Effect of aluminum toxicity and phosphorus deficiency on the growth and photosynthesis of oil tea (<Emphasis Type="Italic">Camellia oleifera</Emphasis> Abel.) seedlings in acidic red soils

Wild and cultivated varieties of Camellia oleifera Abel. were studied for the response of their photosynthetic apparatus to Al toxicity and low-P stress in pot experiments with medium of acidic red soil. The effect was measured using physiological processes (growth, photosynthesis, chlorophyll a fluorescence), and pigment contents. The results showed that Al toxicity and low-P stress affected the seedlings’ growth and leaves’ photosynthesis, and the differences could be found between the two varieties. Lime plus P fertilizer treatment led to higher increase in the net photosynthetic rate (Pn) in the cultivar than in the wild variety. Pn increase was positively related to the increase of stomatal conductance (gs) and negatively correlated to intercellular CO₂ concentration (Ci) in both varieties. The maximum PSII quantum yield (Fv/Fm), the efficiency of excitation energy capture by open PSII reaction centers (Fv’/Fm’), the photochemical quenching (qP) and the efficiency of open PSII centers (Φ_PSII) significantly increased almost in all the treatment groups of both varieties, with the exception of an insignificant change in qP value for P₁Al₁ group of cultivar. The insensitive qP and lower Pn for cultivar indicate a higher photosynthetic efficiency for the wild variety, though the Φ_PSII was not significant between the two varieties. The pigment contents of oil tea seedlings under treatments changed significantly when lime and P were added, especially the Car/Chl ratio, suggesting carotenoid plays the role of photoprotection under high-Al and low-P stresses.     

Cyclic electron flow plays an important role in photoprotection of tropical trees illuminated at temporal chilling temperature

Our previous study indicated that PSII is more sensitive to chilling and light stress than PSI in tropical trees, and Erythrophleum guineense is more sensitive to chilling stress than Dalbergia odorifera and Khaya ivorensis, but the underlying physiological mechanisms are unclear. Although recent studies have reported that cyclic electron flow (CEF) plays an important role in photoprotection, the role of CEF in protecting PSI and PSII of tropical tree species remains unclear. We investigated the effect of temporal chilling temperature on energy distribution in PSII, the redox state of P700 and CEF in the above-mentioned tropical evergreen tree species grown in an open field. Our results indicated that the overclosure of PSII reaction centers at chilling temperature led to excess excitation pressure in PSII. At the temporal chilling temperature under low light, PSI acceptor side limitation [Y(NA)] was lower than those at 25°C for all species. Although the effective quantum yield of CEF [Y(CEF)] was not significantly stimulated in E. guineense and K. ivorensis under temporal chilling at low light levels, the ratio of Y(CEF) to the effective quantum yield of PSII [Y(II)] significantly increased. Under chilling conditions Y(CEF)/Y(II) was stimulated much more in K. ivorensis and D. odorifera compared with that in the chilling-sensitive E. guineense. These results suggested that stimulation of Y(CEF)/Y(II) plays an important role in protecting PSI and PSII from photoinhibition caused by chilling stress.     

Non-photochemical reduction of thylakoid photosynthetic redox carriers in vitro: Relevance to cyclic electron flow around photosystem I?

Non-photochemical (dark) increases in chlorophyll a fluorescence yield associated with non-photochemical reduction of redox carriers (F_npr) have been attributed to the reduction of plastoquinone (PQ) related to cyclic electron flow (CEF) around photosystem I. In vivo, this rise in fluorescence is associated with activity of the chloroplast plastoquinone reductase (plastid NAD(P)H:plastoquinone oxidoreductase) complex. In contrast, this signal measured in isolated thylakoids has been attributed to the activity of the protein gradient regulation-5 (PGR5)/PGR5-like (PGRL1)-associated CEF pathway. Here, we report a systematic experimentation on the origin of F_npr in isolated thylakoids. Addition of NADPH and ferredoxin to isolated spinach thylakoids resulted in the reduction of the PQ pool, but neither its kinetics nor its inhibitor sensitivities matched those of F_npr. Notably, F_npr was more rapid than PQ reduction, and completely insensitive to inhibitors of the PSII Q_B site and oxygen evolving complex as well as inhibitors of the cytochrome b₆f complex. We thus conclude that F_npr in isolated thylakoids is not a result of redox equilibrium with bulk PQ. Redox titrations and fluorescence emission spectra imply that F_npr is dependent on the reduction of a low potential redox component (E_m about − 340 mV) within photosystem II (PSII), and is likely related to earlier observations of low potential variants of Q_A within a subpopulation of PSII that is directly reducible by ferredoxin. The implications of these results for our understanding of CEF and other photosynthetic processes are discussed.     

