Thylakoid Reactions Research Articles

Diffusional and Biochemical Limitations to Photosynthesis Under Water Deficit for Field-Grown Cotton.

Water deficit stress limits net photosynthetic rate (AN), but the relative sensitivities of underlying processes such as thylakoid reactions, ATP production, carbon fixation reactions, and carbon loss processes to water deficit stress in field-grown upland cotton require further exploration. Therefore, the objective of the present study was to assess (1) the diffusional and biochemical mechanisms associated with water deficit-induced declines in AN and (2) associations between water deficit-induced variation in oxidative stress and energy dissipation for field-grown cotton. Water deficit stress was imposed for three weeks during the peak bloom stage of cotton development, causing significant reductions in leaf water potential and AN. Among diffusional limitations, mesophyll conductance was the major contributor to the AN decline. Several biochemical processes were adversely impacted by water deficit. Among these, electron transport rate and RuBP regeneration were most sensitive to AN-limiting water deficit. Carbon loss processes (photorespiration and dark respiration) were less sensitive than carbon assimilation, contributing to the water deficit-induced declines in AN. Increased energy dissipation via non-photochemical quenching or maintenance of electron flux to photorespiration prevented oxidative stress. Declines in AN were not associated with water deficit-induced variation in ATP production. It was concluded that diffusional limitations followed by biochemical limitations (ETR and RuBP regeneration) contributed to declines in AN, carbon loss processes partially contributed to the decline in AN, and increased energy dissipation prevented oxidative stress under water deficit in field-grown cotton.

Genotypic variation in growth, single leaf physiology, and acclimation potential of thylakoid processes in cotton exposed to high temperature extremes

The cotton crop already experiences yield-limiting high temperatures during the growing season, which are predicted to worsen in a changing climate. This makes the identification of heat tolerant cultivars that can acclimate to high temperature conditions a necessity. The objective of this study was to assess the effects of growth temperature and genotype on plant growth, single-leaf physiology, and thermotolerance of thylakoid processes for cotton exposed to optimal and supra-optimal temperature conditions. A diverse collection of cotton cultivars was selected based on previously documented differences in thylakoid-specific thermotolerance, and they were exposed to optimal (30/20 C) and two supra-optimal growth temperatures (35/25 and 40/30 °C). There was a significant interaction between genotype (G) and growth temperature (T) for all growth measures (leaf area, mainstem node production, and plant dry weight), photosynthetic rates, nighttime respiration and stomatal conductance, and thermotolerance of photosystem II. Greater relative growth (plant dry matter) under high temperature was strongly and positively correlated with leaf area and strongly and negatively correlated with nighttime respiration and stomatal conductance. In contrast, photosynthetic parameters were not significantly associated with genotypic variation in growth-specific thermotolerance. Thermotolerance of the thylakoid reactions was not consistently associated with more thermotolerant growth. We conclude that genotypes with heat tolerant vegetative growth have higher relative leaf area and lower relative values for nocturnal respiration and stomatal conductance under high temperature conditions.

Diurnal Response of Photosystem I to Fluctuating Light Is Affected by Stomatal Conductance.

Upon a sudden transition from low to high light, electrons transported from photosystem II (PSII) to PSI should be rapidly consumed by downstream sinks to avoid the over-reduction of PSI. However, the over-reduction of PSI under fluctuating light might be accelerated if primary metabolism is restricted by low stomatal conductance. To test this hypothesis, we measured the effect of diurnal changes in stomatal conductance on photosynthetic regulation under fluctuating light in tomato (Solanum lycopersicum) and common mulberry (Morus alba). Under conditions of high stomatal conductance, we observed PSI over-reduction within the first 10 s after transition from low to high light. Lower stomatal conductance limited the activity of the Calvin–Benson–Bassham cycle and aggravated PSI over-reduction within 10 s after the light transition. We also observed PSI over-reduction after transition from low to high light for 30 s at the low stomatal conductance typical of the late afternoon, indicating that low stomatal conductance extends the period of PSI over-reduction under fluctuating light. Therefore, diurnal changes in stomatal conductance significantly affect the PSI redox state under fluctuating light. Moreover, our analysis revealed an unexpected inhibition of cyclic electron flow by the severe over-reduction of PSI seen at low stomatal conductance. In conclusion, stomatal conductance can have a large effect on thylakoid reactions under fluctuating light.

