Recovery of photosynthetic systems during rewetting is quite rapid in a terrestrial cyanobacterium, Nostoc commune

Recovery processes of photosynthetic systems during rewetting were studied in detail in a terrestrial, highly drought-tolerant cyanobacterium, Nostoc commune. With absorption of water, the weight of N. commune colony increased in three phases with half-increase times of about 1 min, 2 h and 9 h. Fluorescence intensities of phycobiliproteins and photosystem (PS) I complexes were recovered almost completely within 1 min, suggesting that their functional forms were restored very quickly. Energy transfer from allophycocyanin to the core-membrane linker peptide (L(CM)) was recovered within 1 min, but not that from L(CM) to PSII. PSI activity and cyclic electron flow around PSI recovered within 2 min, while the PSII activity recovered in two phases after a time lag of about 5 min, with half times of about 20 min and 2 h. Photosynthetic CO(2) fixation was restored almost in parallel with the first recovery phase of the PSII reaction center activity. Although the amount of absorbed water became more than 20 times the initial dry weight of the N. commune colony in the presence of sufficient water, about twice the initial dry weight was enough for recovery and maintenance of the PSII activity.     

Cold acclimation of Arabidopsis thaliana results in incomplete recovery of photosynthetic capacity, associated with an increased reduction of the chloroplast stroma

The effects of short-term cold stress and long-term cold acclimation on the light reactions of photosynthesis were examined in vivo to assess their contributions to photosynthetic acclimation to low temperature in Arabidopsis thaliana (L.) Heynh.. All photosynthetic measurements were made at the temperature of exposure: 23 degrees C for non-acclimated plants and 5 degrees C for cold-stressed and cold-acclimated plants. Three-day cold-stress treatments at 5 degrees C inhibited light-saturated rates of CO2 assimilation and O2 evolution by approximately 75%. The 3-day exposure to 5 degrees C also increased the proportion of reduced QA by 50%, decreased the yield of PSII electron transport by 65% and decreased PSI activity by 31%. In contrast, long-term cold acclimation resulted in a strong but incomplete recovery of light-saturated photosynthesis at 5 degrees C. The rates of light-saturated CO2 and O2 gas exchange and the in vivo yield of PSII activity under light-saturating conditions were only 35-40% lower, and the relative redox state of QA only 20% lower, at 5 degrees C after cold acclimation than in controls at 23 degrees C. PSI activity showed full recovery during long-term cold acclimation. Neither short-term cold stress nor long-term cold acclimation of Arabidopsis was associated with a limitation in ATP, and both treatments resulted in an increase in the ATP/NADPH ratio. This increase in ATP/NADPH was associated with an inhibition of PSI cyclic electron transport but there was no apparent change in the Mehler reaction activity in either cold-stressed or cold-acclimated leaves. Cold acclimation also resulted in an increase in the reduction state of the stroma, as indicated by an increased total activity and activation state of NADP-dependent malate dehydrogenase, and increased light-dependent activities of the major regulatory enzymes of the oxidative pentose-phosphate pathway. We suggest that the photosynthetic capacity during cold stress as well as cold acclimation is altered by limitations at the level of consumption of reducing power in carbon metabolism.     

Protection of photosystem I during sudden light stress depends on ferredoxin:NADP(H) reductase abundance and interactions

Plant tolerance to high light and oxidative stress is increased by overexpression of the photosynthetic enzyme Ferredoxin:NADP(H) reductase (FNR), but the specific mechanism of FNR-mediated protection remains enigmatic. It has also been reported that the localization of this enzyme within the chloroplast is related to its role in stress tolerance. Here, we dissected the impact of FNR content and location on photoinactivation of photosystem I (PSI) and photosystem II (PSII) during high light stress of Arabidopsis (Arabidopsis thaliana). The reaction center of PSII is efficiently turned over during light stress, while damage to PSI takes much longer to repair. Our results indicate a PSI sepcific effect, where efficient oxidation of the PSI primary donor (P700) upon transition from darkness to light, depends on FNR recruitment to the thylakoid membrane tether proteins: thylakoid rhodanase-like protein (TROL) and translocon at the inner envelope of chloroplasts 62 (Tic62). When these interactions were disrupted, PSI photoinactivation occurred. In contrast, there was a moderate delay in the onset of PSII damage. Based on measurements of ΔpH formation and cyclic electron flow, we propose that FNR location influences the speed at which photosynthetic control is induced, resulting in specific impact on PSI damage. Membrane tethering of FNR therefore plays a role in alleviating high light stress, by regulating electron distribution during short-term responses to light.

