to Weis and Berry (1987), and non-photochemical quenching of chlorophyll fluorescence of plants grown at 19°C and 7°C was measured at 19°C and 7°C. The relation was linear when measured at 19°C, but when measured at 7°C a deviation from linearity was observed at high values of non-photochemical quenching. In plants grown at 7°C this deviation occurred at higher values of non-photochemical quenching than in plants grown at 19°C. The deviations at high light intensity and low temperature are ascribed to an increase in an inhibition-related, non-photochemical quenching component (qI).
The relation between the quantum yield of excitation capture of open photosystem II reaction centers (Φexe), calculated according to Genty et al. (1989), and non-photochemical quenching of chlorophyll fluorescence was found to be non-linear and was neither influenced by growth temperature nor by measuring temperature.
At high PFD the efficiency of overall steady state electron transport measured by oxygen-evolution, correlated well with the product of qN
and the efficiency of excitation capture (Φexe) but it deviated at low PFD. The deviations at low light intensity are attributed to the different populations of chloroplasts measured by gas exchange and chlorophyll fluorescence and to the light gradient within the leaf.
Dissipation of absorbed excitation energy as heat, measured by its effect on the quenching of chlorophyll fluorescence, is induced under conditions of excess light in order to protect the photosynthetic apparatus of plants from light-dependent damage. The spectral characteristics of this quenching have been compared to that due to photochemistry in the Photosystem II reaction centre using leaves of Guzmania monostachia. This was achieved by making measurements at 77K when fluorescence emission bands from each type of chlorophyll protein complex can be distinguished. It was demonstrated that photochemistry and non-photochemical dissipation preferentially quench different emission bands and therefore occur by dissimilar mechanisms at separate sites. It was found that photochemistry was associated with a preferential quenching of emission at 688 nm whereas the spectrum for rapidly reversible non-photochemical quenching had maxima at 683 nm and 698 nm, suggesting selective quenching of the bands originating from the light harvesting complexes of Photosystem II. Further evidence that this was occurring in the light harvesting system was obtained from the fluorescence excitation spectra recorded in the quenched and relaxed states.
This paper discusses biochemical and regulatory aspects of the violaxanthin cycle as well as its possible role in photoprotection. The violaxanthin cycle responds to environmental conditions in the short-term and long-term by adjusting rates of pigment conversions and pool sizes of cycle pigments, respectively. Experimental evidence indicating a relationship between zeaxanthin formation and non-photochemical energy dissipation is reviewed. Zeaxanthin-associated energy dissipation appears to be dependent on transthylakoid ΔpH. The involvement of light-harvesting complex II in this quenching process is indicated by several studies. The current hypotheses on the underlying mechanism of zeaxanthin-dependent quenching are alterations of membrane properties, including conformational changes of the light-harvesting complex II, and singlet-singlet energy transfer from chlorophyll to zeaxanthin.
Photoinhibition of photosynthesis was induced in intact kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson) leaves grown at two photon flux densities (PFDs) of 700 and 1300 μmol·m(-2)·s(-1) in a controlled environment, by exposing the leaves to PFD between 1000 and 2000 μmol·m(-2)·s(-1) at temperatures between 10 and 25°C; recovery from photoinhibition was followed at the same range of temperatures and at a PFD between 0 and 500 μmol·m(-2)·s(-1). In either case the time-courses of photoinhibition and recovery were followed by measuring chlorophyll fluorescence at 692 nm and 77K and by measuring the photon yield of photosynthetic O2 evolution. The initial rate of photoinhibition was lower in the high-light-grown plants but the long-term extent of photoinhibition was not different from that in low-light-grown plants. The rate constants for recovery after photoinhibition for the plants grown at 700 and 1300 μmol·m(-2)·s(-1) or for those grown in shade were similar, indicating that differences between sun and shade leaves in their susceptibility to photoinhibition could not be accounted for by differences in capacity for recovery during photoinhibition. Recovery following photoinhibition was increasingly suppressed by an increasing PFD above 20 μmol·m(-2)·s(-1), indicating that recovery in photoinhibitory conditions would, in any case, be very slow. Differences in photosynthetic capacity and in the capacity for dissipation of non-radiative energy seemed more likely to contribute to differences in susceptibility to photoinhibition between sun and shade leaves of kiwifruit.