Effect of Cotton Leafroll Dwarf Virus on Physiological Processes and Yield of Individual Cotton Plants.

Cotton leafroll dwarf disease (CLRDD) caused by cotton leafroll dwarf virus (CLRDV) is an emerging threat to cotton production in the United States. The disease was first reported in Alabama in 2017 and subsequently has been reported in 10 other cotton producing states in the United States, including Georgia. A field study was conducted at field sites near Tifton, Georgia in 2019 and 2020 to evaluate leaf gas exchange, chlorophyll fluorescence, and leaf temperature responses for a symptomatic cultivar (diseased plants observed at regular frequency) at multiple stages of disease progression and for asymptomatic cultivars (0% disease incidence observed). Disease-induced reductions in net photosynthetic rate (An, decreased by 63–101%), stomatal conductance (gs, decreased by 65–99%), and efficiency of the thylakoid reactions (32–92% decline in primary photochemistry) were observed, whereas leaf temperature significantly increased by 0.5–3.8°C at advanced stages of the disease. Net photosynthesis was substantially more sensitive to disease-induced declines in gs than the thylakoid reactions. Symptomatic plants with more advanced disease stages remained stunted throughout the growing season, and yield was reduced by 99% by CLRDD due to reductions in boll number per plant and declines in boll mass resulting from fewer seeds per boll. Asymptomatic cultivars exhibited more conservative gas exchange responses than apparently healthy plants of the symptomatic cultivar but were less productive. Overall, it is concluded that CLRDV limits stomatal conductance and photosynthetic activity of individual leaves, causing substantial declines in productivity for individual plants. Future studies should evaluate the physiological contributors to genotypic variation in disease tolerance under controlled conditions.

Extreme temperatures affect seedling growth and photosynthetic performance of advanced cotton genotypes

Growth temperature and genotype can influence seedling vigor in cotton (Gossypium hirsutum L.), and identifying genotypes that can perform well under different temperature extremes may broaden the range of temperatures over which optimum growth could be obtained. To this end, a controlled environment study was conducted to evaluate the response of advanced breeding lines to growth temperature and to evaluate the utility of rapid, fluorescence-based measurements (the OJIP test) as indicators of plant growth response to day/night temperature regimes (20/15, 30/20, 35/25, and 40/30 °C). At two weeks after planting, growth analysis, chlorophyll fluorescence measurements, and pigment concentrations were obtained. Significant genotype, temperature, and interaction effects were observed for seedling growth parameters, and some of the key components of the thylakoid reactions. Specifically, the 35/25 °C treatment had the highest values for all growth parameters, with genotypic differences in growth primarily being observed at this regime. Energy trapping by photosystem II (PSII) (φPO), intersystem electron transport (φEO), and photosystem I end electron acceptor reduction (φRO) were significantly affected by temperature, but each component differed in heat sensitivity. All growth parameters were significantly correlated with a number of OJIP parameters including quantum efficiencies and performance indices. However, the strongest positive associations were observed between quantum efficiencies and growth metrics, with φEO exhibiting the strongest correlations with growth. Our observations indicate that rapid OJIP assessments can potentially be used as indicators of early season growth responses to temperature.

Elevated CO2 Concentration Alters Photosynthetic Performances under Fluctuating Light in Arabidopsis thaliana.

In view of the current and expected future rise in atmospheric CO2 concentrations, we examined the effect of elevated CO2 on photoinhibition of photosystem I (PSI) under fluctuating light in Arabidopsis thaliana. At 400 ppm CO2, PSI showed a transient over-reduction within the first 30 s after transition from dark to actinic light. Under the same CO2 conditions, PSI was highly reduced after a transition from low to high light for 20 s. However, such PSI over-reduction greatly decreased when measured in 800 ppm CO2, indicating that elevated atmospheric CO2 facilitates the rapid oxidation of PSI under fluctuating light. Furthermore, after fluctuating light treatment, residual PSI activity was significantly higher in 800 ppm CO2 than in 400 ppm CO2, suggesting that elevated atmospheric CO2 mitigates PSI photoinhibition under fluctuating light. We further demonstrate that elevated CO2 does not affect PSI activity under fluctuating light via changes in non-photochemical quenching or cyclic electron transport, but rather from a rapid electron sink driven by CO2 fixation. Therefore, elevated CO2 mitigates PSI photoinhibition under fluctuating light at the acceptor rather than the donor side. Taken together, these observations indicate that elevated atmospheric CO2 can have large effects on thylakoid reactions under fluctuating light.

