Response Of Photosynthetic Apparatus Research Articles

Stress-induced delay of the I-P rise of the fast chlorophyll a fluorescence transient in tomato

Fast chlorophyll a fluorescence induction curve (OJIP) provides a powerful tool for rapidly assessing the photosynthetic performance of plants. However, the application of this technology in tomato plants is scarce. Here, OJIP curve was used to evaluate the photosynthetic performance of tomato plants under chilling and drought stress, and interpretation of the obtained OJIP curves was improved. The results showed that the performance index (PI-abs) and the amplitude of I-P phase of greenhouse-grown tomato plants decreased rapidly with the decline of ambient temperatures and fluctuated with the changes in temperature during the autumn-winter transition. However, photosystem II efficacy (Fv/Fm) showed slow response. In the chambers, net photosynthetic rate (PN) and photochemical efficiency were inhibited by short-term chilling stress. The amplitudes of O-J phase and I-P phase were slightly affected by chilling stress. Interestingly, the appearance of I points of OJIP curves was delayed by chilling stress. Consequently, the corrected amplitudes of I-P phase were found to be significantly inhibited by chilling. Moreover, the corrected amplitudes of I-P phase correlated well with PN and PI-abs, during chilling and subsequent recovery. Mutation of PGR5, which is involved in cyclic electron flow, resulted in declines of photosynthetic parameters including the corrected amplitudes of I-P phase under control and chilling conditions. Under drought stress, the corrected amplitudes of I-P phase and operational photochemical efficiency [Y(II)] also progressively declined in parallel. However, Fv/Fm remained stable even when plants showed a wilting phenotype. Our results indicate that the delay of I point by stress should be considered when using the amplitude of I-P phase to reflect the status of photosynthetic apparatus in response to stress.

Orange protein (DbOR) from the salt-tolerant green alga Dunaliella bardawil mediates photosynthesis against heat stress via interacting with DbPsbP1

In terrestrial plants, Orange (OR) protein can modulate carotenoid homeostasis, photosynthesis, and improve tolerance to environmental stress. However, the related regulatory mechanisms of OR protein are rarely reported in green algae. Until now, research on photosynthetic regulation mechanism is largely unknown in the halophilic green alga Dunaliella bardawil. In this study, one Orange gene (DbOR, encoding a DnaJ-like protein) and two PsbP genes (DbPsbP1/2, encoding oxygen-evolving enhancer protein 2 of photosystem II) were first isolated and characterized from D. bardawil. DbOR shared a highly conserved DnaJ cysteine-rich domain and two transmembrane domains. DbOR and DbPsbP1 had multiple subcellular localizations including nucleus, cytoplasm, and cell membrane, and they could interact with each other widespread in the cell including in the chloroplast. After short time of heat treatment (32 °C, 37 °C, and 40 °C for 4 h), the maximum quantum efficiency, electron transport rate, photosynthetic efficiency of D. bardawil cells could be quickly recovered in 1 d, although the biomass was decreased in some way at high temperature. We believe that the up-regulation of DbOR and DbPsbP1/2 genes under 37 °C or/and 40 °C for 4 h and the interaction of DbOR and DbPsbP1 could enhance the stability of DbPsbP1, thus protecting the photosynthetic apparatus in response to heat stress and contribute to the increase of electron transport rate and photosynthetic efficiency of D. bardawil during the recovery from heat stress. Our study enhances the understanding of the function and mechanisms of algal OR protein in regulating photosynthesis and stress tolerance in D. bardawil and provides a gene source for the cultivation of algae or plants with high photosynthesis and stress resistance.