Altered location of a key enzyme involved in the post-photosystem I electron transport chain ameliorates damage to photosystem I during increasing light intensity.     

The different effects of chilling stress under moderate light intensity on photosystem II compared with photosystem I and subsequent recovery in tropical tree species

Tropical plants are sensitive to chilling temperatures above zero but it is still unclear whether photosystem I (PSI) or photosystem II (PSII) of tropical plants is mainly affected by chilling temperatures. In this study, the effect of 4°C associated with various light densities on PSII and PSI was studied in the potted seedlings of four tropical evergreen tree species grown in an open field, Khaya ivorensis, Pometia tomentosa, Dalbergia odorifera, and Erythrophleum guineense. After 8 h chilling exposure at the different photosynthetic flux densities of 20, 50, 100, 150 μmol m⁻² s⁻¹, the maximum quantum yield of PSII (F _v /F _m) in all of the four species decreased little, while the quantity of efficient PSI complex (P _m) remained stable in all species except E. guineense. However, after chilling exposure under 250 μmol m⁻² s⁻¹ for 24 h, F _v /F _m was severely photoinhibited in all species whereas P _m was relative stable in all plants except E. guineense. At the chilling temperature of 4°C, electron transport from PSII to PSI was blocked because of excessive reduction of primary electron acceptor of PSII. F _v /F _m in these species except E. guineense recovered to ~90% after 8 h recovery in low light, suggesting the dependence of the recovery of PSII on moderate PSI and/or PSII activity. These results suggest that PSII is more sensitive to chilling temperature under the moderate light than PSI in tropical trees, and the photoinhibition of PSII and closure of PSII reaction centers can serve to protect PSI.     

Photosystem II photoinhibition-repair cycle protects Photosystem I from irreversible damage

Photodamage of Photosystem II (PSII) has been considered as an unavoidable and harmful reaction that decreases plant productivity. PSII, however, has an efficient and dynamically regulated repair machinery, and the PSII activity becomes inhibited only when the rate of damage exceeds the rate of repair. The speed of repair is strictly regulated according to the energetic state in the chloroplast. In contrast to PSII, Photosystem I (PSI) is very rarely damaged, but when occurring, the damage is practically irreversible. While PSII damage is linearly dependent on light intensity, PSI gets damaged only when electron flow from PSII exceeds the capacity of PSI electron acceptors to cope with the electrons. When electron flow to PSI is limited, for example in the presence of DCMU, PSI is extremely tolerant against light stress. Proton gradient (ΔpH)-dependent slow-down of electron transfer from PSII to PSI, involving the PGR5 protein and the Cyt b6f complex, protects PSI from excess electrons upon sudden increase in light intensity. Here we provide evidence that in addition to the ΔpH-dependent control of electron transfer, the controlled photoinhibition of PSII is also able to protect PSI from permanent photodamage. We propose that regulation of PSII photoinhibition is the ultimate regulator of the photosynthetic electron transfer chain and provides a photoprotection mechanism against formation of reactive oxygen species and photodamage in PSI.     

Recovery of photosynthetic reactions after high-temperature treatments of a heat-tolerant cactus

Inhibition and recovery of net CO₂ uptake and three photosynthetic electron transport reactions as well as plant survival following high-temperature treatments were investigated for Opuntia ficus-indica. For plants maintained at 30°C/20°C day/night air temperatures, treatment at 60°C for 1 h irreversibly inhibited net CO₂ uptake and photosynthetic electron transport, resulting in plant death in about 60 days. When a plant maintained at 30°C/20°C was treated at 55°C for 1 h, net CO₂ uptake was completely inhibited 1 d after the treatment but fully recovered in 60 d. Differential inactivation of photosystem (PS) I, PSII, and whole chain electron transport activities occurred; PSI was the most tolerant of 55°C and took the least time (45 d) for total recovery. All 30°C/20°C plants survived a 1-h treatment at 55°C, although some pale green areas were observed on the cladode surfaces. In contrast to growing at 30°C/20°C, plants acclimated to 45°C/35°C survived 60°C for 1 h without showing any necrotic or pale green areas on the cladode surfaces. When such a plant was transferred to 30°C/20°C following the high-temperature treatment, recovery in net CO₂ uptake began in 1 d and progressed to complete recovery by 30 d. Growth temperatures thus influence the possibility for recovery of photosynthetic reactions and ultimately the survival of O. ficus-indica following a high-temperature exposure.Abbreviations DCPIP 2,6-dichlorophenol indophenol - MV methyl viologen - PAR photosynthetically active radiation - PSI or PSII photosystem I or II - WC whole chain     