Temperature-dependent regulation of electron transport and ATP synthesis in chloroplasts in vitro and in silico.

The significance of temperature-dependent regulation of photosynthetic apparatus (PSA) is determined by the fact that plant temperature changes with environmental temperature. In this work, we present a brief overview of temperature-dependent regulation of photosynthetic processes in class B chloroplasts (thylakoids) and analyze these processes using a computer model that takes into account the key stages of electron and proton transport coupled to ATP synthesis. The rate constants of partial reactions were parametrized on the basis of experimental temperature dependences of partial photosynthetic processes: (1) photosystem II (PSII) turnover and plastoquinone (PQ) reduction, (2) the plastoquinol (PQH2) oxidation by the cytochrome (Cyt) b6f complex, (3) the ATP synthase activity, and (4) the proton leak from the thylakoid lumen. We consider that PQH2 oxidation is the rate-limiting step in the intersystem electron transport. The parametrization of the rate constants of these processes is based on earlier experimental data demonstrating strong correlations between the functional and structural properties of thylakoid membranes that were probed with the lipid-soluble spin labels embedded into the membranes. Within the framework of our model, we could adequately describe a number of experimental temperature dependences of photosynthetic reactions in thylakoids. Computer modeling of electron and proton transport coupled to ATP synthesis supports the notion that PQH2 oxidation by the Cyt b6f complex and proton pumping into the lumen are the basic temperature-dependent processes that determine the overall electron flux from PSII to molecular oxygen and the net ATP synthesis upon variations of temperature. The model describes two branches of the temperature dependence of the post-illumination reduction of [Formula: see text] characterized by different activation energies (about 60 and ≤ 3.5kJmol-1). The model predicts the bell-like temperature dependence of ATP formation, which arises from the balance of several factors: (1) the thermo-induced acceleration of electron transport through the Cyt b6f complex, (2) deactivation of PSII photochemistry at sufficiently high temperatures, and (3) acceleration of the passive proton outflow from the thylakoid lumen bypassing the ATP synthase complex. The model describes the temperature dependence of experimentally measured parameter P/2e, determined as the ratio between the rates of ATP synthesis and pseudocyclic electron transport (H2O → PSII → PSI → O2).

Impact of First True Leaf Photosynthetic Efficiency on Peanut Plant Growth under Different Early-Season Temperature Conditions

ABSTRACT Selecting planting dates with optimal temperatures for plant growth and development is important for successful crop production. Photosynthetic rates of peanut leaves under adverse environmental conditions have been widely studied; however, characterization of photosynthetic efficiency of first true leaves as well as its contribution to plant growth is not well elucidated. The objectives of this research were to assess the influence of first true leaves of peanut cultivars on plant growth under different temperature conditions during early growth and at the onset of flowering and to identify the photosynthetic components more closely linked with photosynthetic efficiency of the first true leaves. Experiments were conducted with April (early), May (optimum), and June (late) planting dates in 2017 and 2018. Cultivars Georgia-06G, Georgia-14N, and TifNV-High O/L were evaluated. Measurements were taken at three and five wks after planting, early season and the onset of flowering, respectively. Rapid development of first true leaves of peanut plants contributed to whole-plant growth in the early season and at the onset of flowering across the temperature conditions. Net photosynthesis of first true leaves was not impacted by temperature conditions in the early season or at the onset of flowering primarily due to greater activity of non-stomatal components associated with the thylakoid reactions. Whole-plant growth was more considerably associated with first true leaf area development than photosynthetic efficiency of those leaves in peanut plants.