Proteomics and photosynthetic apparatus response to vermicompost attenuation of salinity stress Vicia faba leaves

Crop production and growth are severely affected by salt stress. Nevertheless, the bio-fertilizer vermicompost (VC) can be participated as a potent inhibitor of salinity on plant growth and crop production by regulating photosynthetic efficiency. We investigated the effect of VC on photosynthetic performance of salt-stressed broad bean (Vicia faba L. Aspani cultivar). Seeds were grown in soil mixture; clay and sand in ratio 1:2 by volume with five different volumetric ratios of VC; 0, 2.5, 5, 10 and 15% irrigated with either water and/or 200 mM NaCl. Leaf area, Na and K contents, chlorophylls, photosystem II efficiency, Rubisco content, soluble sugars, chloroplasts’ organization and proteomics were analyzed. The imposed stress decrease leaf area, chlorophyll contents, maximum quantum efficiency (Fv/Fm), Rubisco content, increase soluble sugars and damage chloroplasts organization. Salinity upregulated glucose-1-phosphate adenylyl transferase, ribulose bisphosphate carboxylase large subunit and chloroplastic peptidyl-prolyl cis–trans isomerase. The increased leaf area, chlorophyll a, b and carotenoids, maximum quantum efficiency of photosystem II, Rubisco content, improving the degeneration of thylakoid lamellae and lessening plastoglobuli number in thylakoid membranes are the major benefits attained with vermicompost treatments under salt stress.Analysis of proteomic revealed that VC upregulated chloroplastic ferredoxin–NADP reductase, plastocyanin, polyphenol oxidase, peptidyl-prolyl cis–trans isomerase, alpha-glucan phosphorylase H isozyme and maturase expression under salt stress. The results suggest that VC controls protein expression at the level of transcriptional and translational which may conserve photosynthetic components and prevent salt-induced harmful effects in broad bean plants.

Photosynthetic electron transport rate and root dynamics of finger millet in response to Trichoderma harzianum

ABSTRACT Finger millet (ragi) is the main food grain for many people, especially in the arid and semiarid regions of developing countries in Asia and Africa. The grains contain an exceptionally higher amount of Ca (>300 mg/100 g) when compared to other major cereals. For sustainable production of ragi in the current scenario of climate change, this study aimed to evaluate the impact of Trichoderma harzianum (TRI) on ragi performance. The performance of photosynthetic pigment pool, photosynthetic apparatus, and root dynamics of three varieties of ragi (PRM-1, PRM-701, and PRM-801) in response to four treatments viz., C (soil), S+ TRI (soil + Trichoderma), farmyard manure (soil+ FYM), and FYM+TRI (Soil + FYM + Trichoderma) were studied. Results have shown a significant increase in the photosynthetic pigment pool and optimized functional and structural integrity of the photosynthetic apparatus in response to the combination of farmyard manure (FYM) with TRI. Higher yield parameters viz., φ(Po) and φ(Eo), δ(Ro), efficiency ψ(Eo), performance indices – PIabs and PItotal, and enhanced root canopy and biomass were observed in all three varieties. Improved electron transport from PSII to PSI, root canopy and biomass, may also suitably favor biological carbon sequestration to retain soil health and plant productivity in case grown in association with FYM and TRI.

Synergistic Effect of Plant-Growth-Promoting Rhizobacteria Improves Strawberry Growth and Flowering with Soil Salinization and Increased Atmospheric CO2 Levels and Temperature Conditions

Biofertilization with plant-growth-promoting rhizobacteria (PGPR) can positively affect the growth and health of host plants and reinforce their tolerance of stressors. Here, we investigate the use of isolated PGPR consortia from halophytes to improve strawberry growth and flowering performance under saline and elevated CO2 and temperature conditions. Growth, flower bud production, and the photosynthetic apparatus response were determined in strawberry plants grown at 0 and 85 mmol L−1 NaCl and in two atmospheric CO2-temperature combinations (400/700 ppm and 25/+4 °C, respectively). Biofertilization improved strawberry plant growth and flower bud production, independently of salinity conditions, at ambient CO2 and 25 °C, while bacterial inoculation only had a positive effect on plant growth in the presence of salt in high CO2 and at +4 °C. Biofertilizers 1 and 3 generated the largest biomass of strawberries at 400 ppm CO2 and 0 and 85 mmol L−1 NaCl, respectively, while biofertilizer 1 did so in the presence of salt and in an atmosphere enriched with CO2 and at +4 °C. The effect of the consortia was mediated by bacterial strain PGP properties, rather than by an improvement in the photosynthetic rate of the plants. Furthermore, biofertilizers 1 and 2 increased the number of flower buds in the absence of salt, while biofertilizers 3 and 4 did so for salt-inoculated plants at 400 ppm CO2 and at 25 °C. There was no effect of inoculation on flower bud production of plants grown at high CO2 and at +4 °C. Finally, we concluded that the effect of bacterial inoculation on strawberry growth and flowering depended on the type of bacterial strain and growth conditions. This highlights the importance of developing studies considering stress interaction to assess the real potential of biofertilizers.

