Changes in Photosystem II Complex and Physiological Activities in Pea and Maize Plants in Response to Salt Stress.

Salt stress significantly impacts the functions of the photosynthetic apparatus, with varying degrees of damage to its components. Photosystem II (PSII) is more sensitive to environmental stresses, including salinity, than photosystem I (PSI). This study investigated the effects of different salinity levels (0 to 200 mM NaCl) on the PSII complex in isolated thylakoid membranes from hydroponically grown pea (Pisum sativum L.) and maize (Zea mays L.) plants treated with NaCl for 5 days. The data revealed that salt stress inhibits the photochemical activity of PSII (H2O → BQ), affecting the energy transfer between the pigment-protein complexes of PSII (as indicated by the fluorescence emission ratio F695/F685), QA reoxidation, and the function of the oxygen-evolving complex (OEC). These processes were more significantly affected in pea than in maize under salinity. Analysis of the oxygen evolution curves after flashes and continuous illumination showed a stronger influence on the PSIIα than PSIIß centers. The inhibition of oxygen evolution was associated with an increase in misses (α), double hits (ß), and blocked centers (SB) and a decrease in the rate constant of turnover of PSII reaction centers (KD). Salinity had different effects on the two pathways of QA reoxidation in maize and pea. In maize, the electron flow from QA- to plastoquinone was dominant after treatment with higher NaCl concentrations (150 mM and 200 mM), while in pea, the electron recombination on QAQB- with oxidized S2 (or S3) of the OEC was more pronounced. Analysis of the 77 K fluorescence emission spectra revealed changes in the ratio of the light-harvesting complex of PSII (LHCII) monomers and trimers to LHCII aggregates after salt treatment. There was also a decrease in pigment composition and an increase in oxidative stress markers, membrane injury index, antioxidant activity (FRAP assay), and antiradical activity (DPPH assay). These effects were more pronounced in pea than in maize after treatment with higher NaCl concentrations (150 mM-200 mM). This study provides insights into how salinity influences the processes in the donor and acceptor sides of PSII in plants with different salt sensitivity.

Molecular Mechanisms of Plant Defense against Abiotic Stress.

The climatic changes and anthropogenic factors in recent decades (global warming, drought, salinity, extreme temperature, environmental pollution) have led to an increase in the negative impact of environmental factors on plants. Abiotic stress strongly influences the important processes of plants and thus affects their growth and development. The effects of stressors on the plants depend on the intensity, frequency, and duration of stress, plant species as well as a combination of various stressors. Plants have developed different mechanisms to limit adverse environmental conditions. In the publications in this Special Issue, Molecular Mechanisms of Plant Defense against Abiotic Stress, new information on plant defense mechanisms against abiotic and biotic stress is presented. The studies help us better understand plants' protection mechanisms again global climate change.

Protective Effects of Sodium Nitroprusside on Photosynthetic Performance of Sorghum bicolor L. under Salt Stress.

In this study, the impacts of the foliar application of different sodium nitroprusside (SNP, as a donor of nitric oxide) concentrations (0-300 µM) on two sorghum varieties (Sorghum bicolor L. Albanus and Sorghum bicolor L. Shamal) under salt stress (150 mM NaCl) were investigated. The data revealed that salinity leads to an increase in oxidative stress markers and damage of the membrane integrity, accompanied by a decrease in the chlorophyll content, the open photosystem II (PSII) centers, and the performance indexes (PI ABS and PI total), as well as having an influence on the electron flux reducing photosystem I (PSI) end acceptors (REo/RC). Spraying with SNP alleviated the NaCl toxicity on the photosynthetic functions; the protection was concentration-dependent, and greater in Shamal than in Albanus, i.e., variety specific. Furthermore, the experimental results revealed that the degree of SNP protection under salt stress also depends on the endogenous nitric oxide (NO) amount in leaves, the number of active reaction centers per PSII antenna chlorophylls, the enhanced electron flux reducing end acceptors at the acceptor side of PSI, as well as the stimulation of the cyclic electron transport around PSI. The results showed better protection in both varieties of sorghum for SNP concentrations up to 150 µM, which corresponds to about a 50% increase in the endogenous NO leaf content in comparison to the control plants. Our study provides valuable insight into the molecular mechanisms underlying SNP-induced salt tolerance in sorghum varieties and might be a practical approach to correcting salt intolerance.