Photoinhibition of photosynthesis was induced in attached leaves of kiwifruit grown in natural light not exceeding a photon flux density (PFD) of 300 μmol·m-2·s-1, by exposing them to a PFD of 1500 μmol·m-2·s-1. The temperature was held constant, between 5 and 35° C, during the exposure to high light. The kinetics of photoinhibition were measured by chlorophyll fluorescence at 77K and the photon yield of photosynthetic O2 evolution. Photoinhibition occurred at all temperatures but was greatest at low temperatures. Photoinhibition followed pseudo first-order kinetics, as determined by the variable fluorescence (F
v) and photon yield, with the long-term steady-state of photoinhibition strongly dependent on temperature wheareas the observed rate constant was only weakly temperature-dependent. Temperature had little effect on the decrease in the maximum fluorescence (F
m) but the increase in the instantaneous fluorescence (F
o) was significantly affected by low temperatures in particular. These changes in fluorescence indicate that kiwifruit leaves have some capacity to dissipate excessive excitation energy by increasing the rate constant for non-radiative (thermal) energy dissipation although temperature apparently had little effect on this. Direct photoinhibitory damage to the photosystem II reaction centres was evident by the increases in F
o and extreme, irreversible damage occurred at the lower temperatures. This indicates that kiwifruit leaves were most susceptible to photoinhibition at low temperatures because direct damage to the reaction centres was greatest at these temperatures. The results also imply that mechanisms to dissipate excess energy were inadequate to afford any protection from photoinhibition over a wide temperature range in these shade-grown leaves.
The relationship between components of non-photochemical quenching of chlorophyll fluorescence yield (qNP) and dissipation of excessive excitation energy was determined in cotton leaves using concurrent measurements of fluorescence and gas-exchange at 2% and 20% O2 under a range of photon flux densities and CO2 pressures. A nearly stoichiometric relationship was obtained between dissipation of energy not used in photosynthetic CO2 fixation or photorespiration and qNP provided that a component, probably associated with state transitions, was not included in qNP. Although two distinct components of qNP were resolved on the basis of their relaxation kinetics, both components appear effective in energy dissipation. The photon yield of "open" photosystem-II reaction centers decreased linearly with increases in qNP, indicating that much of the energy dissipation occurs in the pigment bed. However, increases in qNP appear dependent on the redox state of these centers. The results are discussed in relation to current hypotheses of the molecular basis of non-radiative energy dissipation. It is concluded that determinations of qNP can provide a quantitative measure of the dissipation of excessive excitation energy if precautions are taken to ensure that the maximum fluorescence yield is measured under conditions that provide complete closure of the photosystem-II reaction centers. It is also concluded that such dissipation can prevent photoinhibitory damage in cotton leaves even under extreme conditions where as much as 80% of the excitation energy is excessive.
Photosynthetic activity, in leaf slices and isolated thylakoids, was examined at 25° C after preincubation of the slices at either 25° C or 4° C at a moderate photon flux density (PFD) of 450 μmol·m(-2)·s(-1), or at 4° C in the dark. The plants used wereSpinacia oleracea L.,Cucumis sativus L. andNerium oleander L. which was acclimated to growth at 20° C or 45° C. The plants were grown at a PFD of 550 μmol·m(-2)·s(-1). Photosynthesis, measured as CO2-dependent O2 evolution, was not inhibited in leaf slices from any plant after preincubation at 25° C at a moderate PFD or at 4° C in the dark. However, exposure to 4° C at a moderate PFD induced an inhibition of CO2-dependent O2 evolution within 1 h inC. sativus, a chilling-sensitive plant, and in 45° C-grownN. oleander. The inhibition in these plants after 5 h reached 80% and 40%, respectively, and was independent of the CO2 concentration but was reduced at O2 concentrations of less than 3%. Methyl-viologen-dependent O2 exchange in leaf slices from these plants was not inhibited. There was no photoxidation of chlorophyll, in isolated thylakoids, or any inhibition of electron transport at photosystem (PS)II, PSI or through both photosystems which would account for the inhibition of photosynthesis. The conditions which inhibit photosynthesis in chilling-sensitive plants do not cause inhibition inS. oleracea, a chilling-insensitive plant, or in 20° C-grownN. oleander. The CO2-dependent photosynthesis, measured at 5° C, was reduced to about 3% of that recorded at 25° C in chilling-sensitive plants but only to about 30% in the chilling-insensitive plants. Methyl-viologen-dependent O2 exchange, measured at 5° C, was greater than 25% of the activity at 25° C in all the plants. The results indicate that the mechanism of the chilling-induced inhibition of photosynthesis does not involve damage to PSII. That inhibition of photosynthesis is observed only in the chilling-sensitive plants indicates it is related, in some way, to the disproportionate decrease in photosynthetic activity in these plants at chilling temperatures.