Comparative Study between the Photosynthetic Parameters of Two Avocado (Persea americana) Cultivars Reveals Natural Variation in Light Reactions in Response to Frost Stress

Avocado is a commercially important fruit tree which is sold worldwide. Originating in subtropical regions of the South America, this species is now grown worldwide and is sometimes exposed to cold temperatures. Specifically, frost stress harms the crop yield and its quality. While it is known in general that the photosynthetic apparatus changes in response to cold conditions, there is still not much information regarding the photosynthetic apparatus response to sporadic frost stress. In this study, we tracked the photosynthetic apparatus’ light reaction of ‘Hass’ and ‘Ettinger’ avocado cultivars to frost stress, with Ettinger being known to be more resilient to cold than Hass. We found that in avocado trees, the photosynthetic apparatus’ response to frost occurs at the level of photosystem II (PSII) itself, rather than a photoprotective response to a stress. The Hass apparatus incorrectly interprets the reduction in electron transport rate activity and by that increases its light harvesting complex size at the expense of its reaction centers which then increases the apparatus’ probability to generate reactive oxygen species. The results of this study open opportunities to further research the process which regulates the feedback mechanism that controls the photosynthetic unit’s size in Hass when compared to the Ettinger cultivar, and whether it is part of a feedback regulation from the carbon assimilation step or indirectly from a stomatal limitation which arises in these subtropical species. While corroborating past studies performed on avocados, this study suggests using advanced chlorophyll a fluorescence protocols when researching natural variation in crops.

Consortia of Plant-Growth-Promoting Rhizobacteria Isolated from Halophytes Improve the Response of Swiss Chard to Soil Salinization

Inadequate fertilization or the indiscriminate use of water with high salt concentrations have led to salinization of agricultural soils. In this context, biofertilization with plant-growth-promoting rhizobacteria (PGPR) is an environmentally benign strategy to stimulate plant growth, even under salt stress. Thus, we studied the use of isolated PGPR consortia from halophytes to enhance Swiss chard growth under saline conditions. Growth, photosynthetic apparatus response, nutrient status, pigment concentrations, and secondary metabolites with antioxidant activity were determined in Swiss chard plants grown at 0 and 85 mmol L−1 NaCl. In general, inoculation of plants with PGPR has been shown to be an effective strategy to stimulate the growth of Swiss chard and improve its tolerance to salt stress. Inoculated plants watered with 85 mmol L−1 NaCl showed higher values of leaf dry weight than control plants. Furthermore, PGPR inoculation reduced electrolyte leakage and Na+ uptake and improved chlorophyll a fluorescence parameters, chlorophyll and carotenoid concentrations, stomatal conductance, and antioxidant capacity of Swiss chard. Finally, our findings highlight the potential of isolated PGPR from halophytes to counterbalance the deleterious effect of salinity and stimulate crop growth.

Eco‐physiological responses of Calamagrostis epigejos L (Roth) and Solidago gigantea Aition to complex environmental stresses in coal‐mine spoil heaps