Enhanced function of non-photoinhibited photosystem II complexes upon PSII photoinhibition

Light induced photosystem (PS)II photoinhibition inactivates and irreversibly damages the reaction center protein(s) but the light harvesting complexes continue the collection of light energy. Here we addressed the consequences of such a situation on thylakoid light harvesting and electron transfer reactions. For this purpose, Arabidopsis thaliana leaves were subjected to investigation of the function and regulation of the photosynthetic machinery after a distinct portion of PSII centers had experienced photoinhibition in the presence and absence of Lincomycin (Lin), a commonly used agent to block the repair of damaged PSII centers. In the absence of Lin, photoinhibition increased the relative excitation of PSII and decreased NPQ, together enhancing the electron transfer from still functional PSII centers to PSI. In contrast, in the presence of Lin, PSII photoinhibition increased the relative excitation of PSI and led to strong oxidation of the electron transfer chain. We hypothesize that plants are able to minimize the detrimental effects of high-light illumination on PSII by modulating the energy and electron transfer, but lose such a capability if the repair cycle is arrested. It is further hypothesized that dynamic regulation of the LHCII system has a pivotal role in the control of excitation energy transfer upon PSII damage and repair cycle to maintain the photosynthesis safe and efficient.     

Study of tobacco transformants to assess the role of chloroplastic NAD(P)H dehydrogenase in photoprotection of photosystems I and II

Nicotiana tabacum L. wild-type plants and transformants (DeltandhCKJ), deficient in functional NAD(P)H dehydrogenase (NDH), were subjected to high light at 20 degrees C and 4 degrees C for 2 h to examine a possible role of NDH-mediated cyclic electron flow in protecting photosystems I and II from photoinhibition. Photochemical activity of photosystem I (PSI) was assessed by means of P700 absorbance changes at 810 nm. In addition, potential photosystem II (PSII) efficiency was determined by measuring the 'dark-adapted' ratio of variable to maximum chlorophyll fluorescence, F(v)/ F(m). Both photosystems were more susceptible to photoinhibition at 4 degrees C than at 20 degrees C. However, the degree of photoinhibition was not less in the wild type than in the NDH-deficient plants. To evaluate the efficiency of P700 oxidation in far-red light, a saturation constant, K(s), was determined, representing the far-red irradiance at which half of the maximum P700 absorbance change was reached. In photoinhibited leaves, a decrease in the efficiency of P700 oxidation (increase in K(s)) was observed. The increase in K(s) was more pronounced at 4 degrees C than at 20 degrees C, but not significantly different between wild-type and DeltandhCKJ plants. Re-reduction kinetics of oxidised P700 in the dark were accelerated to a similar extent in photoinhibited samples of both genotypes and at the two temperatures tested. The data indicate that NDH-mediated cyclic electron flow does not protect PSI against short-term light stress. It is proposed that the observed increase in K(s) represents a protective mechanism that is based on accelerated charge recombination in PSI and facilitates thermal dissipation of excessive light energy.     

Electron flow to photosystem I from stromal reductants in vivo: the size of the pool of stromal reductants controls the rate of electron donation to both rapidly and slowly reducing photosystem I units