Projected day/night temperatures specifically limits rubisco activity and electron transport in diverse rice cultivars

Ambient temperature has a profound effect on agricultural production. Generally, sub- and supra-optimal temperatures have a negative effect on growth/yield, provided the deviation for optimal is large enough. On average, mean global temperatures are projected to increase by 0.3 to 0.6 °C per decade over the next century. Nighttime temperatures (TN) are projected to increase faster (20%–40% by 2100) than day maximum temperatures (TD). Therefore, an experiment was conducted to determine the effects of projected sub- and supra-optimal day/night temperatures on two diverse rice (Oryza sativa) cultivars, IR6 and Rex, during reproductive growth. At flowering, nine different day/night temperature treatments (TTs) were imposed and maintained until harvesting using sunlit growth chambers (SPAR units). Alterations to plant metabolism were evaluated using CO2 response functions, also, to survey gas exchange readings under ambient conditions. Net photosynthesis (AN) declined under both TD and TN effects. The decline in maximum carboxylation rate allowed by Rubisco (Vcmax) and maximal rate of electron transport allowed by the thylakoid reactions (Jmax), with increasing TN mainly contributed to the decline in AN. Dark respiration rates (RD) declined with increasing temperatures could be a cultivars acclimation mechanism to prevent excessive carbon losses. Cultivars varied mainly for primary photochemistry, with cultivar Rex exhibited significantly higher RD, and pigment contents but lower Jmax than IR6 across TTs. The information on genetic, biochemical and physiological processes in these two diverse cultivars under varied temperature regimes could be useful for developing breeding tools to enhance thermotolerance in US rice cultivars for future climatic conditions.

Sub-optimal emergence temperature alters thermotolerance of thylakoid component processes in cotton seedlings

Cotton (Gossypium hirsutum) is often planted under sub-optimal early season temperatures, yet it is unknown whether this increases susceptibility to acute high temperature exposure. To address this, cotton seedlings were grown under optimal (30/20 °C) and sub-optimal (20/15 °C) growth temperature regimes, and comprehensive fluorescence analysis was conducted for cotyledons from both temperature regimes in response to incubation temperatures ranging from 30 to 50 °C. Results indicated that low growth temperature did not alter reaction center density (RC/CS0), but increased antenna size (ratio of antenna chlorophyll to reaction center chlorophyll; ABS/RC) and decreased the amplitude of the I to P phase of the fluorescence transient [ΔVIP; associated with the photosystem (PS) I content]. Photosynthetic performance indices (PItotal and PIABS), quantum yield of energy trapping (φPo), the contribution of the thylakoid reactions to PIABS (Fv/F0), electron transport to intersystem electron acceptors (φE0) and reduction of PSI end electron acceptors (φRo) were all substantially lower in plants grown under sub-optimal temperature than in plants grown at optimal temperature. Furthermore, thermotolerance of all the aforementioned parameters was significantly reduced by low early season growth temperatures. Among different OJIP-derived parameters, PIABS and PItotal were more heat sensitive than other fluorescence-based parameters, and φPo and φRo were the most heat tolerant. In addition, thermotolerance of PItotal (overall photosynthetic thermotolerance) was significantly correlated with thermotolerance of all other OJIP-derived parameters. Regression analysis indicated that heat tolerance trends for φE0 were more consistent with the changes in thermotolerance of PItotal. We conclude that 1) low growth temperature in the seedling phase of cotton results in decreased photosynthetic thermotolerance and 2) overall photosynthetic thermotolerance is primarily constrained by the ability of intersystem electron transport to acclimate to growth environment.

Plastoquinol generates and scavenges reactive oxygen species in organic solvent: Potential relevance for thylakoids