Cotton (Gossypium hirsutum L.) plants were grown in flowing-culture solutions containing 0%, 26% and 55% natural seawater under controlled and otherwise identical conditions. Leaf Na(+) content rose to 360 mM in 55% seawater, yet the K(+) content was maintained above 100 mM. The K(+)/Na(+) selectivity ratio was much greater in the saline plants than in the control plants. All plants were healthy and able to complete the life cycle but relative growth rate fell by 46% in 26% seawater and by 83% in 55% seawater. Much of this reduction in growth was caused by a decreased allocation of carbon to leaf growth versus root growth. The ratio of leaf area/plant dry weight fell by 32% in 26% seawater and by 50% in 55 % seawater while the rate of carbon gain per unit leaf area fell by only 20% in 26% seawater and by as much as 66% in 55% seawater. Partial stomatal closure accounted for nearly all of the fall in the photosynthesis rate in 26% seawater but in 55% seawater much of the fall also can be attributed to non-stomatal factors. As a result of the greater effect of salinity on stomatal conductance than on CO2-uptake rate, photosynthetic water-use efficiency was markedly improved by salinity. This was also confirmed by stablecarbon-isotope analyses of leaf sugar and of leaf cellulose and starch. - Although non-stomatal photosynthetic capacity at the growth light was reduced by as much as 42% in 55% seawater, no effects were detected on the intrinsic photon yield of photosynthesis nor on the efficiency of photosystem II photochemistry, chlorophyll a/b ratio, carotenoid composition or the operation of the xanthophyll cycle. Whereas salinity caused in increase in mesophyll thickness and content of chloroplast pigments it caused a decrease in total leaf nitrogen content. The results indicate that the salinity-induced reduction in non-stomatal photosynthetic capacity was not caused by any detrimental effect on the photosynthetic apparatus but reflects a decreased allocation to enzymes of carbon fixation. - Rates of energy dissipation via CO2 fixation and photorespiration, calculated from gas-exchange measurements, were insufficient to balance the rate of light-energy absorption at the growth light. Salinity therefore would have been expected to cause the excess excitation energy to rise, leading to an increased nonradiative dissipation in the pigment bed and resulting increases in non-photochemical fluorescence quenching and zeaxanthin formation. However, no such changes could be detected, implying that salinity may have increased energy dissipation via a yet unidentified energy-consuming process. This lack of a response to salinity stress is in contrast to the responses elicited by short-term water stress which caused strong non-photochemical quenching and massive zeaxanthin formation.
Thylakoid vesicles enriched in peroxidized lipids by 2-to 3-fold in comparison with thylakoid membranes were isolated from the stroma of intact chloroplasts obtained from primary leaves of two-week-old bean seedlings (Phaseolus vulgaris L. cv. Kinghorn). The fatty acid composition of the vesicles is similar to that of thylakoid membranes. The vesicles also exhibit photosystem II activity, but the level of activity is ~4-fold lower than that measured for thylakoid membranes. The chlorophyll a/b ratio for the vesicles is 6.59±0.21 compared with corresponding ratios of 2.85±0.02 and 9.85±0.62 for granal and stromal lamellae, respectively. It is clear from SDS-PAGE that the polypeptide composition of the vesicles is distinguishable from that of thylakoid membranes, and immunological analyses of several thylakoid membrane proteins have indicated that there are marked differences in protein composition between thylakoid vesicles and fractionated granal and stromal lamellae. Nonetheless, it is evident from their polypeptide and lipid composition that these vesicles originate from thylakoids, presumably by vesiculation. Moreover, the vesicles appear to be in situ elements of the stroma inasmuch as they are isolated from intact chloroplasts by a procedure that does not involve homogenization, and similar vesicles are visible in situ by electron microscopy. Indeed, the finding that the vesicles are enriched in peroxidized lipids suggests that vesiculation may be a means of removing destabilizing lipid catabolites from thylakoid membranes.