Electron donation from stromal reductants to photosystem I (PSI) was studied using the kinetics of P700(+) (the oxidized primary donor of PSI) reduction in the dark after irradiation of barley ( Hordeum vulgare L.) leaves. The leaves were treated with diuron and methyl viologen to abolish both the electron flow from PSII and PSI-driven cyclic electron transport. The redox state of P700 was monitored using the absorbance changes at 830 nm (Delta A(830)). Two exponentially decaying components with half-times of about 3 s (the slow component) and about 0.6 s (the fast one) were distinguished in the kinetic curves of Delta A(830) relaxation after a 1-s pulse of far-red light. The complex kinetics of P700(+) reduction thus manifested two types of PSI unit differing in the rate of electron input from stromal reductants. The rates of both kinetic components assayed after 1-s pulses were increased about 20-fold by a short (2-5 min) heat-pretreatment of leaves, indicating the accelerated input of electrons to both types of PSI unit. The increased rates of electron flow to P700(+) were even observed 1.5 h after the action of heat had been completed. Both kinetic components were dramatically slowed down upon irradiation of heat-treated leaves for 20-30 s. Their rates were restored after a short (20-30 s) period of darkness. A 5-min leaf exposure at 38 degrees C was sufficient to stimulate by severalfold the reduction of P700(+) pre-oxidized by a brief light pulse. In contrast, the acceleration of P700(+) reduction after a 1-min irradiation was observed only if leaves were subjected to temperatures above 40 degrees C. Neither heat treatment of leaves nor light-dark modulations in the rates of the fast and the slow components of P700(+) dark reduction influenced the relative magnitudes of the two kinetic components, providing strong additional evidence in favor of two distinct types of PSI existing per se in barley leaves. The key role in the control of the activity of electron donation to P700(+) in both rapidly and slowly reducing PSI units was attributed to the amount of stromal reductants available for P700(+) reduction. The latter was expected to be reduced under illumination in the presence of methyl viologen, while increased again in the dark. The regeneration of the pool of stromal reductants in the dark was likely provided by starch breakdown within the chloroplast stroma, but not by import of reducing equivalents from the cytosol. This was evidenced by much lower rates, compared with 1-h dark-adapted leaves, of dark reduction of both components of P700(+) in leaves stored for 24 h in the dark and thus depleted of starch but containing large amounts of glucose, the respiratory substrate.     

High root temperature blocks both linear and cyclic electron transport in the dark during chilling of the leaves of rice seedlings

The most photosynthetically active leaves of rice seedlings were severely damaged when shoots but not roots were chilled (10°C/25°C, respectively), but no such injury was observed when the whole seedling was chilled (10°C/10°C). To elucidate the mechanisms, we compared the photosynthetic characteristics of the seedlings during the dark chilling treatments. Simultaneous analyses of Chl fluorescence and the change in absorbance of P700 showed that electron transport almost disappeared in both PSII and PSI in the 10°C/25°C leaves, whereas the electron transport rate in PSI in the 10°C/10°C leaves was similar to or higher than that in non-chilled control leaves. Light-induced non-photochemical quenching in PSII was inhibited in the 10°C/25°C leaves, occurring at only half the level in the 10°C/10°C leaves, whereas non-light-induced non-photochemical quenching remained high in the 10°C/25°C leaves. The light induction of Chl a fluorescence (OJIP curves) in the 10°C/25°C leaves was similar to that in leaves treated with DCMU. The fluorescence decay after a single turnover saturating flash in the 10°C/25°C leaves was much slower than in the 10°C/10°C leaves. In vivo analyses of the 550-515 nm difference signal indicated decreased formation of a proton gradient across the thylakoid membrane and decreased zeaxanthin formation in the 10°C/25°C leaves. Our results suggest that electron transport was blocked between Q(A) and Q(B) in the dark 10°C/25°C leaves, but without irreversible damage to the components of this system. The consequent light-dependent losses of electron transport, proton gradient formation across the thylakoids and thermal dissipation may therefore be responsible for the visible injury.     

Light-dependent reversal of dark-chilling induced changes in chloroplast structure and arrangement of chlorophyll-protein complexes in bean thylakoid membranes