AbstractWe present a study of short and long‐term responses of photosynthetic apparatus and anti‐oxidant capacity to complex abiotic stresses of Calamagrostis epigejos and Solidago gigantea commonly occurring in seminatural habitats and novel ecosystems of coal‐mine spoil heaps. Drought/salinity and elevated temperature were dominant abiotic stressors triggering both species‐ and habitat‐specific responses, confirmed by ChlF induction curves analysis. Although in both species a decrease in net photosynthetic rate (A) and transpiration rate (E) in spoil heap were observed, Ce showed higher (A) on both habitats in comparison to Sg. Moreover, we found higher H2O2 concentration in Sg leaves as compared to Ce leaves, large differences in catalase (CAT) activity and the reverse pattern of lipid peroxidation in Sg and Ce populations, suggesting species‐specific differences in antioxidative mechanisms. The Sg individuals developed structural and functional adaptations to protect PSA against drought/salinity stresses (lower leaf chlorophyll, higher flavonoids content, ChlF parameters: Vi, Vj, dVG/dto). The Ce populations have higher values of JIP parameters related to the electron transfer site within PSI. Mechanisms of plant species adaptation to industrial areas are crucial for species selection and planning effective reclamation of them. In novel ecosystems of spoil heaps both species responded differently to complex abiotic stresses in comparison to seminatural ones that enable them to gain success on both sites. They can spontaneously colonize such areas, create permanent plant cover, and produce large amounts of biomass. Further research on plant traits response and adaptation to complex environmental stresses on industrial habitats are needed.

Photosynthetic apparatus performance of tomato seedlings grown under various combinations of LED illumination.

It is already known that the process of photosynthesis depends on the quality and intensity of light. However, the influence of the new light sources recently used in horticulture, known as Light Emitting Diodes (LEDs), on this process is not yet fully understood. Chlorophyll a fluorescence measurement has been widely used as a rapid, reliable, and noninvasive tool to study the efficiency of the photosystem II (PSII) and to evaluate plant responses to various environmental factors, including light intensity and quality. In this work, we tested the responses of the tomato photosynthetic apparatus to different light spectral qualities. Our results showed that the best performance of the photosynthetic apparatus was observed under a mixture of red and blue light (R7:B3) or a mixture of red, green and blue light (R3:G2:B5). This was demonstrated by the increase in the effective photochemical quantum yield of PSII (Y[II]), photochemical quenching (qP) and electron transport rate (ETR). On the other hand, the mixture of red and blue light with a high proportion of blue light led to an increase in non-photochemical quenching (NPQ). Our results can be used to improve the production of tomato plants under artificial light conditions. However, since we found that the responses of the photosynthetic apparatus of tomato plants to a particular light regime were cultivar-dependent and there was a weak correlation between the growth and photosynthetic parameters tested in this work, special attention should be paid in future research.

A Holistic Approach to Study Photosynthetic Acclimation Responses of Plants to Fluctuating Light.

Plants in natural environments receive light through sunflecks, the duration and distribution of these being highly variable across the day. Consequently, plants need to adjust their photosynthetic processes to avoid photoinhibition and maximize yield. Changes in the composition of the photosynthetic apparatus in response to sustained changes in the environment are referred to as photosynthetic acclimation, a process that involves changes in protein content and composition. Considering this definition, acclimation differs from regulation, which involves processes that alter the activity of individual proteins over short-time periods, without changing the abundance of those proteins. The interconnection and overlapping of the short- and long-term photosynthetic responses, which can occur simultaneously or/and sequentially over time, make the study of long-term acclimation to fluctuating light in plants challenging. In this review we identify short-term responses of plants to fluctuating light that could act as sensors and signals for acclimation responses, with the aim of understanding how plants integrate environmental fluctuations over time and tailor their responses accordingly. Mathematical modeling has the potential to integrate physiological processes over different timescales and to help disentangle short-term regulatory responses from long-term acclimation responses. We review existing mathematical modeling techniques for studying photosynthetic responses to fluctuating light and propose new methods for addressing the topic from a holistic point of view.

Differential response of photosynthetic apparatus towards alkaline pH treatment in NIES-39 and PCC 7345 strains of Arthrospira platensis.

