Components Of Photosynthetic Apparatus Research Articles

The origins of photosynthetic systems: Clues from the phosphorus and sulphur chemical scenarios

Photosynthesis is the predominant biochemical process of carbon dioxide assimilation in the biosphere. To reduce carbon dioxide into organic compounds, photosynthetic organisms have one or two distinct photochemical reaction centre complexes with which they capture solar energy and generate ATP and reducing power. The core polypeptides of the photosynthetic reaction centres show low homologies but share overlapping structural folds, overall architecture, similar functional properties and highly conserved positions in protein sequences suggesting a common ancestry. However, the other biochemical components of photosynthetic apparatus appear to be a mosaic resulting from different evolutionary trajectories. The current proposal focusses on the nature and biosynthetic pathways of some organic redox cofactors that participate in the photosynthetic systems: quinones, chlorophyll and heme rings and their attached isoprenoid side chains, as well as on the coupled proton motive forces and associated carbon fixation pathways. This perspective highlights clues about the involvement of the phosphorus and sulphur chemistries that would have shaped the different types of photosynthetic systems.

Photosynthetic apparatus of barley plants treated with 5-aminolevulinic acid: mechanisms of adaptation to drought

A significant effect of soil drought on the morphometric parameters of the leaves of barley plants of the Brovar and Avans varieties, the accumulation of reactive oxygen species, as well as the content of photosynthetic pigments in them has been established. It has been shown that during drought, the treatment of leaves of barley plants of the Brovar variety with 5-aminolevulinic acid causes a decrease in the content of proteins of photosystem antenna complexes, which leads to a decrease in the size of the light-harvesting antenna and is an effective mechanism for protecting the photosynthetic apparatus from oxidative stress. Fine adjustment of the photosynthetic apparatus components of leaves of barley plants ofthe Brovar variety to drought may be a key factor in determining the resistance of this variety to this type of abiotic stress. In the Avans variety, these adaptation mechanisms are either absent or manifest to a lesser extent, which leads to a more intensive development of oxidative stress in plants of this variety under the action of soil drought.

Application of titanium regulates the functional components of photosynthetic apparatus in grafted seedlings of Carya cathayensis Sarg. under shade

Light acts as a key environmental factor for normal growth and development of plants. Carya cathayensis Sarg. (hickory) faces low light conditions, especially those caused by cloudy or rainy days during the rapid growth period, which has caused adverse effects on its growth. In the current investigation, to alleviate the adverse effects of insufficient light on the cultivation of hickory, anti-hydrolyze stabilized ionic titanium (ASIT) was sprayed on the leaves of the three kinds of grafted seedlings and the non-grafted seedlings of hickory grown under different shade conditions. Results showed that the leaf mass per area and chlorophyll content of grafted hickory seedlings were increased after ASIT application. Rubisco content and photosynthetic rate (Pn) of seedlings grown under shading conditions were positively affected by ASIT treatment, especially on the 45th day of treatment, while the interaction effects of the two parameters between ASIT application and different shade treatments were significant. Titanium accumulation was the highest in roots, followed by leaves, and then in stems, while ASIT had the most significant effects on roots and leaves under 50 ± 5% shade. Severe shading inhibited growth and lead to serious destruction of chloroplast ultrastructure. In addition, the role of ASIT was rootstock-dependent, since ASIT had the weakest mitigation effect on the C/H grafted seedlings. To sum up, the application of ASIT to the grafted seedlings of hickory could improve its ability to resist shade stress.

Nitrogen deficiency regulates premature senescence by modulating flag leaf function, ROS homeostasis, and intercellular sugar concentration in rice during grain filling