Changes in chloroplast structure and rearrangement of chlorophyll-protein (CP) complexes were investigated in detached leaves of bean (Phaseolus vulgaris L. cv. Eureka), a chilling-sensitive plant, during 5-day dark-chilling at 1 degrees C and subsequent 3-h photoactivation under white light (200 mumol photons m(-2) s(-1)) at 22 degrees C. Although, no change in chlorophyll (Chl) content and Chl a/b ratio in all samples was observed, overall fluorescence intensity of fluorescence emission and excitation spectra of thylakoid membranes isolated from dark-chilled leaves decreased to about 50%, and remained after photoactivation at 70% of that of the control sample. Concomitantly, the ratio between fluorescence intensities of PSI and PSII (F736/F681) at 120 K increased 1.5-fold upon chilling, and was fully reversed after photoactivation. Moreover, chilling stress seems to induce a decrease of the relative contribution of LHCII fluorescence to the thylakoid emission spectra at 120 K, and an increase of that from LHCI and PSI, correlated with a decrease of stability of LHCI-PSI and LHCII trimers, shown by mild-denaturing electrophoresis. These effects were reversed to a large extent after photoactivation, with the exception of LHCII, which remained partly in the aggregated form. In view of these data, it is likely that dark-chilling stress induces partial disassembly of CP complexes, not completely restorable upon photoactivation. These data are further supported by confocal laser scanning fluorescence microscopy, which showed that regular grana arrangement observed in chloroplasts isolated from control leaves was destroyed by dark-chilling stress, and was partially reconstructed after photoactivation. In line with this, Chl a fluorescence spectra of leaf discs demonstrated that dark-chilling caused a decrease of the quantum yield PSII photochemistry (F(v)/F(m)) by almost 40% in 5 days. Complete restoration of the photochemical activity of PSII required 9 h post-chilling photoactivation, while only 3 h were needed to reconstruct thylakoid membrane organization and chloroplast structure. The latter demonstrated that the long-term dark-chilled bean leaves started to suffer from photoinhibition after transfer to moderate irradiance and temperature conditions, delaying the recovery of PSII photochemistry, independently of photo-induced reconstruction of PSII complexes.     

The site of photoinhibition in leaves of Cucumis sativus L. at low temperatures is photosystem I,not photosystem II

Maximum quantum yields (QY) of photosynthetic electron flows through PSI and PSII were separately assessed in thylakoid membranes isolated from leaves of Cucumis sativus L. (cucumber) that had been chilled in various ways. The QY(PSI) in the thylakoids prepared from the leaves treated at 4° C in moderate light at 220 mol quanta·m^–2·s^–1 (400–700 nm) for 5 h, was about 20–30% of that in the thylakoids prepared from untreated leaves, while QY(PSII) decreased, at most, by 20% in response to the same treatment. The decrease in QY(PSI) was observed only when the leaves were chilled at temperatures below 10° C, while such a marked temperature dependency was not observed for the decrease in QY(PSII). In the chilling treatment at 4° C for 5 h, the quantum flux density that was required to induce 50% loss of QY (PSI) was ca. 50 umol quanta·m^–2·s^–1. When the chilling treatment at 4° C in the light was conducted in an atmosphere of N₂, photoinhibition of PSI was largely suppressed, while the damage to PSII was somewhat enhanced. The ferricyanide-oxidised minus ascorbate-reduced difference spectra and the light-induced absorbance changes at 700 nm obtained with the thylakoid suspension, indicated the loss of P700 to extents that corresponded to the decreases in QY(PSI). Accordingly, the decreases in QY(PSI) can largely be attributed to destruction of the PSI reaction centre itself. These results clearly show that, at least in cucumber, a typical chillingsensitive plant, PSI is much more susceptible to aerobic photoinhibition than PSII.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - P700 primary electron donor of PSI - PPFD photosynthetically active photon flux density - QY quantum yield We are grateful to invaluable comments by Prof. S. Katoh, K. Hikosaka and the members of our laboratory. We also thank A. Aoyama for technical assistance. This work was partly supported by the grants from the Ministry of Education, Science, and Culture, Japan, to I. Terashima (#03740342 and #04640621).     

The physiological links of the increased photosystem II activity in moderately desiccated Porphyra haitanensis (Bangiales, Rhodophyta) to the cyclic electron flow during desiccation and re-hydration

Photosynthetic electron flow changed considerably during desiccation and re-hydration of the intertidal macroalgae Porphyra haitanensis. Activities of both photosystem (PSI) and photosystem (PSII) increased significantly at moderate desiccation levels. Whereas PSII activity was abolished at an absolute water content (AWC) <24 %, PSI remained active with progressive decreases in AWC to values as low as 16 %. This result suggested that cyclic electron flow around PSI was still active after inactivation of linear electron flow following severe desiccation. Moreover, the PSI activity was restored more rapidly than that of PSII upon re-hydration. Pretreatment of the blades with 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) suppressed PSII activity following desiccation to an AWC of ~16 % AWC. Cyclic electron flow around PSI decreased markedly in blades pretreated with DCMU than in blades without pretreatment of DCMU during re-hydration in seawater containing DCMU. All results suggested that the activity of PSII under desiccation conditions plays an important role in the operation of cyclic electron flow during desiccation and its recovery during re-hydration. Therefore, we proposed the PSII activity during desiccation could eventually lead to the accumulation of NADPH, which could serve as electron donor for P700⁺ and promote its recovery during re-hydration, thereby favoring the operation of cyclic electron flow.     