Impact of Salinity on the Energy Transfer between Pigment-Protein Complexes in Photosynthetic Apparatus, Functions of the Oxygen-Evolving Complex and Photochemical Activities of Photosystem II and Photosystem I in Two Paulownia Lines.

The present study shows the effect of salinity on the functions of thylakoid membranes from two hybrid lines of Paulownia: Paulownia tomentosa x fortunei and Paulownia elongate x elongata, grown in a Hoagland solution with two NaCl concentrations (100 and 150 mM) and different exposure times (10 and 25 days). We observed inhibition of the photochemical activities of photosystem I (DCPIH2 → MV) and photosystem II (H2O → BQ) only after the short treatment (10 days) with the higher NaCl concentration. Data also revealed alterations in the energy transfer between pigment-protein complexes (fluorescence emission ratios F735/F685 and F695/F685), the kinetic parameters of the oxygen-evolving reactions (initial S0-S1 state distribution, misses (α), double hits (ß) and blocked centers (SB)). Moreover, the experimental results showed that after prolonged treatment with NaCl Paulownia tomentosa x fortunei adapted to the higher concentration of NaCl (150 mM), while this concentration is lethal for Paulownia elongata x elongata. This study demonstrated the relationship between the salt-induced inhibition of the photochemistry of both photosystems and the salt-induced changes in the energy transfer between the pigment-protein complexes and the alterations in the Mn cluster of the oxygen-evolving complex under salt stress.

Impact of Sodium Nitroprusside on the Photosynthetic Performance of Maize and Sorghum.

Nitric oxide (NO) is an important molecule in regulating plant growth, development and photosynthetic performance. This study investigates the impact of varying concentrations (0-300 µM) of sodium nitroprusside (SNP, a donor of NO) on the functions of the photosynthetic apparatus in sorghum (Sorghum bicolor L. Albanus) and maize (Zea mays L. Kerala) under physiological conditions. Analysis of the chlorophyll fluorescence signals (using PAM and the JIP-test) revealed an increased amount of open PSII reaction centers (qP increased), but it did not affect the number of active reaction centers per PSII antenna chlorophyll (RC/ABS). In addition, the smaller SNP concentrations (up to 150 µM) alleviated the interaction of QA with plastoquine in maize, while at 300 µM it predominates the electron recombination on QAQB-, with the oxidized S2 (or S3) states of oxygen evolving in complex ways in both studied plant species. At the same time, SNP application stimulated the electron flux-reducing end electron acceptors at the PSI acceptor side per reaction center (REo/RC increased up to 26%) and the probability of their reduction (φRo increased up to 20%). An increase in MDA (by about 30%) and H2O2 contents was registered only at the highest SNP concentration (300 µM). At this concentration, SNP differentially affected the amount of P700+ in studied plant species, i.e., it increased (by 10%) in maize but decreased (by 16%) in sorghum. The effects of SNP on the functions of the photosynthetic apparatus were accompanied by an increase in carotenoid content in both studied plants. Additionally, data revealed that SNP-induced changes in the photosynthetic apparatus differed between maize and sorghum, suggesting species specificity for SNP's impact on plants.

Assessment of the Photosynthetic Apparatus Functions by Chlorophyll Fluorescence and P700 Absorbance in C3 and C4 Plants under Physiological Conditions and under Salt Stress.

Functions of the photosynthetic apparatus of C3 (Pisum sativum L.) and C4 (Zea mays L.) plants under physiological conditions and after treatment with different NaCl concentrations (0-200 mM) were investigated using chlorophyll a fluorescence (pulse-amplitude-modulated (PAM) and JIP test) and P700 photooxidation measurement. Data revealed lower density of the photosynthetic structures (RC/CSo), larger relative size of the plastoquinone (PQ) pool (N) and higher electron transport capacity and photosynthetic rate (parameter RFd) in C4 than in C3 plants. Furthermore, the differences were observed between the two studied species in the parameters characterizing the possibility of reduction in the photosystem (PSI) end acceptors (REo/RC, REo/CSo and δRo). Data revealed that NaCl treatment caused a decrease in the density of the photosynthetic structures and relative size of the PQ pool as well as decrease in the electron transport to the PSI end electron acceptors and the probability of their reduction as well as an increase in the thermal dissipation. The effects were stronger in pea than in maize. The enhanced energy losses after high salt treatment in maize were mainly from the increase in the regulated energy losses (ΦNPQ), while in pea from the increase in non-regulated energy losses (ΦNO). The reduction in the electron transport from QA to the PSI end electron acceptors influenced PSI activity. Analysis of the P700 photooxidation and its decay kinetics revealed an influence of two PSI populations in pea after treatment with 150 mM and 200 mM NaCl, while in maize the negligible changes were registered only at 200 mM NaCl. The experimental results clearly show less salt tolerance of pea than maize.