Differential responses of photosystems I and II to seasonal drought in two Ficus species

Hemiepiphytic Ficus species exhibit more conservative water use strategy and are more drought-tolerant compared with their non-hemiepiphytic congeners, but a difference in the response of photosystem I (PSI) and photosystem II (PSII) to drought stress has not been documented to date. The enhancement of non-photochemical quenching (NPQ) and cyclic electron flow (CEF) have been identified as important mechanisms that protect the photosystems under drought conditions. Using the hemiepiphytic Ficus tinctoria and the non-hemiepiphytic Ficus racemosa, we studied the water status and the electron fluxes through PSI and PSII under seasonal water stress. Our results clearly indicated that the decline in the leaf predawn water potential (ψ_pd), the maximum photosynthetic rate (A_max) and the predawn maximum quantum yield of PSII (F_v/F_m) were more pronounced in F. racemosa than in F. tinctoria at peak drought. The F_v/F_m of F. racemosa was reduced to 0.69, indicating net photoinhibition of PSII. Concomitantly, the maximal photo-oxidizable P700 (P_m) decreased significantly in F. racemosa but remained stable in F. tinctoria. The fraction of non-photochemical quenching [Y(NPQ)] and the ratio of effective quantum yield of PSI to PSII [Y(I)/Y(II)] increased for both Ficus species at peak drought, with a stronger increase in F. racemosa. These results indicated that the enhancement of NPQ and the activation of CEF contributed to the photoprotection of PSI and PSII for both Ficus species under seasonal drought, particularly for F. racemosa.     

The effect of CaCl<Subscript>2</Subscript> on calcium content,photosynthesis, and chlorophyll fluorescence of tung tree seedlings under drought conditions

The study investigated the effects of different CaCl₂ concentrations (2, 5, and 10 mM) on photosynthetic enzymatic activities, photosynthesis, and chlorophyll fluorescence of tung tree seedlings under drought conditions. Plants were sprayed with either CaCl₂ or distilled water until run-off. Irrigation was then withheld to induce drought stress. The strength of drought stress was evaluated by relative leaf water content and soil water content, which was 27.3 and 9.5% on day 0 and day 12, respectively. Drought stress decreased activities of ribulose-1,5-bisphosphate carboxylase/oxygenase and phosphoenolpyruvate carboxylase, chlorophyll (a+b) content, net photosynthetic rate, stomatal conductance, transpiration rate, electron transport rate, the maximal quantum yield of PSII photochemistry, and effective quantum yield of PSII in tung tree seedlings. The CaCl₂ pretreatments alleviated the negative effect of drought stress to some degree on all the parameters mentioned above.     

Inflorescences vs leaves: a distinct modulation of carbon metabolism process during Botrytis infection

Plant growth and survival depends critically on photo assimilates. Pathogen infection leads to changes in carbohydrate metabolism of plants. In this study, we monitored changes in the carbohydrate metabolism in the grapevine inflorescence and leaves using Botrytis cinerea and Botrytis pseudo cinerea. Fluctuations in gas exchange were correlated with variations in chlorophyll a fluorescence. During infection, the inflorescences showed an increase in net photosynthesis (Pn) with a stomatal limitation. In leaves, photosynthesis decreased, with a non‐stomatal limitation. A decrease in the effective photosystem II (PSII) quantum yield (ΦPSII) was accompanied by an increase in photochemical quenching (qP) and non‐photochemical quenching (qN). The enhancement of qP and ΦPSII could explain the observed increase in Pn. In leaves, the significant decline in ΦPSII and qP, and increase in qN suggest that energy was mostly oriented toward heat dissipation instead of CO₂ fixation. The accumulation of glucose and sucrose in inflorescences and glucose and fructose in the leaves during infection indicate that the plant's carbon metabolism is differently regulated in these two organs. While a strong accumulation of starch was observed at 24 and 48 hours post‐inoculation (hpi) with both species of Botrytis in the inflorescences, a significant decrease with B. cinerea at 24 hpi and a significant increase with B. pseudo cinerea at 48 hpi were observed in the leaves. On the basis of these results, it can be said that during pathogen attack, the metabolism of grapevine inflorescence and leaf is modified suggesting distinct mechanisms modifying gas exchange, PSII activity and sugar contents in these two organs.     