Xanthophyll Cycle and Inactivation of Photosystem Ⅱ Reaction Centers Alleviating Reducing Pressure to Photosystem Ⅰ in Morning Glory Leaves under Short-term High Irradiance

investigated through chlorophyll fluorescence parameters in morning glory (Ipomoea setosa) leaves, which were dipped into water, dithiothreitol (DTT) and lincomycin (LM), respectively. During the stress, both the xanthophyll cycle and D1 protein turnover could protect PSI from photoinhibition. In DTT leaves, non-photochemical quenching (NPQ) was inhibited greatly and the oxidation level of P700 (P700+) was the lowest one. However, the maximal photochemical efficiency of PSII (Fv/Fm) in DTT leaves was higher than that of LM leaves and was lower than that of control leaves. These results suggested that PSI was more sensitive to the loss of the xanthophyll cycle than PSII under high irradiance. In LM leaves, NPQ was partly inhibited, Fv/Fm was the lowest one among three treatments under high irradiance and P700+ was at a similar level as that of control leaves. These results implied that inactivation of PSII reaction centers could protect PSI from further photoinhibition. Additionally, the lowest of the number of active reaction centers to one inactive reaction center for a PSII cross-section (RC/CSo), maximal trapping rate in a PSII cross-section (TRo/CSo), electron transport in a PSII cross-section (ETo/CSo) and the highest of 1-qP in LM leaves further indicated that severe photoinhibition of PSII in LM leaves was mainly induced by inactivation of PSII reaction centers, which limited electrons transporting to PSI. However, relative to the LM leaves the higher level of RC/CSo, TRo/CSo, Fv/Fm and the lower level of 1-qP in DTT leaves indicated that PSI photoinhibition was mainly induced by the electron accumulation at the PSI acceptor side, which induced the decrease of P700+ under high irradiance.     

NaCl胁迫加重强光胁迫下超大甜椒叶片的光系统II和光系统I的光抑制

快速叶绿素荧光动力学可以在无损情况下探知叶片光合机构的损伤程度, 快速叶绿素荧光测定和分析技术(JIP-test)将测量值转化为多种具有生物学意义的参数, 因而被广泛应用于植物光合机构对环境的响应机制研究。该文研究了超大甜椒(Capsicum annuum)幼苗在强光及不同NaCl浓度胁迫下的荧光响应情况。与单纯强光胁迫相比, NaCl胁迫引起了叶绿素荧光诱导曲线的明显改变, 光系统II (PSII)光抑制加重, 同时PSII反应中心和受体侧受到明显影响, 而且高NaCl浓度胁迫下PSII供体侧受伤害明显, 同时PSI反应中心活性(P700⁺)在盐胁迫下明显降低。这些结果表明, NaCl胁迫会增强强光对超大甜椒光系统的光抑制, 并且浓度越高抑制越明显, 但对PSI的抑制作用低于PSII。高NaCl浓度胁迫易对PSII供体侧造成破坏, 且PSI光抑制严重。     

Cold acclimation of the Arabidopsis dgd1 mutant results in recovery from photosystem I-limited photosynthesis

We compared the thylakoid membrane composition and photosynthetic properties of non- and cold-acclimated leaves from the dgd1 mutant (lacking >90% of digalactosyl-diacylglycerol; DGDG) and wild type (WT) Arabidopsis thaliana. In contrast to warm grown plants, cold-acclimated dgd1 leaves recovered pigment-protein pools and photosynthetic function equivalent to WT. Surprisingly, this recovery was not correlated with an increase in DGDG. When returned to warm temperatures the severe dgd1 mutant phenotype reappeared. We conclude that the relative recovery of photosynthetic activity at 5 degrees C resulted from a temperature/lipid interaction enabling the stable assembly of PSI complexes in the thylakoid.     