Alkaline stress is one of the severe abiotic stresses, which is not well studied so far, especially among cyanobacteria. To affirm the characteristics of alkaline stress and the subsequent adaptive responses in Arthrospira platensis NIES-39 and Arthrospira platensis PCC 7345, photosynthetic pigments, spectral properties of thylakoids, PSII and PSI activities, and pigment-protein profiles of thylakoids under different pH regimes were examined. The accessory pigments showed a pH-mediated sensitivity. The pigment-protein complexes of thylakoids are also affected, resulting in the altered fluorescence emission profile. At pH 11, a possible shift of the PBsome antenna complex from PSII to PSI is observed. PSII reaction center is found to be more susceptible to alkaline stress in comparison to the PSI. In Arthrospira platensis NIES-39 at pH 11, a drop of 68% in the oxygen evolution with a significant increase of PSI activity by 114% is recorded within 24 h of pH treatment. Alterations in the cellular ultrastructure of Arthrospira platensis NIES-39 at pH 11 were observed, along with the increased number of plastoglobules attached with the thylakoid membranes. Arthrospira platensis NIES-39 is more adaptable to pH variation than Arthrospira platensis PCC 7345.

Functioning of the Photosynthetic Apparatus in Response to Drought Stress in Oat × Maize Addition Lines.

The oat × maize chromosome addition (OMA) lines, as hybrids between C3 and C4 plants, can potentially help us understand the process of C4 photosynthesis. However, photosynthesis is often affected by adverse environmental conditions, including drought stress. Therefore, to assess the functioning of the photosynthetic apparatus in OMA lines under drought stress, the chlorophyll content and chlorophyll a fluorescence (CF) parameters were investigated. With optimal hydration, most of the tested OMA lines, compared to oat cv. Bingo, showed higher pigment content, and some of them were characterized by increased values of selected CF parameters. Although 14 days of drought caused a decrease of chlorophylls and carotenoids, only slight changes in CF parameters were observed, which can indicate proper photosynthetic efficiency in most of examined OMA lines compared to oat cv. Bingo. The obtained data revealed that expected changes in hybrid functioning depend more on the specific maize chromosome and its interaction with the oat genome rather than the number of retained chromosomes. OMA lines not only constitute a powerful tool for maize genomics but also are a source of valuable variation in plant breeding, and can help us to understand plant susceptibility to drought. Our research confirms more efficient functioning of hybrid photosynthetic apparatus than oat cv. Bingo, therefore contributes to raising new questions in the fields of plant physiology and biochemistry. Due to the fact that the oat genome is not fully sequenced yet, the mechanism of enhanced photosynthetic efficiency in OMA lines requires further research.

Special issue in honour of Prof. Reto J. Strasser - Structural and functional response of photosynthetic apparatus of radish plants to iron deficiency

In this work, we tried to identify some specific chlorophyll a fluorescence (ChlF) parameters, that could enable detection of iron deficiency (Fedef) in radish plants (Raphanus sativus L.), before any visual symptoms appear. Changes in ChlF kinetics, JIP-test parameters, and chlorophyll content revealed that iron deficiency negatively affected PSII activity mainly via disruption of light absorption in light-harvesting complexes and by decreasing the activity of the primary quinone acceptor of PSII (QA). Iron deficiency was clearly reflected in the changes of some JIP-test parameters, such as time to reach maximal fluorescence (FM), Area, normalized total area under the OJIP curve, and number of QA redox turnovers until FM is reached. The visible symptoms of Fedef appeared after 7 d of stress application, while ChlF measurements allowed us to detect iron deficiency during 1-3 d. Our results suggest that analysis of ChlF signals has a high potential for early detection of iron deficiency in radish plants.

Effects of drought at different periods of wheat development on the leaf photosynthetic apparatus and productivity