Differences in the stimulation of cyclic electron flow in two tropical ferns under water stress are related to leaf anatomy

Cyclic electron flow (CEF) plays an important role in photoprotection for angiosperms under environmental stresses. However, ferns are more sensitive to drought and their water transport systems are not as efficient as those of angiosperms, it is unclear whether CEF also contributes to photoprotection in these plants. Using Microsorum punctatum and Paraleptochillus decurrens, we studied the electron fluxes through both photosystem I (PSI) and photosystem II (PSII) under water stress and their leaf anatomies. Our goal was to determine if CEF functions in the photoprotection of these ferns and, if so, whether CEF stimulation is related to leaf anatomy. Compared with P. decurrens, M. punctatum had thicker leaves and cuticles and higher water storage capacity, but lower stomatal density and slower rate of water loss. During induced drought, the decrease in leaf water potential (Ψ_leaf) was more pronounced in P. decurrens than in M. punctatum. For both species, the decline in Ψ_leaf was associated with a lower effective PSII quantum yield, photochemical quantum yield of PSI and electron transport rate (ETR), whereas increases were found in the quantum yield of regulated energy dissipation, CEF and CEF/ETR(II) ratio. Values for CEF and the CEF/ETR(II) ratio peaked in M. punctatum at a light intensity of 500–600 µmol m^?2 s^?1 vs only 150–200 µmol m^?2 s^?1 in P. decurrens. Therefore, our results indicate that the stimulation of CEF in tropical ferns contributes to their photoprotection under water stress, and is related to their respective drought tolerance and leaf anatomy.     

Effect of ATP on the Content of Inactive PSII Complexes in Chlorella

A prolonged (20 h) dark incubation of Chlorella pyrenoidosa algae at 37°C resulted in a twofold rise of the slowly rising phase (10–15 min), sF _v, in the kinetics of variable chlorophyll fluorescence, F _v (F _v = F _m – F ₀) in diuron-treated cells. This effect suggests the accumulation of inactive photosystem II (PSII) complexes with low efficiency of primary quinone acceptor of electron of PSII (Q_A) reduction. The presence of methylamine (MA), a thylakoid membrane uncoupler, or N, N-dicyclohexylcarbodiimide, an inhibitor of ATPase, precluded the accumulation of inactive PSII complexes. When salicylhydroxamate promoted the reduction of the plastoquinone (PQ) pool, exogenous ATP accelerated the accumulation of inactive complexes. Dark PQ oxidation in the presence of nonmetabolized glucose analog, 2-deoxy-D-glucose, lowered the content of inactive PSII complexes, and NaF, an inhibitor of chloroplast phosphatases, retarded this process. These data are considered as evidence for a mechanism regulating the content of inactive PSII complexes in the process of redox-dependent phosphorylation of D1- and/or D2-proteins of PSII.     

氮调控对盐环境下甜菜功能叶光系统Ⅱ荧光特性的影响

采用盆栽试验方法,以NaCl为盐分模拟不同盐度环境,研究了施氮(N)对盐环境下生长的甜菜(Beta vulgaris)功能叶光系统Ⅱ (PSⅡ)荧光特性的影响及光合色素含量的变化.结果表明:在轻度、中度及重度盐环境下,施N均能增大PSⅡ最大光化学效率(Fv/Fm)、PSⅡ潜在活性(Fv/Fo)、PSⅡ实际光量子产量(Y(Ⅱ))、非调节性能量耗散的量子产量(Y(NO))、相对电子传递速率(ETR)及光化学猝灭系数(qp),且在适宜的施N范围内(0-1.2 g·kg-1)上述参数随施N量的增加而增大.各叶绿素荧光参数光响应的结果表明,随着光强的增加,各处理下调节性能量耗散的量子产量(KNPQ))、ETR及非光化学猝灭系数(NPQ)旱上升趋势,相反,Y(Ⅱ)、Y(NO)及qp则呈下降趋势,在有效的光强范围内(0-1 000 μmol·m-2·s-1)施N提高了甜菜功能叶PSⅡ反应中心的开放程度,并且在高光强下调节PSⅡ耗散掉过剩的光能以避免对其反应中心造成伤害.各盐度环境下施N也显著增加了甜菜功能叶叶绿素与类胡萝卜素含量,增大了叶绿素a/叶绿素b值,且叶绿素与类胡萝卜素含量随施N水平的增加而增加.说明盐环境下施N能够增强甜菜功能叶PSⅡ的活性,提高PSⅡ光能利用率,从而增强其对盐渍环境的适应性.     