Photosynthetic electron transport and proton flux under moderate heat stress

Moderate heat stress has been reported to increase PSI cyclic electron flow (CEF). We subjected leaves of Arabidopsis (Arabidopsis thaliana) mutants disrupted in the regulation of one or the other pathway of CEF flow—crr2 (chlororespiratory reduction, deficient in regulation of chloroplast NAD(P)H dehydrogenase-dependent CEF) and pgr5 (proton gradient regulation, proposed to have reduced efficiency of antimycin-A-sensitive-CEF regulation) to moderate heat stress. Light-adapted leaves were switched from 23 to 40°C in 2 min. Gas exchange, chlorophyll fluorescence, the electrochromic shift (ECS), and P700 were measured. Photosynthesis of crr2 and pgr5 was more sensitive to heat and had less ability to recover than the genetic background gl. The proton conductance in light was increased by heat and it was twice as much in pgr5, which had much smaller light-induced proton motive force. We confirmed that P700 becomes more reduced at high temperature and show that, in contrast, the proportion of PSII open centers (with Q _A oxidized) increases. The two mutants had much slower P700⁺ reduction rate during and after heat than gl. The proportion of light absorbed by PSI versus PSII was increased in gl and crr2 during and after heat treatment, but not in pgr5. We propose that heat alters the redox balance away from PSII and toward PSI and that the regulation of CEF helps photosynthesis tolerate heat stress.     

The difficulty of estimating the electron transport rate at photosystem I

It is still a controversial issue how the electron transport reaction is carried out around photosystem I (PSI) in the photosynthetic electron transport chain. The measurable component in PSI is the oxidized P700, the reaction center chlorophyll in PSI, as the absorbance changes at 820–830 nm. Previously, the quantum yield at PSI [Y(I)] has been estimated as the existence probability of the photo-oxidizable P700 by applying the saturated-pulse illumination (SP; 10,000–20,000 µmol photons m^?2 s^?1). The electron transport rate (ETR) at PSI has been estimated from the Y(I) value, which was larger than the reaction rate at PSII, evaluated as the quantum yield of PSII, especially under stress-conditions such as CO₂-limited and high light intensity conditions. Therefore, it has been considered that the extra electron flow at PSI was enhanced at the stress condition and played an important role in dealing with the excessive light energy. However, some pieces of evidence were reported that the excessive electron flow at PSI would be ignorable from other aspects. In the present research, we confirmed that the Y(I) value estimated by the SP method could be easily misestimated by the limitation of the electron donation to PSI. Moreover, we estimated the quantitative turnover rate of P700⁺ by the light-to-dark transition. However, the turnover rate of P700 was much slower than the ETR at PSII. It is still hard to quantitatively estimate the ETR at PSI by the current techniques.

     

Xanthophyll Cycle and Inactivation of Photosystem Ⅱ Reaction Centers Alleviating Reducing Pressure to Photosystem Ⅰ in Morning Glory Leaves under Short-term High Irradiance

Under 30-min high irradiance （1500μmol m^-2 s^-1）, the roles of the xanthophyll cycle and D1 protein turnover were investigated through chlorophyll fluorescence parameters in morning glory （Ipomoea setosa） leaves, which were dipped into water, dithiothreitol （DTT） and lincomycin （LM）, respectively. During the stress, both the xanthophyll cycle and D1 protein turnover could protect PSI from photoinhibition. In DTT leaves, non-photochemical quenching （NPQ） was inhibited greatly and the oxidation level of P700 （P700^＋） was the lowest one. However, the maximal photochemical efficiency of PSII （Fv/Fm） in DTT leaves was higher than that of LM leaves and was lower than that of control leaves. These results suggested that PSI was more sensitive to the loss of the xanthophyll cycle than PSII under high irradiance. In LM leaves, NPQ was partly inhibited, Fv/Fm was the lowest one among three treatments under high irradiance and P700^＋ was at a similar level as that of control leaves. These results implied that inactivation of PSII reaction centers could protect PSI from further photoinhibition. Additionally, the lowest of the number of active reaction centers to one inactive reaction center for a PSII cross-section （RC/CSo）, maximal trapping rate in a PSll cross-section （TRo/CSo）, electron transport in a PSll cross-section （ETo/CSo） and the highest of 1-qP in LM leaves further indicated that severe photoinhibition of PSII in LM leaves was mainly induced by inactivation of PSII reaction centers, which limited electrons transporting to PSh However, relative to the LM leaves the higher level of RC/CSo, TRo/CSo, Fv/Fm and the lower level of 1-qP in DTT leaves indicated that PSI photoinhibition was mainly induced by the electron accumulation at the PSI acceptor side, which induced the decrease of P700^＋ under high irradiance.