Different Sensitivity Levels of the Photosynthetic Apparatus in Zea mays L. and Sorghum bicolor L. under Salt Stress.

The impacts of different NaCl concentrations (0-250 mM) on the photosynthesis of new hybrid lines of maize (Zea mays L. Kerala) and sorghum (Sorghum bicolor L. Shamal) were investigated. Salt-induced changes in the functions of photosynthetic apparatus were assessed using chlorophyll a fluorescence (PAM and OJIP test) and P700 photooxidation. Greater differences between the studied species in response to salinization were observed at 150 mM and 200 mM NaCl. The data revealed the stronger influence of maize in comparison to sorghum on the amount of closed PSII centers (1-qp) and their efficiency (Φexc), as well as on the effective quantum yield of the photochemical energy conversion of PSII (ΦPSII). Changes in the effective antenna size of PSII (ABS/RC), the electron flux per active reaction center (REo/RC) and the electron transport flux further QA (ETo/RC) were also registered. These changes in primary PSII photochemistry influenced the electron transport rate (ETR) and photosynthetic rate (parameter RFd), with the impacts being stronger in maize than sorghum. Moreover, the lowering of the electron transport rate from QA to the PSI end electron acceptors (REo/RC) and the probability of their reduction (φRo) altered the PSI photochemical activity, which influenced photooxidation of P700 and its decay kinetics. The pigment content and stress markers of oxidative damage were also determined. The data revealed a better salt tolerance of sorghum than maize, associated with the structural alterations in the photosynthetic membranes and the stimulation of the cyclic electron flow around PSI at higher NaCl concentrations. The relationships between the decreased pigment content, increased levels of stress markers and different inhibition levels of the function of both photosystems are discussed.

Cadmium toxicity in Salvia sclarea L.: An integrative response of element uptake, oxidative stress markers, leaf structure and photosynthesis.

The herbal plant Salvia sclarea L. (clary sage) is classified to cadmium (Cd) accumulators and considered as a potential plant for phytoremediation of heavy metal polluted soil. However, the effect of Cd only treatment on the function of the photosynthetic apparatus of S. sclarea, as well as the mechanisms involved in Cd tolerance have not yet been studied in detail. This study was conducted to examine the integrative responses of S. sclarea plants exposed to a high Cd supply (100 µM) for 3 and 8 days by investigating element nutrient uptake, oxidative stress markers, pigment composition, photosynthetic performance and leaf structure. Measurements of the functional activities of photosystem I (PSI, by P700 photooxidation), photosystem II (PSII, by chlorophyll fluorescence parameters), the oxygen-evolving complex (oxygen evolution by Joliot- and Clark-type electrodes), as well as the leaf pigment and phenolic contents, were used to evaluate the protective mechanisms of the photosynthetic apparatus under Cd stress. Data suggested that the molecular mechanisms included in the photosynthetic tolerance to Cd toxicity involve strongly increased phenolic and anthocyanin contents, as well as an increased non-photochemical quenching and accelerated cyclic electron transport around PSI up to 61%, which protect the function of the photosynthetic apparatus under stress. Furthermore, the tolerance of S. sclarea to Cd stress is also associated with increased accumulation of Fe in leaves by 25%. All the above, clearly suggest that S. sclarea plants employ several different mechanisms to protect the function of the photosynthetic apparatus against Cd stress, which are discussed here.

The wheat mutant DELLA-encoding gene (Rht-B1c) affects plant photosynthetic responses to cadmium stress.