Effects of low night temperature on pigments,chl <Emphasis Type="Italic">a</Emphasis> fluorescence and energy allocation in two bitter gourd (<Emphasis Type="Italic">Momordica charantia</Emphasis> L.) genotypes

Using two different inbred lines of Momordica charantia (bitter gourd), Y-106-5 and Z-1-4, the cell membrane stability, leaf water potential, pigment contents and the chlorophyll a fluorescence were investigated with different low night temperature (LNT) treatments over a 7 day time period and the sequent a 7 day recovery. Under LNT treatments, electrolyte leakage increased in both inbred lines and it increased more significantly in Y-106-5 plants than that in Z-1-4. The content of Chl b and total Chl decreased, while the Chl a/b ratio increased in stressed plants of the two lines. Almost all LNT treatments induced little change in Chl a content in Z-1-4 whereas obvious decreases in 5 and 8°C treated Y-106-5 plants were observed. Chilling changed the water status of plants and induced decreases of leaf water potential (LWP) in 5 and 8°C treated plants. LNT treatments also resulted in changes in the chlorophyll fluorescence parameters in bitter gourd leaves. The potential PSII activity (F _v/F _o) was reduced obviously by LNT stress and showed more sensitive to LNT than the maximum quantum efficiency of PSII primary photochemistry (F _v/F _m). The efficiency of open PSII centers exhibited a slight decrease whereas the photochemical quenching efficient (q _P) was affected more seriously by LNT stress in both two inbred lines. The allocation of energy was rearranged by LNT stress. The light fraction used for PSII photochemistry (P) was reduced, while that used for heat dissipation (D) and the third fraction of absorbed light defines excess energy (E) increased due to the chilling stress. The impacts of LNT stress on bitter gourd generally increased with the number of LNT chilling and the severe night chilling. Plants were little affected by 12°C night chilling and the most acute damage was found in 5°C night chilling treatments. A 7 day recovery mitigated the adverse effects of LNT for both lines and almost all LNT treated plants restored to control levels except 5°C night chilling treated Y-106-5 plants. The two lines have a variance in tolerance to LNT stress and display obvious differences of phenotypes under extreme conditions.     

Modulation of Exogenous Glutathione in Phytochelatins and Photosynthetic Performance Against Cd Stress in the Two Rice Genotypes Differing in Cd Tolerance

Greenhouse hydroponic experiments were conducted using Cd-sensitive (Xiushui63) and tolerant (Bing97252) rice genotypes to evaluate genotypic differences in response of photosynthesis and phytochelatins to Cd toxicity in the presence of exogenous glutathione (GSH). Plant height, chlorophyll content, net photosynthetic rate (Pn), and biomass decreased in 5 and 50 μM Cd treatments, and Cd-sensitive genotype showed more severe reduction than the tolerant one. Cadmium stress caused decrease in maximal photochemical efficiency of PSII (Fv/Fm) and effective PSII quantum yield [Y(II)] and increase in quantum yield of regulated energy dissipation [Y(NPQ)], with changes in Cd-sensitive genotype being more evident. Cadmium-induced phytochelatins (PCs), GSH, and cysteine accumulation was observed in roots of both genotypes, with markedly higher level in PCs and GSH on day 5 in Bing97252 compared with that measured in Xiushui63. Exogenous GSH significantly alleviated growth inhibition in Xiushui63 under 5 μM Cd and in both genotypes in 50 μM Cd. External GSH significantly increased chlorophyll content, Pn, Fv/Fm, and Y(II) of plants exposed to Cd, but decreased Y(NPQ) and the coefficient of non-photochemical quenching (qN). GSH addition significantly increased root GSH content in plants under Cd exposure (except day 5 of 50 μM Cd) and induced up-regulation in PCs of 5 μM-Cd-treated Bing97252 throughout the 15-day and Xiushui63 of 5-day exposure. The results suggest that genotypic difference in the tolerance to Cd stress was positively linked to the capacity in elevation of GSH and PCs, and that alleviation of Cd toxicity by GSH is related to significant improvement in chlorophyll content, photosynthetic performance, and root GSH levels.     