BackgroundLeaf senescence occurs in an age-dependent manner, but the rate and timing of leaf senescence may be influenced by various biotic and abiotic factors. In the course of stress, the function, composition, and different components of photosynthetic apparatus occur to be synthesized homogeneously or degraded paradoxically due to different senescence-related processes. Nitrogen (N) deficiency is one of the critical environmental factors that induce leaf senescence, and its incidence may curtail leaf photosynthetic function and markedly alter the genetic information of plants that might result in low grain yield. However, the physiological and genetic mechanism underlying N deficiency regulates premature senescence, and flag leaf function, ROS homeostasis, and intercellular sugar concentration in rice during grain filling are not well understood. In this paper, Zhehui7954 an excellent indica restorer line (wildtype) and its corresponding mutant (psf) with the premature senescence of flag leaves were used to study the effect of different N supplies in the alteration of physiological and biochemical components of flag leaf organ and its functions during grain filling. ResultsThe results showed that the psf mutant appeared to be more susceptible to the varying N supply levels than WT. For instance, the psf mutant showed considerably lower Pn, Chl a, Chl b, and Car contents than its WT. N deficiency (LN) decreased leaves photosynthetic activities, N metabolites, but significantly burst O2•−, H2O2, and relative conductivity (R1/R2) concentrations, which was consistent with the expression levels of senescence-associated genes. Sucrose, glucose, and C/N ratio concentrations increased with a decrease in N level, which was closely associated with N and non-structural carbohydrate translocation rates. Increases in POD activity were positively linked with the senescence-related enhancement of ROS generation under LN conditions, whereas, SOD, CAT, and APX activities showed opposite trends. High N (HN) supply significantly inhibits the transcripts of carbohydrate biosynthesis genes, while N assimilation gene transcripts gradually increased along with leaf senescence. The psf mutant had a relatively higher grain yield under HN treatment than LN, while WT had a higher grain yield under MN than HN and LN. ConclusionsThis work revealed that the C/N ratio and ROS undergo a gradual increase driven by interlinking positive feedback, providing a physiological framework connecting the participation of sugars and N assimilation in the regulation of leaf senescence. These results could be useful for achieving a higher yield of rice production by appropriate N supply and plant senescence regulation.

Field Study of the Effects of Two Different Environmental Conditions on Wheat Productivity and Chlorophyll Fluorescence Induction (OJIP) Parameters

Wheat is one of the main cereal crops for ensuring food supply. Thus, increasing grain yield is a major target for plant breeders, where insights into wheat productivity can be gained by studying the activity of the components of photosynthetic apparatus. The objectives of this study were to evaluate the agronomical performance of three winter wheat varieties and test photosynthetic efficiency over two different locations. Chlorophyll fluorescence was used to evaluate the maximum quantum yield of photosystem II (PSII) (TR0/ABS) and performance index on absorption basis (PIabs) of flag leaves and glumes of heads at the flowering stage until the mid-senescence stage. The grain yield of all varieties on average was significantly higher at Osijek compared to Tovarnik. Variety Tika Taka exhibited the highest yield reduction (27.1%) at Tovarnik compared to Osijek, followed by El Nino (20.5%) and Vulkan (18.7%), respectively. A higher amount of precipitation in June at Tovarnik provoked higher Fusarium head blight disease intensity, which could be seen as the bleaching of plant heads at the plots and resulted in an earlier decrease in photosynthetic activity. Therefore, earlier senescence and contracted grain fill duration could occur.

Photosynthetic performance and photosynthesis-related gene expression coordinated in a shade-tolerant species Panax notoginseng under nitrogen regimes

BackgroundNitrogen (N) is an essential component of photosynthetic apparatus. However, the mechanism that photosynthetic capacity is suppressed by N is not completely understood. Photosynthetic capacity and photosynthesis-related genes were comparatively analyzed in a shade-tolerant species Panax notoginseng grown under the levels of low N (LN), moderate N (MN) and high N (HN).ResultsPhotosynthetic assimilation was significantly suppressed in the LN- and HN-grown plants. Compared with the MN-grown plants, the HN-grown plants showed thicker anatomic structure and larger chloroplast accompanied with decreased ratio of mesophyll conductance (gm) to Rubisco content (gm/Rubisco) and lower Rubisco activity. Meanwhile, LN-grown plants displayed smaller chloroplast and accordingly lower internal conductance (gi). LN- and HN-grown individuals allocated less N to light-harvesting system (NL) and carboxylation system (NC), respectively. N surplus negatively affected the expression of genes in Car biosynthesis (GGPS, DXR, PSY, IPI and DXS). The LN individuals outperformed others with respect to non-photochemical quenching. The expression of genes (FBA, PGK, RAF2, GAPC, CAB, PsbA and PsbH) encoding enzymes of Calvin cycle and structural protein of light reaction were obviously repressed in the LN individuals, accompanying with a reduction in Rubisco content and activity. Correspondingly, the expression of genes encoding RAF2, RPI4, CAB and PetE were repressed in the HN-grown plants.ConclusionsLN-induced depression of photosynthetic capacity might be caused by the deceleration on Calvin cycle and light reaction of photosynthesis, and HN-induced depression of ones might derive from an increase in the form of inactivated Rubisco.