The effect of salinity (short term), high light and interaction of the two was investigated in leaves and isolated chloroplasts from these leaves of barley (C3) and sorghum (C4) plants. The light saturated rate of CO2 uptake and maximum quantum yield decreased with increasing salt concentrations. Stomatal conductance was more sensitive to salt stress than photoinhibition. Photoinhibition treatment of salt stressed plants resulted in a further decrease of the CO2 assimilation rate and quantum efficiency of photosynthesis. The recovery of the CO2 assimilation rate was considerably lower in salt and high light stressed plants compared to plants which were given a photoinhibition treatment only (no salt).
It is concluded that salt stress itself does not exert adverse effects on the primary photosynthesis process since the Fv/Fm ratio and electron transport activity did not decrease with higher salt concentrations. However, salinity stress enhanced substantially the susceptibility to photoinhibition; this may be due to the predisposing of the photosynthetic apparatus to damage under salt stress, resulting in lower light saturated rates of photosynthesis a decreased ability to recover from photodamage.
The role of semiquinone anion radicals in photoinhibition of isolated wheat (Triticum aestivum) chloroplasts was investigated by subjecting the chloroplasts to high light stress in the presence or absence of DCMU and DBMIB. The decrease in the efficiency of PS II photochemistry measured as Fv/Fm ratio and oxygen evolution after photoinhibition of isolated wheat chloroplasts was less in the presence of DCMU. We suggest that the protective effect of DCMU is due to its binding to the 32 kDa QB-binding protein and reducing the probability of formation of semiquinone anion and other free radical species that have been suggested to be involved in photoinhibition damage. The hypothesis was also tested by using DBMIB during photoinhibition. DBMIB is known to reduce the plastoquinone pool, resulting in an increase in the semiquinone ion population. A greater extent of reduction of Fv/Fm and oxygen evolution was observed when chloroplasts were photoinhibited in the presence of DBMIB. The results suggest an involvement of reduced semiquinones in the photoinhibition of wheat chloroplasts. A partial recovery of variable chlorophyll fluorescence in the presence of 20 mM hydroxylamine was also observed in chloroplasts subjected to light stress.
The effect of high light stress and salt stress, individually and in combination, was investigated in the leaves of Sorghum bicolor (1.). High light treatment at 5°C and 20°C caused an increase in zeaxanthin and a decrease in violaxanthin contents. Decreases in β-carotene were also observed, which was followed by increased formation of 5,6-epoxide-β-carotene. Violaxanthin + antheraxanthin + zeaxanthin (total) was not the same in control and stressed plants. Salt stress (no photoinhibition) also resulted in the formation of zeaxanthin but it was significantly less than that observed in the high light stressed plants. Neoxanthin content showed a slight decrease, while 5,6-epoxide lutein increased under salt stress. Changes in the xanthophyll cycle (violaxanthin + antheraxanthin + zeaxanthin) were reversed during recovery after photoinhibition. The CO2 assimilation rate and fluorescence Fv/Fm ratio were measured in intact leaves and isolated chloroplasts, respectively, from control and stressed plants. Formation of zeaxanthin and degradation of β-carotene are apparently involved in protection against photoinhibition.
The photosynthetic characteristics of eleven mutant lines of barley deficient in enzymes of the photorespiratory nitrogen
cycle were examined at 1%, 21% and 50% oxygen. The mutant lines could be separated into two classes: (1) those that displayed
normal rates of photosynthetic CO2 assimilation when compared to wild-type rates and (2) those that exhibited low rates of CO2 assimilation at high O2 concentrations and where there was no restoration of the original rate on return to 1% O2. Chlorophyll fluorescence kinetics of the mutant lines were examined following the transfer to high O2 concentrations. Mutant lines showed more similarities than differences and it is suggested that the depletion of metabolite
pools within the chloroplast is the main effect, with the rate of depletion varying not only with the mutation, but also with
light intensity and O2 concentration.