The present work reports reactions of plastoquinol (PQH2-9) and plastoquinone (PQ-9) in organic solvents and summarizes the literature to understand similar reactions in thylakoids. In thylakoids, PQH2-9 is oxidized by the cytochrome b6/f complex (Cyt b6/f) but some PQH2-9 is also oxidized by reactions in which oxygen acts as an electron acceptor and is converted to reactive oxygen species (ROS). Furthermore, PQH2-9 reacts with ROS. Light enhances oxygen-dependent oxidation of PQH2-9. We examined the oxidation of PQH2-9 via dismutation of PQH2-9 and PQ-9 and scavenging of the superoxide anion radical (O2−) and hydrogen peroxide (H2O2) by PQH2-9. Oxidation of PQH2-9 via dismutation to semiquinone was slow and independent of pH in organic solvents and in solvent/buffer systems, suggesting that intramembraneous oxidation of PQH2-9 in darkness mainly proceeds via reactions catalyzed by the plastid terminal oxidase and cytochrome b559. In the light, oxidation of PQH2-9 by singlet oxygen and by O2− formed in PSI contribute significantly. In addition, Cyt b6/f forms H2O2 with a PQH2-9 dependent mechanism. Measurements of the reaction of O2− with PQH2-9 and PQ-9 in acetonitrile showed that O2− oxidizes PQH2-9, forming PQ-9 and several PQ-9-derived products. The rate constant of the reaction between PQH2-9 and O2− was found to be 104 M−1 s−1. H2O2 was found to oxidize PQH2-9 to PQ-9, but failed to oxidize all PQH2-9, suggesting that the oxidation of PQH2-9 by H2O2 proceeds via deprotonation mechanisms producing PQH−-9, PQ2−-9 and the protonated hydrogen peroxide cation, H3O2+.

Integration of energy and electron transfer reactions in plant thylakoids

Integration of energy and electron transfer reactions in plant thylakoids

Calcification and ocean acidification: new insights from the coccolithophore Emiliania huxleyi

Calcification and ocean acidification: new insights from the coccolithophore <i>Emiliania huxleyi</i>

The effects of moderately high temperature on zeaxanthin accumulation and decay

Moderately high temperature reduces photosynthetic capacities of leaves with large effects on thylakoid reactions of photosynthesis, including xanthophyll conversion in the lipid phase of the thylakoid membrane. In previous studies, we have found that leaf temperature of 40°C increased zeaxanthin accumulation in dark-adapted, intact tobacco leaves following a brief illumination, but did not change the amount of zeaxanthin in light-adatped leaves. To investigate heat effects on zeaxanthin accumulation and decay, zeaxanthin level was monitored optically in dark-adapted, intact tobacco and Arabidopsis thaliana leaves at either 23 or 40°C under 45-min illumination. Heated leaves had more zeaxanthin following 3-min light but had less or comparable amounts of zeaxanthin by the end of 45min of illumination. Zeaxanthin accumulated faster at light initiation and decayed faster upon darkening in leaves at 40°C than leaves at 23°C, indicating that heat increased the activities of both violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase (ZE). In addition, our optical measurement demonstrated in vivo that weak light enhances zeaxanthin decay relative to darkness in intact leaves of tobacco and Arabidopsis, confirming previous observations in isolated spinach chloroplasts. However, the maximum rate of decay is similar for weak light and darkness, and we used the maximum rate of decay following darkness as a measure of the rate of ZE during steady-state light. A simulation indicated that high temperature should cause a large shift in the pH dependence of the amount of zeaxanthin in leaves because of differential effects on VDE and ZE. This allows for the reduction in ΔpH caused by heat to be offset by increased VDE activity relative to ZE.

Moderate heat stress reduces the pH component of the transthylakoid proton motive force in light‐adapted, intact tobacco leaves

We measured the DeltaPsi and DeltapH components of the transthylakoid proton motive force (pmf) in light-adapted, intact tobacco leaves in response to moderate heat. The DeltaPsi causes an electrochromic shift (ECS) in carotenoid absorbance spectra. The light-dark difference spectrum has a peak at 518 nm and the two components of the pmf were separated by following the ECS for 25 s after turning the light off. The ECS signal was deconvoluted by subtracting the effects of zeaxanthin formation (peak at 505 nm) and the qE-related absorbance changes (peak at 535 nm) from a signal measured at 520 nm. Heat reduced DeltapH while DeltaPsi slightly increased. Elevated temperature accelerated ECS decay kinetics likely reflecting heat-induced increases in proton conductance and ion movement. Energy-dependent quenching (qE) was reduced by heat. However, the reduction of qE was less than expected given the loss of DeltapH. Zeaxanthin did not increase with heat in light-adapted leaves but it was higher than would be predicted given the reduced DeltapH found at high temperature. The results indicate that moderate heat stress can have very large effects on thylakoid reactions.