Examining effects of salt stress on leaf photosynthesis of cotton based on the FvCB model

To understand the responsive mechanism of leaf photosynthesis of cotton to salinity stress, we investigated the effects of salt stress on leaf photosynthetic characteristics of cotton seedlings with the FvCB model under five levels of salt concentration, i.e., 0 (CK), 50, 100, 150 and 200 mmol·L-1. Results showed that, compared with CK, the salt concentrations of 50 and 100 mmol·L-1 increased the maximum carboxylation rate (Vc max) and the maximum electron transport rate (Jmax), while the salt concentrations of 150 and 200 mmol·L-1 significantly decreased Vc max and Jmax. The net photosynthetic rate (Pn), mesophyll conductance (gm) and dark respiration rate (Rd) gradually decreased with the increases of salt concentration. Compared with CK, the salt concentrations of 50 and 100 mmol·L-1 did not affect gm, but significantly decreased Pn and Rd. The salt concentrations of 150 and 200 mmol·L-1 significantly decreased Pn, gm and Rd, which were significantly different from the salt concentrations of 0, 50 and 100 mmol·L-1. Pn of cotton seedlings under different salt concentrations was simulated by the FvCB model. Compared with the results from the FvCB model without considering gm, the FvCB model with gm improved the determination coefficient between the simulated and measured values and decreased the mean absolute error. The salinity threshold of cotton seedlings ranged between 100 and 150 mmol·L-1. With the increases of salt concentration, the limiting factor of leaf photosynthesis changed from mesophyll conductance to impaired components of photosynthetic apparatus. The FvCB model combined gm could improve the accuracy of photosynthesis simulation.

Comparative photosynthetic responses of Norway spruce and Scots pine seedlings to prolonged water deficiency

Stressors of different natures, including drought stress, substantially compromise the ability of plants to effectively and safely utilize light energy. We investigated the influence of water stress on the photosynthetic processes in Picea abies and Pinus sylvestris, two species with contrasting drought sensitivities. Spruce and pine seedlings were exposed to polyethylene glycol 6000-induced water deficits of different intensities and durations. The maintenance of photosystem I (PSI) oxidation in spruce required increased photosynthetic control and led to the increased reduction of the plastoquinone pool, which was not the case in pine seedlings. As a result of increased excitation pressure, photosystem II (PSII) inactivation was observed in spruce plants, whereas in pine, the decreased PSII photochemistry was likely due to sustained non-photochemical quenching. Downregulation of PSII photochemistry and maintenance of PSI in an oxidized state were linked with the prevention of oxidative stress, even under severe water deficit. The decreased photosynthetic pigment content and photosynthetic gene expression suggested the coordinated downregulation of photosynthetic apparatus components under water stress to reduce light energy absorption. In summary, the observed adaptative mechanisms of pine and spruce to water stress may be similar to the well-studied adaptative mechanisms to winter stress, which may indicate the universality of protective mechanisms under various stresses in conifers.

Water-stress induced downsizing of light-harvesting antenna complex protects developing rice seedlings from photo-oxidative damage