Cyclic electron flow plays an important role in photoprotection for the resurrection plant <Emphasis Type="Italic">Paraboea</Emphasis><Emphasis Type="Italic">rufescens</Emphasis> under drought stress

Resurrection plants could survive severe drought stress, but the underlying mechanism for protecting their photosynthetic apparatus against drought stress is unclear. Cyclic electron flow (CEF) has been documented as a crucial mechanism for photoprotection in Arabidopsis and tobacco. We hypothesized that CEF plays an important role in protecting photosystem I (PSI) and photosystem II (PSII) against drought stress for resurrection plants. To address this hypothesis, the effects of mild drought stress on light energy distribution in PSII and P700 redox state were examined in a resurrection plant Paraboea rufescens. Cyclic electron flow was not activated below the photosynthetic photon flux density (PPFD) of 400 μmol m⁻² s⁻¹ in leaves without drought stress. However, CEF was activated under low light in leaves with mild drought stress, and the effective quantum yield of PSII significantly decreased. Meanwhile, non-photochemical quenching (NPQ) was significantly stimulated not only under high light but also under low light. Compared with the control, the fraction of overall P700 that cannot be oxidized in a given state (PSI acceptor side limitation) under high light was maintained at low level of 0.1 in leaves with water deficit, indicating that the over-reduction of the PSI acceptor side was prevented by the significant stimulation of CEF. Furthermore, methyl viologen could significantly increase the PSII photo-inhibition induced by high light compared with chloramphenicol. These results suggested that CEF is an important mechanism for protecting PSI and PSII from drought stress in resurrection plants.     

Photoinhibition-induced reduction in photosynthesis is alleviated by abscisic acid,cytokinin and brassinosteroid in detached tomato leaves

Detached leaves of tomato (Lycopersicon esculentum Mill.) experienced photoinhibition associated with sharp reductions in net photosynthetic rate (Pn), quantum efficiency of PSII (Φ_PSII) and photochemical quenching (qP) even though they were exposed to mild light intensity (400 μmol m⁻² s⁻¹ PPFD) at 28°C. Photoinhibition and the reduction in Pn, Φ_PSII and qP, however, were significantly alleviated by 1 mg l⁻¹ ABA, 0.1 mg l⁻¹ N-(2-chloro-4-pyridyl)-N′-phenylurea (CPPU) and 0.01 mg l⁻¹ 24-epibrassinolide (EBR). Higher concentrations, however, reduced the effects or even exacerbated the occurrence of photoinhibition. Superoxide dismutase and ascorbate peroxidase activity in leaves increased with the increases in ABA concentration within 1–100 mg l⁻¹, CPPU concentration within 0.1–10 mg l⁻¹ and EBR concentration within 0.01–1.0 mg l⁻¹. Catalase and guaiacol peroxidase activity also increased with the increase in EBR concentration but CPPU and ABA treatments at higher concentrations caused a decrease. Malondialdehyde (MDA) content decreased with the increase in CPPU concentration. ABA and EBR, however, decreased MDA concentration only at 1 and 0.01 mg l⁻¹, respectively. In conclusion, detached leaves had increased sensitivity to PSII photoinhibition. Photoinhibition-induced decrease in photosynthesis, however, was significantly alleviated by EBR, CPPU and ABA at a proper concentration.     

The Effect of Calcium Chloride on Growth,Photosynthesis, and Antioxidant Responses of Zoysia japonica under Drought Conditions

Few attempts have been made to study the alleviating effects of signal molecules on zoysiagrass ( Zoysia japonica ) under drought stress. Calcium chloride has been shown to ameliorate the adverse effects of drought stress on many plants. It is necessary to investigate how to enhance drought tolerance of zoysiagrass using calcium chloride. The study elucidated the effects of calcium chloride on zoysiagrass under drought conditions by investigating the following parameters: biomass, chlorophyll (Chl) content, net photosynthetic rate (Pn), chlorophyll fluorescence, antioxidant enzymes, proline content, and malondialdehyde (MDA) content. Experimental conditions consisted of an aqueous CaCl₂ solution at 5, 10, and 20 mM sprayed on zoysiagrass leaves for 3 d, following by an inducement of drought conditions by withholding water for 16 d. Under drought conditions, all CaCl₂ pretreatments were found to increase the above-ground fresh biomass, as well as below-ground fresh and dry biomass. The resulting Chl (a, b, a+b) contents of the 5 and 10 mM CaCl₂ pretreatment groups were higher than those of the control. In the later stages of drought conditions, the chlorophyll fluorescence parameter Fv/Fm was higher in leaves treated with 10 mM CaCl₂ than in the leaves of the other two treatment groups. Zoysiagrass pretreated with 10 mM CaCl₂ possessed both the maximum observed Pn and antioxidant enzyme activities. Meanwhile, lower MDA and proline contents were recorded in the plants pretreated with 5 and 10 mM CaCl₂ under drought conditions. As a whole, the drought tolerance of zoysiagrass was improved to some extent by the application of a moderate calcium concentration.     