Effects of moderate heat stress on photosynthesis: importance of thylakoid reactions, rubisco deactivation, reactive oxygen species, and thermotolerance provided by isoprene

ABSTRACTPhotosynthesis is particularly sensitive to heat stress and recent results provide important new insights into the mechanisms by which moderate heat stress reduces photosynthetic capacity. Perhaps most surprising is that there is little or no damage to photosystem II as a result of moderate heat stress even though moderate heat stress can reduce the photosynthetic rate to near zero. Moderate heat stress can stimulate dark reduction of plastoquinone and cyclic electron flow in the light. In addition, moderate heat stress may increase thylakoid leakiness. At the same time, rubisco deactivates at moderately high temperature. Relationships between effects of moderate heat on rubisco activation and thylakoid reactions are not yet clear. Reactive oxygen species such as H2O2 may also be important during moderate heat stress. Rubisco can make hydrogen peroxide as a result of oxygenase side reactions and H2O2 production by rubisco was recently shown to increase substantially with temperature. The ability to withstand moderately high temperature can be improved by altering thylakoid lipid composition or by supplying isoprene. In my opinion this indicates that thylakoid reactions are important during moderate heat stress. The deactivation of rubisco at moderately high temperature could be a parallel deleterious effect or a regulatory response to limit damage to thylakoid reactions.

Electron transport is the functional limitation of photosynthesis in field‐grown Pima cotton plants at high temperature

ABSTRACTRestrictions to photosynthesis can limit plant growth at high temperature in a variety of ways. In addition to increasing photorespiration, moderately high temperatures (35–42 °C) can cause direct injury to the photosynthetic apparatus. Both carbon metabolism and thylakoid reactions have been suggested as the primary site of injury at these temperatures. In the present study this issue was addressed by first characterizing leaf temperature dynamics in Pima cotton (Gossypium barbadense) grown under irrigation in the US desert south‐west. It was found that cotton leaves repeatedly reached temperatures above 40 °C and could fluctuate as much as 8 or 10 °C in a matter of seconds. Laboratory studies revealed a maximum photosynthetic rate at 30–33 °C that declined by 22% at 45 °C. The majority of the inhibition persisted upon return to 30 °C. The mechanism of this limitation was assessed by measuring the response of photosynthesis to CO2 in the laboratory. The first time a cotton leaf (grown at 30 °C) was exposed to 45 °C, photosynthetic electron transport was stimulated (at high CO2) because of an increased flux through the photorespiratory pathway. However, upon cooling back to 30 °C, photosynthetic electron transport was inhibited and fell substantially below the level measured before the heat treatment. In the field, the response of assimilation (A) to various internal levels of CO2 (Ci) revealed that photosynthesis was limited by ribulose‐1,5‐bisphosphate (RuBP) regeneration at normal levels of CO2 (presumably because of limitations in thylakoid reactions needed to support RuBP regeneration). There was no evidence of a ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) limitation at air levels of CO2 and at no point on any of 30 A–Ci curves measured on leaves at temperatures from 28 to 39 °C was RuBP regeneration capacity measured to be in substantial excess of the capacity of Rubisco to use RuBP. It is therefore concluded that photosynthesis in field‐grown Pima cotton leaves is functionally limited by photosynthetic electron transport and RuBP regeneration capacity, not Rubisco activity.

The role of trace metals in photosynthetic electron transport in O2-evolving organisms