We studied the diurnal photosynthetic changes of Populus euphratica, a tree with distinct leaf shape polymorphism, in a typical desert environment. The photosynthetic CO2 assimilation rate in lanceolate leaves (LL), located at the bottom of the tree's canopy, is dependent on the diurnal changes in irradiation and reaches a maximum at 13:00 h, while a severe midday depression was observed in broad-ovate leaves (BOL) at the top of the tree's canopy at 13:00 h, most probably due to high temperature, irradiance, and water deficit. Though non-photochemical quenching increased dramatically at 13:00 h in BOL, the photosystem II (PS II) photochemical efficiency decreased. Western blot analysis showed that both PS II reaction center D1 protein and light harvesting complex II (LHC II) protein decreased when the midday depression occurred in BOL, which suggests that the decreased PS II proteins may account for the photosynthetic depression. Together with the low stomatal conductance observed, when this occurs, both stomatal and non-stomata] factors contribute to the midday depression in BOL. The physiological significance of leaf polymorphism is also discussed.
Recovery of photoinhibition in intact leaves of shade-grown kiwifruit was followed at temperatures between 10° and 35° C. Photoinhibition was initially induced by exposing the leaves for 240 min to a photon flux density (PFD) of 1 500 µmol·m-2·s-1 at 20° C. In additional experiments to determine the effect of extent of photoinhibition on recovery, this period of exposure was varied between 90 and 400 min. The kinetics of recovery were followed by chlorophyll fluorescence at 77K. Recovery was rapid at temperatures of 25–35° and slow or negligible below 20° C. The results reinforce those from earlier studies that indicate chilling-sensitive species are particularly susceptible to photoinhibition at low temperatures because of the low rates of recovery. At all temperatures above 15° C, recovery followed pseudo first-order kinetics. The extent of photoinhibition affected the rate constant for recovery which declined in a linear fashion at all temperatures with increased photoinhibition. However, the extent of photoinhibition had little effect on the temperature-dependency of recovery. An analysis of the fluorescence characteristics indicated that a reduction in non-radiative energy dissipation and repair of damaged reaction centres contributed about equally to the apparent recovery though biochemical studies are needed to confirm this. From an interpretation of the kinetics of photoinhibition, we suggest that recovery occurring during photoinhibition is limited by factors different from those that affect post-photoinhibition recovery.
A pulse amplitude-modulated fluorometric technique was employed to separate photochemical (qP) and non-photochemical (qN) chlorophyll fluorescence quenching in attached leaves of wild-type and a chlorophyll-b-less mutant of barley (Hordeum vulgare L.). Whereas a significantly higher qN developed in the wild type in the intensity range at which the photosynthetic light response became non-linear, there were virtually no differences in qP until pronounced photoinhibition was apparent. On monitoring the “dark” recovery of the maximum (FM) and dark level (F0) fluorescence yield, three distinct kinetic phases were resolved, which are ascribed to relaxation of high energy state quenching (qE), state 1-state 2 transitions (qT) and quenching due to photoinhibition (qI). The results provide evidence for heterogeneity of qE. The major part of qE (related to a fast-relaxing phase of FM quenching) is strongly reduced in mutant leaves. A fast-relaxing phase of F0 quenching, readily observed in wild-type leaves under high light, is absent in the mutant under the same conditions. Hence, qE appears to be associated with the photosystem II light-harvesting complex (LHC II). A medium component of “dark” relaxation kinetics was observed in both mutant and wild-type leaves. However, it appears attributable to qT only in the latter at low light. Supported by the finding that there was no difference in xanthophyll pool size (on a chlorophyll a basis) between wild type and mutant, under high light conditions the medium phase may reflect a slower-relaxing portion of qE, probably due to xanthophyll conversion. Under supersaturating light the very slowly relaxing qI component became dominating, affecting mutant leaves to a considerably greater extent. The results stress the key role of LHC II not only as a solar energy collector but also in protecting the photosynthetic apparatus from adverse effects of excess excitation input.