The problem of drought impact on crop productivity is especially relevant for the leading cereal – wheat, since significant cultivation areas of this crop are located in risky farming zones. The aim of our studies was to compare the peculiarities of drought impact during the period of vegetative growth and reproductive development on the chlorophyll content, activity of chloroplast antioxidant enzymes, the rate of carbon dioxide gas exchange and transpiration as well as productivity of the wheat plants. The studies were carried out on bread winter wheat plants (Triticum aestivum L.) of the Astarta and Natalka varieties grown in a pot experiment. The plants were watered daily to maintain the soil moisture level in the pots in the range 70–60% of field capacity. Drought treatment involving decrease in soil moisture to 30% of the field capacity was applied to some of the plants at late stem elongation stage (BBCH 37-45) and to other plants at heading-anthesis (BBCH 59-65) while control plants were watered as usual. After 7 days of drought, the watering of treated plants was resumed to maintain the soil moisture at the level of control plants until the harvest. The penultimate leaf at the late stem elongation stage and flag leaf at the heading–anthesis period were used for measurements of photosynthetic apparatus response to drought. Plants of both varieties treated at stem elongation stage showed lower decline in the photosynthetic activity, chlorophyll content and grain productivity than those treated at heading-anthesis. The results obtained indicated that photosynthetic apparatus of wheat has a higher drought tolerance at the stem elongation stage than during the heading-anthesis stage. Furthermore, plants stressed at the stem elongation stage revealed better compensatory growth of lateral productive shoots due to superior supply of plants with assimilates after drought termination. Astarta variety had a more drought-tolerant photosynthetic apparatus and higher grain productivity in both well-watered and drought conditions as compared to Natalka variety. The differences between varieties were more contrasting when drought was applied at heading-anthesis.

Responses of photosynthetic apparatus in sunflower cultivars to combined drought and salt stress

Response of photosynthetic apparatus in some sunflower cultivars, i.e., S.28111, Hysun-33, Hysun-39, and SF0049, to salt, drought, and combined stresses were studied. The combined stress caused severe damage to photosynthetic apparatus as compared to single stress. The maximum quantum yield of PSII, phenomenological fluxes, plastoquinone pool size, performance indexes, and driving force of absorption were greatly affected by the combined stress. Among the cultivars, the combined stress produced synergistic effect (greater damage) in Hysun-33 and cross-tolerance (lesser damage) in S.28111. Similarly, concerning the ion imbalance, S.28111 and SF0049 showed lower Na+ and Cl- concentrations with lesser electrolyte leakage as compared to Hysun-33 and Hysun-39 under salt and combined stress. Results revealed that the disturbance in photosynthetic performance could be easily determined using JIP test by measuring chlorophyll a fluorescence. This information can be useful for the screening of oil-seed crop plants having better photosynthetic performance under salinized and desertified conditions.

Different response of photosynthetic apparatus to high-light stress in sporotrophophyll and nest leaves of Platycerium bifurcatum

Tropical forest deforestation leads to sudden changes in light conditions, which is a problem for the development of epiphytes, such as Platycerium bifurcatum. High light (HL) results in disturbances of the light phase of photosynthesis and changes in leaf pigment composition. The current study determined differences in fast chlorophyll fluorescence transients and changes in radiation reflectance in sporotrophophylls and nest leaves after one and seven days of HL. In addition, a spectral range was identified with the largest change in reflectance induced by HL. Sporotrophophylls exhibited a stronger response of the photosynthetic apparatus to photoinhibition than that of the nest leaves. Moreover, in sporotrophophylls, disorders of electron transport were visible in the whole O-P phase, and in the nest leaves mainly in the O-J phase. Higher accumulation of photoprotective compounds in sporotrophophylls underlined a different reaction to HL.

Response of photosynthetic apparatus of Isochrysis galbana to different nitrogen concentrations

The chlorophyll fluorescence parameters of Isochrysis. galbana at different nitrogen levels were determined by using chlorophyll fluorescence with the aim of better understanding the operative status of photosynthetic system II (PSII) and electron transport chain in oleaginous microalgae. In this experiment, the growth and photosynthetic efficiency were restricted in N-deprived I. galbana, which were mainly presented in significant decrease of cell numbers and chlorophyll content, the dramatic decline of Fv/Fm, P.I., ΦPSII and rETR, and commensurate increase of NPQ and Wk. The growth of algal cells gradually increased with rising nitrogen concentration, especially reached the top in N-1.76 mM L1 medium, but were inhibited in N-3.52 mM L1 medium. The results suggested that N-deprivation and N-deficiency could inhibit the growth and photosynthesis of I. galbana, but excessive nitrogen could also restrict the growth of algal cells. The nitrogen concentration plays an important role in the algal growth and photosynthesis.