Evidence for leaf fold to remedy the deficiency of physiological photoprotection for photosystem II

An interesting phenomenon is that some light-demanding plants fold their leaves when exposed to high light. Since high light could induce selective photodamage to photosystem II (PSII), we suggest that the leaves fold themselves to diminish the absorption of light energy and remedy the deficiency of physiological photoprotection for PSII. To test this hypothesis, we determined light responses of non-photochemical quenching (NPQ) and cyclic electron flow (CEF) and the effect of high light on PSII activity in Microcos paniculata (non-foldable species) and Bauhinia tenuiflora (foldable species). Under high light B. tenuiflora showed much lower NPQ and CEF than M. paniculata. Meanwhile, the excess light energy that cannot be harmlessly dissipated in B. tenuiflora was more compared with that in M. paniculata. After exposure to a high light of 1,900 μmol photons m⁻² s⁻¹ for 2 h, the maximum quantum yield of PSII, as estimated by variable to maximal fluorescence (F _v /F _m) decreased from 0.7 to 0.52 in the foldable species B. tenuiflora but was stable at 0.7 in the nonfoldable species M. paniculata. These results indicate that the foldable species B. tenuiflora has more sensitivity of PSII to high light stress than the nonfoldable species M. paniculata, partly as a result of less CEF and NPQ in B. tenuiflora. Our results suggest that sun leaves fold themselves under high light to remedy the deficiency of physiological photoprotection for PSII.     

Comparative chloroplast proteome analysis of exogenously supplied trehalose to wheat seedlings under heat stress

The aim of our study was to investigate the underlying molecular mechanisms of exogenously supplied trehalose affecting wheat photosynthesis under heat stress. The amount of ATP synthase (ATPase), oxygen-evolving enhancer protein (OEE), PsbP, Rubisco, chloroplast fructose-bisphosphate aldolase (FBPA), and ferredoxin-NADP(H) oxidoreductase (FNR) were downregulated, while PSI reaction center subunits were upregulated under heat stress. However, in the trehalose-pretreated groups, the amount of FNR, cytochrome b₆f complex, PSI reaction center subunits, ATPase, FBPA, and Rubisco were upregulated under normal growth conditions and heat stress. Besides, during the recovery period, the upregulation in CAB, PsbP, OEE2, and ATPase suggested that trehalose pretreatment might help to the recovery of PSII and PSI. These results indicate that trehalose pretreatment effectively regulates the levels of the photosynthesis-related proteins and relieves the damage of heat stress to wheat chloroplast.     

Photosystem II activity of wild type <Emphasis Type="Italic">Synechocystis</Emphasis> PCC 6803 and its mutants with different plastoquinone pool redox states

To assess the role of redox state of photosystem II (PSII) acceptor side electron carriers in PSII photochemical activity, we studied sub-millisecond fluorescence kinetics of the wild type Synechocystis PCC 6803 and its mutants with natural variability in the redox state of the plastoquinone (PQ) pool. In cyanobacteria, dark adaptation tends to reduce PQ pool and induce a shift of the cyanobacterial photosynthetic apparatus to State 2, whereas illumination oxidizes PQ pool, leading to State 1 (Mullineaux, C. W., and Holzwarth, A. R. (1990) FEBS Lett., 260, 245-248). We show here that dark-adapted Ox^– mutant with naturally reduced PQ is characterized by slower Q_A^– reoxidation and O₂ evolution rates, as well as lower quantum yield of PSII primary photochemical reactions (F_v/F_m) as compared to the wild type and SDH–mutant, in which the PQ pool remains oxidized in the dark. These results indicate a large portion of photochemically inactive PSII reaction centers in the Ox^– mutant after dark adaptation. While light adaptation increases F_v/F_m in all tested strains, indicating PSII activation, by far the greatest increase in F_v/F_m and O₂ evolution rates is observed in the Ox^– mutant. Continuous illumination of Ox^– mutant cells with low-intensity blue light, that accelerates Q_A^– reoxidation, also increases F_v/F_m and PSII functional absorption cross-section (590 nm); this effect is almost absent in the wild type and SDH–mutant. We believe that these changes are caused by the reorganization of the photosynthetic apparatus during transition from State 2 to State 1. We propose that two processes affect the PSII activity during changes of light conditions: 1) reversible inactivation of PSII, which is associated with the reduction of electron carriers on the PSII acceptor side in the dark, and 2) PSII activation under low light related to the increase in functional absorption cross-section at 590 nm.     