Photosynthesis is reduced at low leaf water potentials (Psi(l)) but repeated water deficits can decrease this reduction, resulting in photosynthetic acclimation. The contribution of the stomata and the chloroplasts to this acclimation is unknown. We evaluated stomatal and chloroplast contributions when soil-grown sunflower (Helianthus annuus L.) plants were subjected to water deficit pretreatments for 2 weeks. The relationship between photosynthesis and Psi(l), determined from gas-exchange and isopiestic thermocouple psychometry, was shifted 3 to 4 bars towards lower Psi(l), in pretreated plants. Leaf diffusive resistance was similarly affected. Chloroplast activity, demonstrated in situ with measurements of quantum yield and the capacity to fix CO(2) at all partial pressures of CO(2), and in vitro by photosystem II activity of isolated organelles, was inhibited at low Psi(l) but less in pretreated plants than in control plants. The magnitude of this inhibition indicated that decreases in chloroplast activity contributed more than closure of stomata both to losses in photosynthesis and to the acclimation of photosynthesis to low Psi(l).
The major objective for studying the impacts of external mechanical forces on agricultural soils is to develop and maintain a well- structured surface that interacts favorably with biological growth; water, heat, and air balances in the soil; and human activities. In this NATO Advanced Research Workshop the discussions were limited to the aspects of soil structure which are influenced by application of external forces, although natural forces such as those created by plant roots, soil fauna, freezing and water content changes are also important. The Workshop offered an opportunity for discussion of information available world-wide and to plan future research, both individually an cooperatively.
Incubation of Chlamydomonas reinhardii cells at light levels that are several times more intense than those at which the cells were grown results in a loss of photosystem II function (termed photoinhibition). The loss of activity corresponded to the disappearance from the chloroplast membranes of a lysine-deficient, herbicide-binding protein of 32,000 daltons which is thought to be the apoprotein of the secondary quinone electron acceptor of photosystem II (the QB protein). In vivo recovery from the damage only occurred following de novo synthesis (replacement) of the chloroplast-encoded QB protein. We believe that the turnover of this protein is a normal consequence of its enzymatic function in vivo and is a physiological process that is necessary to maintain the photosynthetic integrity of the thylakoid membrane. Photoinhibition occurs when the rate of inactivation and subsequent removal exceeds the rate of resynthesis of the QB protein.
When the shrub Nerium oleander L., growing under full natural daylight outdoors, was subjected to water stress, stomatal conductance declined, and so did non-stomatal components of photosynthesis, including the CO2-saturated rate of CO2 uptake by intact leaves and the activity of electron transport by chloroplasts isolated from stressed plants. This inactivation of photosynthetic activity was accompanied by changes in the fluorescence characteristics determined at 77 K (-196°C) for the upper leaf surface and from isolated chloroplasts. The maximum (F M) and the variable (F V) fluorescence yield at 692 nm were strongly quenched but there was little effect on the instantaneous (F O) fluorescence. There was a concomitant quenching of the maximum and variable fluorescence at 734 nm. These results indicate an inactivation of the primary photochemistry associated with photosystem II. The lower, naturally shaded surfaces of the same leaves were much less affected than the upper surfaces and water-stress treatment of plants kept in deep shade had little or no effect on the fluorescence characteristics of either surface, or of chloroplasts isolated from the water-stressed leaves. The effects of subjecting N. oleander plants, growing in full daylight, to water stress are indistinguishable from those resulting when plants, grown under a lower light regime, are exposed to full daylight (photoinhibition). Both kinds of stress evidently cause an inactivation of the primary photochemistry associated with photosystem II. The results indicate that water stress predisposes the leaves to photoinhibition. Recovery from this inhibition, following restoration of favorable water relations, is very slow, indicating that photoinhibition is an important component of the damage to the photosynthetic system that takes place when plants are exposed to water stress in the field. The underlying causes of this water-stress-induced susceptibility to photoinhibition are unknown; stomatal closure or elevated leaf temperature cannot explain the increased susceptibility.