Dissecting the Native Architecture and Dynamics of Cyanobacterial Photosynthetic Machinery

The structural dynamics and flexibility of cell membranes play fundamental roles in the functions of the cells, i.e., signaling, energy transduction, and physiological adaptation. The cyanobacterial thylakoid membrane represents a model membrane that can conduct both oxygenic photosynthesis and respiration simultaneously. In this study, we conducted direct visualization of the global organization and mobility of photosynthetic complexes in thylakoid membranes from a model cyanobacterium, Synechococcus elongatus PCC 7942, using high-resolution atomic force, confocal, and total internal reflection fluorescence microscopy. We visualized the native arrangement and dense packing of photosystem I (PSI), photosystem II (PSII), and cytochrome (Cyt) b6f within thylakoid membranes at the molecular level. Furthermore, we functionally tagged PSI, PSII, Cyt b6f, and ATP synthase individually with fluorescent proteins, and revealed the heterogeneous distribution of these four photosynthetic complexes and determined their dynamic features within the crowding membrane environment using live-cell fluorescence imaging. We characterized red light-induced clustering localization and adjustable diffusion of photosynthetic complexes in thylakoid membranes, representative of the reorganization of photosynthetic apparatus in response to environmental changes. Understanding the organization and dynamics of photosynthetic membranes is essential for rational design and construction of artificial photosynthetic systems to underpin bioenergy development. Knowledge of cyanobacterial thylakoid membranes could also be extended to other cell membranes, such as chloroplast and mitochondrial membranes.

Response of photosynthetic apparatus in Arabidopsis thaliana L. mutant deficient in phytochrome A and B to UV-B

The effects of UV-B radiation (1 W m–2, 1 and 2 h) on PSII activity, chloroplast structure, and H2O2 contents in leaves of 26-d-old Arabidopsis thaliana phyA phyB double mutant (DMut) compared with the wild type (WT) were investigated. UV-B decreased PSII activity and affected the H2O2 content in WT and DMut plants grown under white light (WL). The chloroplast structure changes in DMut plants exposed to UV were more significant than that in WT. Reductions in maximal and real quantum photochemical yields and increase in the value of thermal dissipation of absorbed light energy per PSII RC and the amount of QB-nonreducing centers of PSII were bigger in mutant compared to WT plants grown both under WL and red light. Such difference in action of UV-B on WT and DMut can be explained by higher content of UV-absorbing pigments and carotenoids in WT leaves compared with DMut.

Long-term measurements of chlorophyll a fluorescence using the JIP-test show that combined abiotic stresses influence the photosynthetic performance of the perennial ryegrass (Lolium perenne) in a managed temperate grassland.

Several experiments have highlighted the complexity of stress interactions involved in plant response. The impact in field conditions of combined environmental constraints on the mechanisms involved in plant photosynthetic response, however, remains understudied. In a long-term field study performed in a managed grassland, we investigated the photosynthetic apparatus response of the perennial ryegrass (Lolium perenne L.) to environmental constraints and its ability to recover and acclimatize. Frequent field measurements of chlorophyll a fluorescence (ChlF) were made in order to determine the photosynthetic performance response of a population of L. perenne. Strong midday declines in the maximum quantum yield of primary photochemistry (FV FM ) were observed in summer, when a combination of heat and high light intensity increased photosynthetic inhibition. During this period, increase in photosystem I (PSI) activity efficiency was also recorded, suggesting an increase in the photochemical pathway for de-excitation in summer. Strong climatic events (e.g. heat waves) were shown to reduce electron transport between photosystem II (PSII) and PSI. This reduction might have preserved the PSI from photo-oxidation. Periods of low soil moisture and high levels of sun irradiance increased PSII sensitivity to heat stress, suggesting increased susceptibility to combined environmental constraints. Despite the multiple inhibitions of photosynthetic functionality in summer, the L. perenne population showed increased PSII tolerance to environmental stresses in August. This might have been a response to earlier environmental constraints. It could also be linked to the selection and/or emergence of well-adapted individuals.