Iron is the quantitatively most important trace metal involved in thylakoid reactions of all oxygenic organisms since linear (= non-cyclic) electron flow from H2O to NADP+ involves PS II (2–3 Fe), cytochrome b6-f (5 Fe), PS I (12 Fe), and ferredoxin (2 Fe); (replaceable by metal-free flavodoxin in certain cyanobacteria and algae under iron deficiency). Cytochrome c6 (1 Fe) is the only redox catalyst linking the cytochrome b6-f complex to PS I in most algae; in many cyanobacteria and Chlorophyta cytochrome c6 and the copper-containing plastocyanin are alternatives, with the availability of iron and copper regulating their relative expression, while higher plants only have plastocyanin. Iron, copper and zinc occur in enzymes that remove active oxygen species and that are in part bound to the thylakoid membrane. These enzymes are ascorbate peroxidase (Fe) and iron-(cyanobacteria, and most al gae) and copper-zinc- (some algae; higher plants) superoxide dismutase. Iron-containing NAD(P)H-PQ oxidoreductase in thylakoids of cyanobacteria and many eukaryotes may be involved in cyclic electron transport around PS I and in chlororespiration. Manganese is second to iron in its quantitative role in the thylakoids, with four Mn (and 1 Ca) per PS II involved in O2 evolution. The roles of the transition metals in redox catalysts can in broad terms be related to their redox chemistry and to their availability to organisms at the time when the pathways evolved. The quantitative roles of these trace metals varies genotypically (e.g. the greater need for iron in thylakoid reactions of cyanobacteria and rhodophytes than in other O2-evolvers as a result of their lower PS II:PS I ratio) and phenotypically (e.g. as a result of variations in PS II:PS I ratio with the spectral quality of incident radiation).

Adaptation to Salinity as Monitored by PSII Oxygen Evolving Reactions in Barley Thylakoids

The effects of NaCl salinity and Abscisic acid pretreatment on the polypeptide composition, photochemical activity and kinetic characteristics of oxygen evolving reactions in isolated thylakoids from barley ( Hordeum vulgare L., var. Alfa ) seedlings were studied. It was shown that thylakoids isolated from plants previously subjected to a prolonged NaCl treatment were more resistant to high concentration of NaCl after in vitro incubation. Pretreatment with ABA 3 days before salinization diminished the inhibitory effect of the NaCl. Both ABA and NaCl treatment lead to an increase in the internal ABA level in the plant leaves. ABA seems to play a specific role in this adaptive process, promoting some structural changes in the chloroplast membranes, which in turn reflect on the oxygen evolving mechanisms. The obtained results are discussed in terms of an effect of salinity and ABA treatment leading to an increased participation of the stroma situated photosystem II centers, possessing higher resistance to damaging factors.

Enhancement of photosystem II photoreactions and high pH stability in thylakoids from cotyledonary leaves of Brassica juncea raised under sodium chloride stress

Enhancement in the rates of photosystem (PS) II‐catalysed O2 evolution was observed with thylakoids isolated from cotyledonary leaves of Brassica juncea seedlings raised in medium containing 200 mM NaCl compared to controls. The nature of this enhancement in PS II activity by NaCl treatment has been investigated. The pH profile showed an optimal PS 11(H2O→FeCN) activity at pH 7.5 in thylakoids of both controls and NaCl‐raised seedlings. However, the thylakoids from NaCl‐raised plants showed significantly higher PS II activity at all the pHs (from 6.0 to 9.0) with maximal difference occurring at alkaline pHs. Besides the enhanced electron transport rates, light‐induced proton uptake was about 30% higher in the thylakoids from NaCl‐raised plants than in the control thylakoids. The 3‐(3,4‐dichlorophenyl)‐1, 1‐dimethylurea (DCMU)‐induced enhancement in the room temperature fluorescence was significantly higher in thylakoids from NaCl‐grown plants compared to controls, suggesting more efficient PS II photochemical reactions in thylakoids from NaCl‐raised seedlings. The analysis of the changes in the fluorescence yield of Chl also showed an enhancement in Fv level in NaCl‐raised plants, suggesting that NaCl treatment stimulates the photochemistry of PS II. Analysis of light saturation kinetics of PS II [H2O→2,6‐dichloro‐p‐benzoquinone (DCBQ) or phenylenediamine (PD)] activity showed that the extent of stimulation by NaCl under rate‐limiting light intensities was greater than at rate‐saturating intensities. This result indicates that NaCl treatment not only alters the electron transport rates but also the relative quantum yield. Incubation of thylakoids at pH 9.0 inhibited FeCN‐supported PS II activity more drastically in control than in NaCl‐treated thylakoids. Recovery from alkaline‐pH‐induced inhibition occurred when the thylakoids were resuspended at pH 7.5. Reversal in PS II activity was significantly higher in the thylakoids of NaCl‐raised plants than in the control thylakoids. These results suggest that the seedlings raised at moderately elevated levels of NaCl have thylakoids with enhanced stability and PS II photochemical functions.