The biochemistry of the violaxanthin cycle in relationship to photosynthesis is reviewed. The cycle is a component of the thylakoid and consists of a reaction sequence in which violaxanthin is converted to zeaxanthin (de-epoxidation) and then regenerated (epoxidation) through separate reaction mechanisms. The arrangement of the cycle in the thylakoid is transmembranous with the de-epoxidation system situated on the loculus side and epoxidation on the outer side of the membrane. Photosynthetic activities affect turnover of the cycle but the cycle itself consists entirely of dark reactions. Light has at least two roles in de-epoxidation. It establishes through the proton pump the acidic pH in the loculus that is required for de-epoxidase activity and it induces a presumed conformational change in the inner membrane surface which determines the fraction of violaxanthin in the membrane that enters the cycle. De-epoxidation, which requires ascorbate, is presumed to proceed by a reductive-dehydration mechanism. Non-cyclic electron transport can provide the required reducing potential through the dehydroascorbate-ascorbate couple. Whether ascorbate reduces the de-epoxidase system directly or through an intermediate has not been settled. Epoxidation requires NADPH and 02 which suggests a reductive mechanism. In contrast with de-epoxidation, it has a pH optimum near neutrality. The coupling of photosynthetically generated NADPH to epoxidation has been shown. Turnover of the cycle under optimal conditions is estimated to be about two orders of magnitude below optimal electron transport rates. This low rate appears to exclude a direct role of the cycle in photosynthesis or a role in significantly affecting photosynthate levels in a back reaction. The fact that the cycle is sensitive to events both before and after Photosystem I suggests a regulatory role, possibly through effects on membrane properties. A model showing the various relationships of the cycle to photosynthesis is presented. The contrasting view that the cycle can participate directly in photosynthesis, such as in oxygen evolution, is discussed. Violaxanthin de-epoxidase has been purified. It is a lipoprotein which contains monogalactosyldiglyceride (MG) exclusively. The enzyme is a mono-de-epoxidase which is specific for 3-0H, 5-6-epoxy carotenoids that are in a 3R, 5S, 6R configuration. In addition, the polyene chain must be all-trans. A model has been presented which depicts enzymic MG in a receptor role and the stereospecific active center situated in a narrow well-like depression that can accommodate only the all-trans structure.
The nature of the light-induced ΔpH-dependent decline of chlorophyll a fluorescence in intact and broken spinach chloroplasts was investigated. Fluorescence spectra at 77 K of chloroplasts frozen in the low-fluorescent (high ΔpH) state showed increased ratios of the band peak at 735 nm (Photosystem (PS) I fluorescence) to the peak at 695 nm (PS II fluorescence). The increase in the ratio at 77 K was related to the extent of fluorescence quenching at room temperature. Normalization of low-temperature spectra with fluorescein as an internal standard revealed a lowering of F695 that was not accompanied by an increase in F735: preillumination before freezing decreased both F695 and, to a lesser extent, F735 in the spectra recorded at 77 K. Fluorescence induction of chloroplasts frozen in the low-fluorescent state showed a markedly decreased variable fluorescence (Fv) of PS II, but no concomitant increase in initial fluorescence (F0) of PS I. Thus, the buildup of a proton gradient at the thylakoid membrane, as reflected by fluorescence quenching at room temperature, affects low-temperature fluorecence emission in a manner entirely different from the effect of removal of Mg2+, which is thought to alter the distribution of excitation energy in favor of PS I. The ΔpH-dependent quenching therefore cannot be caused by such change in energy distribution and is suggested to reflect increased thermal deactivation.
The quenching action of dibromothymoquinone on fluorescence and on primary photochemistry was examined in chloroplasts at minus 196 degrees C. Both the initial (F0) and final (FM) levels of fluorescence as well as the fluorescence of variable yield (FV equals FM minus FO) were quenched at minus 196 degrees C to a degree which depended on the concentration of dibromothymoquinone added prior to freezing. The initial rate of photoreduction of C-550 at minus 196 degrees C, which was assumed to be proportional to maximum yield for primary photochemistry, phipo, was also decreased in the presence of dibromothymoquinone. Simple theory predicts that the ratio FV/FM should equal phipo. Excellent agreement was found in a comparison of relative values of phipo with relative values of FV/FM at various degrees of quenching by dibromothymoquinone. These results are taken to indicate that FO and FV are the same type of fluorescence, both emanating from the bulk chlorophyll of Photosystem II. Dibromothymoquinone appears to create quenching centers in the bulk chlorophyll of Photosystem II which compete with the reaction centers for excitation energy. The rate constant for the quenching of excitation energy by dibromothymoquinone is directly proportional to the concentration of the quencher. Rate constants for the de-excitation of excited chlorophyll molecules by fluorescence, kF, by nonradiative decay processes, kD, by photochemistry, kP, and by the specific quenching of dibromothymoquinone, kQ, were calculated assuming the absolute yield of fluorescence at FO to be either 0.02 or 0.05.
Manuscript submitted to Planta
- B Demmig
- O Björkman