The impact of water-stress on chloroplast development was studied by applying polyethylene glycol 6000 to the roots of 5-day-old etiolated rice (Oryza sativa) seedlings that were subsequently illuminated up to 72 h. Chloroplast development in drought environment led to down-regulation of light-harvesting Chl-proteins. Photosynthetic proteins of Photosystem II (PSII) and oxygen evolving complex i.e., Cytb559, OEC16, OEC23 and OEC33 as well as those of PSI such as PSI-III, PSI-V, and PSI-VI, decreased in abundance. Consequently, due to reduced light absorption by antennae, the electron transport rates of PSII and PSI decreased by 55% and 25% respectively. Further, seedling development in stress condition led to a decline in the ratio of variable (Fv) to maximum (Fm) Chl a fluorescence, as well in the quantum yield of PSII photochemistry. Addition of Mg2+ to the thylakoid membranes suggested that Mg2+-induced grana stacking was not affected by water deficit. Proteomic analysis revealed the down-regulation of proteins involved in electron transport and in carbon reduction reactions, and up-regulation of antioxidative enzymes. Our results demonstrate that developing seedlings under water deficit could downsize their light-harvesting capacity and components of photosynthetic apparatus to prevent photo-oxidative stress, excess ROS generation and membrane lipid peroxidation.

Physiological and biochemical characterization of sheepgrass (Leymus chinensis) reveals insights into photosynthetic apparatus coping with low-phosphate stress conditions

Sheepgrass [Leymus chinensis (Trin.) Tzvel] is a valuble forage plant highly significant to regional grassland productivity of Euro-Asia steppes. Although effects of environmental stress including drought have been studied, impact of nutrient deficiency in particular phosphate (Pi), one of the essential macronutrient, is not reported. Here, we investigated low-Pi effect on its photosynthetic apparatus via physiological and biochemical analysis. We show that PSII activity was significantly reduced based on chlorophyll fluorescence measurements. We observed decreased amount of core proteins of PSII by immunoblot analysis. Further analysis of thylakoid membranes using 2D-BN/SDS-PAGE and immunoblot detection demonstrated that the amount of PSII complexes was closely correlated with the Pi levels within the range. Together with reduced number of thylakoid grana stackings observed, we suggest that the maintenance of PSII is impaired under Pi-limited condition. Moreover, enzyme activity assays revealed that the activity of several ROS scavenger enzymes was stimulated by low-Pi treatment. Based on these experimental results we conclude that PSII is the component of photosynthetic apparatus most sensitive to Pi supply and the enhanced anti-ROS activity is mainly subjective to protection of PSII against low Pi-induced photo-oxidative stress in the organism.

Regulation of Chloroplast Protein Import by the Ubiquitin E3 Ligase SP1 Is Important for Stress Tolerance in Plants

SummaryChloroplasts are the organelles responsible for photosynthesis in plants [1, 2]. The chloroplast proteome comprises ∼3,000 different proteins, including components of the photosynthetic apparatus, which are highly abundant. Most chloroplast proteins are nucleus-encoded and imported following synthesis in the cytosol. Such import is mediated by multiprotein complexes in the envelope membranes that surround each organelle [3, 4]. The translocon at the outer envelope membrane of chloroplasts (TOC) mediates client protein recognition and early stages of import. The TOC apparatus is regulated by the ubiquitin-proteasome system (UPS) in a process controlled by the envelope-localized ubiquitin E3 ligase SUPPRESSOR OF PPI1 LOCUS1 (SP1) [5, 6]. Previous work showed that SP1-mediated regulation of chloroplast protein import contributes to the organellar proteome changes that occur during plant development (e.g., during de-etiolation). Here, we reveal a critical role for SP1 in plant responses to abiotic stress, which is a major and increasing cause of agricultural yield losses globally [7]. Arabidopsis plants lacking SP1 are hypersensitive to salt, osmotic, and oxidative stresses, whereas plants overexpressing SP1 are considerably more stress tolerant than wild-type. We present evidence that SP1 acts to deplete the TOC apparatus under stress conditions to limit the import of photosynthetic apparatus components, which may attenuate photosynthetic activity and reduce the potential for reactive oxygen species production and photo-oxidative damage. Our results indicate that chloroplast protein import is responsive to environmental cues, enabling dynamic regulation of the organellar proteome, and suggest new approaches for improving stress tolerance in crops.