Тhe sensitivity to cadmium (Cd) stress of two near-isogenic wheat lines with differences at the Rht-B1 locus, Rht-B1a (tall wild type, encoding DELLA proteins) and Rht-B1c (dwarf mutant, encoding modified DELLA proteins), was investigated. The effects of 100 µM CdCl2 on plant growth, pigment content and functional activity of the photosynthetic apparatus of wheat seedlings grown on a nutrient solution were evaluated through a combination of PAM chlorophyll fluorescence, oxygen evolution, oxidation-reduction kinetics of P700 and 77 K fluorescence. The results showed that the wheat mutant (Rht-B1c) was more tolerant to Cd stress compared to the wild type (Rht-B1a), as evidenced by the lower reductions in plant growth and pigment content, lower inhibition of photosystem I (PSI) and photosystem II (PSII) photochemistry and of the oxygen evolution measured with Clark-type and Joliot-type electrodes. Furthermore, the enhanced Cd tolerance was accompanied by increased Cd accumulation within mutant plant tissues. The molecular mechanisms through which the Rht-B1c mutation improves plant tolerance to Cd stress involve structural alterations in the mutant photosynthetic membranes leading to better protection of the Mn cluster of oxygen-evolving complex and increased capacity for PSI cyclic electron transport, protecting photochemical activity of the photosynthetic apparatus under stress. This study suggests a role for the Rht-B1c-encoded DELLA proteins in protective mechanisms and tolerance of the photosynthetic apparatus in wheat plants exposed to heavy metals stress.

Different sensitivities of photosystem II in green algae and cyanobacteria to phenylurea and phenol-type herbicides: effect on electron donor side.

The effects of short-term treatment with phenylurea (DCMU, isoproturon) and phenol-type (ioxynil) herbicides on the green alga Chlorella kessleri and the cyanobacterium Synechocystis salina with different organizations of photosystem II (PSII) were investigated using pulse amplitude modulated (PAM) chlorophyll fluorescence and photosynthetic oxygen evolution measured by polarographic oxygen electrodes (Clark-type and Joliot-type). The photosynthetic oxygen evolution showed stronger inhibition than the PSII photochemistry. The effects of the studied herbicides on both algal and cyanobacterial cells decreased in the following order: DCMU>isoproturon>ioxynil. Furthermore, we observed that the number of blocked PSII centers increased significantly after DCMU treatment (204-250 times) and slightly after ioxynil treatment (19-35 times) in comparison with the control cells. This study suggests that the herbicides affect not only the acceptor side but also the donor side of PSII by modifications of the Mn cluster of the oxygen-evolving complex. We propose that one of the reasons for the different PSII inhibitions caused by herbicides is their influence, in different extents, on the kinetic parameters of the oxygen-evolving reactions (the initial S0-S1 state distribution, the number of blocked centers SB, the turnover time of Si states, misses and double hits). The relationship between the herbicide-induced inhibition and the changes in the kinetic parameters is discussed.

Effects of salinity on the photosynthetic apparatus of two Paulownia lines.

The effects of soil salinity on the functional activity of photosynthetic apparatus and pigment composition of two Paulownia lines (Paulownia tomentosa x fortunei and Paulownia elongata x elongata) were investigated. PAM chlorophyll fluorescence measurements revealed that salinity leads to: (i) an increase of the photochemical quenching coefficient (qP) and the linear electron transport rate (ETR) in both lines of Paulownia, while the maximum quantum yield of the primary photochemistry of PSII in the dark adapted state (Fv/Fm) was unaffected; (ii) improved the efficiency of the photochemical energy conversion (ФPSII); (iii) an impact on the chlorophyll fluorescence decrease ratio (RFd), which correlates to the net CO2 assimilation rate; (iv) an impact on [Formula: see text] reoxidation. The analysis of the kinetics of P700(+) reduction upon turning off far-red irradiation revealed that salinization lead to a delay of the cyclic electron transport around PSI in both studied lines as the effect on this process is more pronounced in P. tomentosa x fortunei than in (in comparison with) P. elongata x elongata. The present experimental results suggested high salt tolerance of the studied lines Paulownia, but P. tomentosa x fortunei is more tolerant to salinity than P. elongata x elongata. Molecular mechanisms involved in the Paulownia response to the soil salinity are discussed.

Damage and protection of the photosynthetic apparatus from UV-B radiation. II. Effect of quercetin at different pH.