Effects of salicylic acid on protein kinase activity and chloroplast D1 protein degradation in wheat leaves subjected to heat and high light stress

In the north of China, wheat plants are often stressed by heat and high light during grain-filling stage, which leads to injury in photosynthetic apparatus and decline in photosynthetic rate. In order to develop a method to protect photosynthetic apparatus in wheat leaves subjected to heat and high light stress, the effects of SA (salicylic acid) and FSBA (5′-p-fluorosulfonylbenzoyl adenosine) on PK (protein kinase) activity, D1 protein degradation and the performance of PSII were investigated in present work. Our results showed that PK activity enhanced under heat and high light stress and declined when stress was removed. FSBA pretreatment resulted in marked decreases in PK activity and D1 protein level, suggesting a correlationship between degradation of D1 protein and phosphorylation. After 2 h of stress, D1 protein level in water-pretreated leaves decreased to 79% of control and then recovered to 81% after 3 h of recovery. This clearly indicated that the damage of D1 protein induced by heat and high light stress was reversible. Compared to the control, SA pretreatment could not only increase PK activity, retard the degradation of D1 protein during heat and high light stress, but also accelerate the recovery of D1 protein level when the stress was removed. Correspondingly, F_v/F_m (maximum photochemical efficiency of PSII), Φ_PSII (actual photochemical efficiency of PSII), ETR (electron transfer rate) and Pn (net photosynthetic rate) in SA-treated leaves were higher than that in leaves of control under both stress and non-stress conditions. Taken together, our results revealed that SA pretreatment could significantly alleviate damages of heat and high light stress on D1 protein and PSII of wheat leaves, and accelerate restoration of photosynthetic function.     

Characterization of the wave phenomenon of flash-induced chlorophyll fluorescence in Chlamydomonas reinhardtii

Flash-induced chlorophyll fluorescence relaxation is a powerful tool to monitor the reoxidation reactions of the reduced primary quinone acceptor, Q_A^? by Q_B and the plastoquinone (PQ) pool, as well as the charge recombination reactions between the donor and acceptor side components of Photosystem II (PSII). Under certain conditions, when the PQ pool is highly reduced (e.g. in microaerobic conditions), a wave phenomenon appears in the fluorescence relaxation kinetics, which reflects the transient reoxidation and re-reduction of Q_A^? by various electron transfer processes, which in cyanobacteria is mediated by NAD(P)H dehydrogenase (NDH-1). The wave phenomenon was also observed and assigned to the operation of type 2 NAD(P)H dehydrogenase (NDH-2) in the green alga Chlamydomonas reinhardtii under hydrogen-producing conditions, which required a long incubation of algae under sulphur deprivation (Krishna et al. J Exp Bot 70 (21):6321–6336, 2019). However, the conditions that induce the wave remained largely uncharacterized so far in microalgae. In this work, we investigated the wave phenomenon in Chlamydomonas reinhardtii under conditions that lead to a decrease of PSII activity by applying hydroxylamine treatment, which impacts the donor side of PSII in combination with a strongly reducing environment of the PQ pool (microaerobic conditions). A similar wave phenomenon could be induced by photoinhibitory conditions (illumination with strong light in the presence of the protein synthesis inhibitor lincomycin). These results indicate that the fluorescence wave phenomenon is activated in green algae when the PSII activity decreases relative to Photosystem I (PS I) activity and the PQ pool is strongly reduced. Therefore, the fluorescence wave could be used as a sensitive indicator of altered intersystem electron transfer processes, e.g. under stress conditions.