Proteomic characterization of the Rph15 barley resistance gene-mediated defence responses to leaf rust

BackgroundLeaf rust, caused by the biotrophic fungal pathogen Puccinia hordei, is one of the most important foliar disease of barley (Hordeum vulgare) and represents a serious threat in many production regions of the world. The leaf rust resistance gene Rph15 is of outstanding interest for resistance breeding because it confers resistance to over 350 Puccinia hordei isolates collected from around the world. Molecular and biochemical mechanisms responsible for the Rph15 effectiveness are currently not investigated. The aim of the present work was to study the Rph15-based defence responses using a proteomic approach.ResultsProtein pattern changes in response to the leaf rust pathogen infection were investigated in two barley near isogenic lines (NILs), Bowman (leaf rust susceptible) and Bowman-Rph15 (leaf rust resistant), differing for the introgression of the leaf rust resistance gene Rph15. Two infection time points, 24 hours and four days post inoculation (dpi), were analysed. No statistically significant differences were identified at the early time point, while at 4 dpi eighteen protein spots were significantly up or down regulated with a fold-change equal or higher than two in response to pathogen infection. Almost all the pathogen-responsive proteins were identified in the Bowman-Rph15 resistant NIL. Protein spots were characterized by LC-MS/MS analysis and found to be involved in photosynthesis and energy metabolism, carbohydrate metabolism, protein degradation and defence. Proteomic data were complemented by transcriptional analysis of the respective genes. The identified proteins can be related to modulation of the photosynthetic apparatus components, re-direction of the metabolism to sustain defence responses and deployment of defence proteins.ConclusionsThe identification of leaf rust infection-modulated defence responses restricted to the resistant NIL support the hypothesis that basal defence responses of Bowman, but not the Rph15 resistance gene-based ones, are suppressed or delayed by pathogen effectors to levels below the detection power of the adopted proteomic approach. Additionally, Rph15-mediated resistance processes identified mainly resides on a modulation of primary metabolism, affecting photosyntesis and carbohydrate pool.

Responses of chlorophyll b-less chlorina 3613 mutant of barley to a prolonged decrease in illuminance: 2. Dynamics of carotenoids in leaf chloroplasts

Barley (Hordeum vulgare L.) mutant chlorina 3613 is notable for a lack of chlorophyll b (Chl b), low content of chlorophyll a (Chl a) and carotenoids in the chloroplasts, as well as reduction in the majority of components of LHCI and LHCII. Incompletely developed photosynthetic machinery of chlorina 3613 results in suppressed growth, lower biomass, and the declined rate of photosynthesis (as compared with the wild-type cv. Donaria). The lack of Chl b and greater part of peripheral antenna suggests that this mutant will have difficulties during acclimation to long-term shading because the light-harvesting role of Chl b-containing antenna becomes more important under the shortage of light. Earlier, our experiments with the mature chlorina 3613 plants shaded for one week at PAR photon flux density of 60 and 40% of that in full sunlight showed a stimulating effect of shading on growth, biomass accumulation, and Chl a synthesis in chlorina 3613 when biosynthesis of Chl b did not occur [1]. In this work, we investigated in more detail the changes in the content of carotenoids in chlorina 3613. We found that in Donaria at both investigated levels of illumination (60 and 40% of full sunlight) and in chlorina 3613 at 60% illumination, moderate reversible changes typical of shade-enduring plants occur in the content of carotenoids. In chlorina 3613 at 40% illumination, the content of β-carotene increased considerably (by 3 times) with simultaneous accumulation of Chl a. When full illumination was restored, the content of β-carotene decreased and remained on the level, which exceeded its initial content in the plants without shading by 38%; this level, was maintained by the end of vegetation. The changes in the contents of β-carotene and Chl a in chlorina 3613 were not accompanied by any accumulation of xanthophylls or changes in the relative content of active violaxanthin. The obtained results suggest that a long-term shading of the leaves of mature chlorina 3613 plants induced the formation of certain components of photosynthetic apparatus: reactive centers and core parts of photosystems’ antennae as well as proteins CP26 and CP29 and in this way contributed to partial restoration of photosynthetic activity and production process in the mutant lacking Chl b.