The effect of the exogenously added quercetin against the UV-B inhibition of the photosystem II (PSII) functions in isolated pea thylakoid membranes suspended at different pH of the medium (6.5, 7.6 and 8.4) was investigated. The data revealed that the interaction of this flavonoid with the membranes depends on the pH and influences the initial S0-S1 state distribution of PSII in the dark, the energy transfer between pigment-protein complexes of the photosynthetic apparatus and the membrane fluidity. Quercetin also displays a different UV-protective effect depending on its location in the membranes, as the effect is more pronounced at pH 8.4 when it is located at the membrane surface. The results suggest that quercetin induces structural changes in thylakoid membranes, one of the possible reasons for its protection of the photosynthetic apparatus.

Effects of exogenous 24-epibrassinolide on the photosynthetic membranes under non-stress conditions.

In the present work the effects of exogenous 24-epibrassinolide (EBR) on functional and structural characteristics of the thylakoid membranes under non-stress conditions were evaluated 48 h after spraying of pea plants with different concentrations of EBR (0.01, 0.1 and 1.0 mg.L(-1)). The results show that the application of 0.1 mg.L(-1) EBR has the most pronounced effect on the studied characteristics of the photosynthetic membranes. The observed changes in 540 nm light scattering and in the calorimetric transitions suggest alterations in the structural organization of the thylakoid membranes after EBR treatment, which in turn influence the kinetics of oxygen evolution, accelerate the electron transport rate, increase the effective quantum yield of photosystem II and the photochemical quenching. The EBR-induced changes in the photosynthetic membranes are most probably involved in the stress tolerance of plants.

Action and target sites of nitric oxide in chloroplasts.

Nitric oxide (NO) is an important signalling molecule in plants under physiological and stress conditions. Here we review the influence of NO on chloroplasts which can be directly induced by interaction with the photosynthetic apparatus by influencing photophosphorylation, electron transport activity and oxido-reduction state of the Mn clusters of the oxygen-evolving complex or by changes in gene expression. The influence of NO-induced changes in the photosynthetic apparatus on its functions and sensitivity to stress factors are discussed.

Effect of partial or complete elimination of light-harvesting complexes on the surface electric properties and the functions of cyanobacterial photosynthetic membranes.

Influence of the modification of the cyanobacterial light-harvesting complex [i.e. phycobilisomes (PBS)] on the surface electric properties and the functions of photosynthetic membranes was investigated. We used four PBS mutant strains of Synechocystis sp. PCC6803 as follows: PAL (PBS-less), CK (phycocyanin-less), BE (PSII-PBS-less) and PSI-less/apcE(-) (PSI-less with detached PBS). Modifications of the PBS content lead to changes in the cell morphology and surface electric properties of the thylakoid membranes as well as in their functions, such as photosynthetic oxygen-evolving activity, P700 kinetics and energy transfer between the pigment-protein complexes. Data reveal that the complete elimination of PBS in the PAL mutant causes a slight decrease in the electric dipole moments of the thylakoid membranes, whereas significant perturbations of the surface charges were registered in the membranes without assembled PBS-PSII macrocomplex (BE mutant) or PSI complex (PSI-less mutant). These observations correlate with the detected alterations in the membrane structural organization. Using a polarographic oxygen rate electrode, we showed that the ratio of the fast to the slow oxygen-evolving PSII centers depends on the partial or complete elimination of light-harvesting complexes, as the slow operating PSII centers dominate in the PBS-less mutant and in the mutant with detached PBS.

Damage and protection of the photosynthetic apparatus from UV-B radiation. I. Effect of ascorbate.

In this work, the effect of the exogenously added ascorbate (Asc) against the UV-B inhibition of the photosystem II (PSII) functions in isolated pea thylakoid membranes was studied. The results reveal that Asc decreases the UV-B induced damage of the donor and the acceptor side of PSII during short treatment up to 60 min. The exogenous Asc exhibits a different UV-protective effect on PSII centers in grana and stroma lamellae, as the effect is more pronounced on the PSIIß centers in comparison to PSIIα centers. Data also suggest that one of the possible protective roles of the Asc in photosynthetic membranes is the modification of the oxygen-evolving complex by influence on the initial S(0)-S(1) state distribution in the dark.

LHCII organization and thylakoid lipids affect the sensitivity of the photosynthetic apparatus to high-light treatment.

Pulse-amplitude-modulated (PAM) chlorophyll fluorescence and photosynthetic oxygen evolution were used to investigate the role of the different amount and organization of light-harvesting complexes of photosystem II (LHCII) in four pea species on the susceptibility of the photosynthetic apparatus to high-light treatment. In this work we analyzed the thylakoid membrane lipid composition of the studied pea plants. A relationship between the structural organization of LHCII proteins, the amount of the main lipid classes and the sensitivity of the photosynthetic apparatus to high-light treatment was found. The results reveal that the photosynthetic apparatus, enriched in oligomeric forms of LHCII concomitant with decreased amount of anionic lipids and increased content of the monogalactosyldiacylglycerol (MGDG), is less sensitive to high light. Our data also suggest that the degree of LHCII oligomerization, as well as the lipid composition do not influence the degree of recovery of the PSII photochemistry after excess light exposure.

Identification of thylakoid membrane thermal transitions in Synechocystis sp. PCC6803 photosynthetic mutants.

We used differential scanning calorimetry (DSC) as a technique capable of identifying photosynthetic complexes on the basis of their calorimetric transitions. Annotation of thermal transitions was carried out with thylakoid membranes isolated from various photosynthetic mutants of Synechocystis sp. PCC6803. The thylakoid membranes exhibited seven major DSC bands between 40 and 85°C. The heat sorption curves were analyzed both by mathematical deconvolution of the overall endotherms and by a subsequent annealing procedure. The successive annealing procedure proved to be more reliable technique than mathematical deconvolution in assigning thermal transitions. The main DSC band, around 47°C, resulting from the high enthalpy change that corresponds to non-interacting complex of PSII, was assigned using the PSI-less/apcE(-) mutant cells. Another band around 68-70°C relates to the denaturation of PSII surrounded by other proteins of the photosynthetic complexes in wild type and PSI-less/apcE(-) cells. A further major transition found at 82-84°C corresponds to the PSI core complex of wild type and PSII-deficient BE cells. Other transition bands between 50-67 and 65-75°C are believed to relate to ATP synthase and cytochrome b(6)f, respectively. These thermal transitions were obtained with thylakoids isolated from PSI(-)/PSII(-) mutant cells. Some minor bands determined at 59 and 83-84°C correspond to an unknown complex and NADH dehydrogenase, respectively. These annotations were done by PSI-less/apcE(-) and PSI(-)/PSII(-) mutants.

Relationship between the degree of carotenoid depletion and function of the photosynthetic apparatus.

Fluridone, an inhibitor of the carotenoid biosynthesis, was used to study the relationship between the degree of carotenoid depletion and the function of the photosynthetic apparatus. The data reveal that, at a small reduction of the carotenoid content (25% decrease of the total carotenoids), the PSII and PSI (oxidation of P700 by far-red light) photochemistry is not influenced, while the oxygen evolution is strongly inhibited. Further reduction of the total carotenoid content (more than 40%) leads to decrease of the chlorophyll content and inhibition of the functions of both photosystems as the effect on the photosynthetic oxygen evolution and primary photochemistry is stronger than the effect on P700 oxidation. The analysis of the oxygen production under continuous illumination and flash oxygen yields suggests that the inhibition of the oxygen evolution is caused mainly by the damage of PSIIalpha centers.

Effect of phosphatidylglycerol depletion on the surface electric properties and the fluorescence emission of thylakoid membranes.

To explore the possible effect of phosphatidylglycerol (PG) on the surface electric properties and chlorophyll fluorescence characteristics we used electric light scattering technique and 77K chlorophyll fluorescence of thylakoid membranes from a cyanobacterium, Synechocystis PCC6803 (wild type) and its pgsA mutant defective in PG synthesis. We found a strong decrease in the permanent and induced electric dipole moments of the mutant thylakoids, following long-term PG depletion parallel with a decrease of the emission peak from PSI and an increase of the emission peak from PSII. Partial recovery of the electric state of thylakoid membranes was observed at re-addition of PG to the mutant cells depleted of PG for 21 days. This change in the electric dipole moments is probably due to a decrease in PG content and progressive structural alterations in the macroorganization of the photosynthetic complexes induced by PG deprivation. Our results suggest that the depletion of a lipid, which carries a negative charge, despite its small contribution to the overall lipid content, significantly perturbs the surface charge of the membranes. These changes are related with the chlorophyll fluorescence emission ratios of two photosystems and may partly explain our earlier results concerning the PG requirement for the function and assembly of photosystems I and II reaction centers.