Involvement of Reactive Oxygen Radicals in Photoinhibition of Primary Photosynthetic Reactions—Effect of Temperature and Oxygen Radical Scavengers

ABSTRACTExposure of leaves or chloroplasts to high light intensity leads to inactivation of photosynthesis. Two processes are observed—inhibition of photochemical activity of both photosystems and photobleaching of pigments. Oxygen evolving complex, located at the oxidizing side of Photosystem II, is the most sensitive component of photosynthetic apparatus to environmental stress factors. In the present work the effect of high light treatment at room and low temperatures on kinetic parameters of flash oxygen yields and oxygen evolution were studied. Isolated thylakoid membranes were subjected to high light illumination for different periods of time at room (22°C) and low (4°C) temperature. Flash oxygen yields were determined using fast oxygen rate electrode. Photochemical activity of photosystem II was measured by Clark oxygen electrode using artificial electron acceptor. Data presented show that the damaging effect of high light treatment on oxygen evolution is lower at 4°C than at 22°C. When high light treatment was carried out in the presence of histidine and DMSO—scavengers of oxygen radicals, the inhibition process was retarded. Data are discussed in terms of different production rate and mobility of oxygen radicals at room and low temperature.

Heterologous synthesis and assembly of functional LHII antenna complexes from Rhodovulum sulfidophilum in Rhodobacter sphaeroides mutant

The light harvesting complexes, including LHII and LHI, are the important components of photosynthetic apparatus. Rhodovulum (Rdv.) sulfidophilum and Rhodobacter (R.) sphaeroides belong to two genera of photosynthetic bacteria, and they are very different in some physiological characteristics and light harvesting complexes structure. The LHII structural genes (pucBsAs) from Rdv. sulfidophilum and the LHI structural genes (pufBA) from R. sphaeroides were amplified, and cloned into an expression vector controlled by puc promoter from R. sphaeroides, which was then introduced into LHI and LHII-minus R. sphaeroides mutants; the transconjugant strains synthesized heterologous LHII and native LHI complexes, which played normal roles in R. sphaeroides. The Rdv. sulfidophilum LHII complex from pucBsAs had near-infrared absorption bands at ~801-853 nm in R. sphaeroides, and was able to transfer energy efficiently to the native LHI complex. The results show that the pucBsAs genes from Rdv. sulfidophilum could be expressed in R. sphaeroides, and the functional foreign LHII and native LHI were assembled into the membrane of R. sphaeroides.

Physicochemical examinations of the mechanisms and regulation of photosynthesis in higher plants: I. Thermoluminescence in examination of photosynthesis

By the end of the XX century, much progress was achieved in examination of photosynthesis. The chemical composition and spatial organization of the photosynthetic apparatus components were elucidated. The sequence of photophysical processes and biochemical reactions, from quantum absorption to carbohydrate formation, was described. The physical mechanisms of energy migration, charge separation, and electron transfer in the photosynthesizing systems were qualitatively identified.

Watching the photosynthetic apparatus in native membranes.

Over the last 9 years, the structures of the various components of the bacterial photosynthetic apparatus or their homologues have been determined by x-ray crystallography to at least 4.8-A resolution. Despite this wealth of structural information on the individual proteins, there remains an urgent need to examine the architecture of the photosynthetic apparatus in intact photosynthetic membranes. Information on the arrangement of the different complexes in a native system will help us to understand the processes that ensure the remarkably high quantum efficiency of the system. In this work we report images obtained with an atomic force microscope of native photosynthetic membranes from the bacterium Rhodospirillum photometricum. Several proteins can be seen and identified at molecular resolution, allowing the analysis and modeling of the lateral organization of multiple components of the photosynthetic apparatus within a native membrane. Analysis of the distribution of the complexes shows that their arrangement is far from random, with significant clustering both of antenna complexes and core complexes. The functional significance of the observed distribution is discussed.

Time and spectral resolved phosphorescence of singlet oxygen and pigments in photosystem II particles

Singlet oxygen generated via chlorophyll triplets in photosynthetic systems can destroy basic components of photosynthetic apparatus. In order to study this process a sensitive spectroscopic experimental set-up was built to detect both weak infrared (IR) chlorophyll phosphorescence (between 900 and 1000nm) and singlet oxygen phosphorescence (at 1270nm). The spectral and time resolution of the used monochromator and photon counting multiscaler are sufficient to provide two-dimensional matrix of temporally and spectrally resolved IR luminescence data. We have studied IR emission of photosystem II particles in D2O and H2O buffer including the effect of the sodium dodecyl sulphate (SDS) detergent. Both sub-microsecond lifetime of chlorophyll triplet states and microsecond lifetime of singlet oxygen were determined. The singlet oxygen lifetime in D2O (of 18μs) was approximately five times longer than that in H2O. Quantum efficiency of singlet oxygen generation in photosystem II particles was significantly lower than that in chlorophyll a in acetone, however, it could be substantially increased by addition of SDS.

Photosystem 2 Efficiency and Thylakoid Protein Pattern in DCMU-Treated Wheat Seedlings during Senescence

Changes in various components of photosynthetic apparatus during the 6-d dark incubation at 25 °C of detached control and DCMU-treated Triticum aestivum L. leaves were examined. The rate of photosystem 2 (PS2) activity was decreased with increase of the time of dark incubation in control leaves. In contrast to this, DCMU-treated leaves demonstrated high stability by slowing down the inactivation processes. Diphenyl carbazide and NH2OH restored the PS2 activity more in control leaves than in DCMU-treated leaves. Mn2+ failed to restore the PS2 activity in both control and DCMU-treated samples. Similar results were obtained when Fv/Fm was evaluated by chlorophyll fluorescence measurements. The marked loss of PS2 activity in dark incubated control leaves was primarily due to the loss of D1, 33, and 23 kDa extrinsic polypeptides and 28-25 kDa LHCP2 polypeptides.

THE ROLE OF NITROGEN NUTRITION IN HIGH‐TEMPERATURE TOLERANCE OF THE KELP, LAMINARIA SACCHARINA (CHROMOPHYTA)1

ABSTRACTMechanisms of high‐temperature tolerance in the kelp Laminaria saccharina (L.) Lamour. were examined by comparing a heat‐tolerant ecotype from Long Island Sound (LIS), New York, and a population from the Atlantic (ATL) coast of Maine. Greater heat tolerance was not attributable to greater thermal stability of the photosynthetic apparatus: LIS and ATL plants exhibited similar short‐term effects of high temperature on photosynthetic capacity (Pmax) and quantum yield (estimated as the ratio of variable to maximum chlorophyll fluorescence, Fv/Fm. As LIS plants had consistently higher N and protein content than ATL plants, the interaction between nitrogen nutrition and high‐temperature tolerance was examined. When grown under high N supply and optimal temperature (12° C), LIS plants had a higher density of photosystem II reaction centers (RCII), higher activity of two Calvin cycle enzymes (ribulose bisphosphate carboxylase oxygenase [RUBISCO] and NADP‐dependent glyceraldehyde‐3‐phosphate dehydrogenase [G3PDH]), and higher Pmax and Fv/Fm than ATL plants. Individual ATL plants, furthermore, exhibited close correlations of RCII density and enzyme activity with N and/or protein content. Variation in RCII density and enzyme activity, in turn, largely accounted for plant‐to‐plant differences in Pmax and Fv/Fm. Relationships among these parameters were generally weak or lacking among individual LIS plants grown under optimal conditions, apparently because luxury N consumption resulted in excess reserves of photosynthetic apparatus components. Exposure of N‐replete LIS and ATL plants to a superoptimal temperature (22° C) for 4 days caused an increase in the minimum turnover time of the photosynthetic apparatus (tau) and a decrease in Pmax, but had no consistent effect on Fv/Fm RCII density, PSU size (chlorophyll a/RCII), or enzyme activities. When plants were subjected to concurrent N limitation and heat stress, however, LIS and ATL populations exhibited quite different responses. All photosynthetic parameters of N‐limited ATL plants declined sharply in response to high temperature, resulting in a negative rate of daily net C fixation. In contrast, LIS plants showed a reduction in PSU size, but maintained other parameters, including daily C fixation, at levels similar to those of N‐limited plants at optimal temperature. Overall, the ability of LIS plants to accumulate and maintain high N reserves appears to be critical for heat tolerance and, therefore, for survival during summer periods of simultaneous low N supply and superoptimal temperature. ATL plants, which also experience low summer N supply but not superoptimal temperatures, do not accumulate large reserves of nitrogenous components and are unable to tolerate the combined stress. Because low N supply often co‐occurs with high temperatures in temperate marine systems, large‐scale declines in algal productivity, such as during El Niño events, are probably due to the interactive effect of N limitation and heat stress.