Acceptor Side Of PSII Research Articles

Response of Photosynthesis in Maize to Drought and Re-Watering

The mechanism by which photosynthetic adaptation occurs in maize (Zea mays L.) during both drought and the subsequent recovery after re-watering is currently unknown. To elucidate this mechanism, two maize cultivars (drought tolerant SD609 and drought sensitive SD902) were subjected to an eight-day drought, followed by re-watering for four days. The photosynthetic electron transport rates and related gene expression were measured in both cultivars. Compared to control plants, progressive drought stress significantly decreased electron transport rates at the donor and acceptor sides of PSI and PSII in SD902, and of PSII in SD609. Meanwhile, the expression of cab, psbP, psbA, psbD, petA, and petB genes involved in electron transport system were significantly down-regulated in both cultivars, particularly in the SD902. Moreover, while expression of psaA and psaB genes encoding PSI was down-regulated in SD902, there were no changes in SD609 during drought stress. After re-watering, measured fluorescence parameters and key photosynthesis-related gene expression levels returned to near control values in SD609, but not in SD902. This finding indicated that SD609 was characterized by reversible down-regulation of PSII, while in SD902, an impairment occurred in two photosystems, and such coordination between PSII and PSI contributed to drought tolerance in SD609. Overall, the higher drought tolerance and rapid recovery capability of SD609 were associated with more effective self-regulation of photochemical activities and photosynthesis-related gene expression, which appears to represent a critical adaptive mechanism to withstand and survive the rapidly changing climate.

Exogenous putrescine alleviates photoinhibition caused by salt stress through cooperation with cyclic electron flow in cucumber.

When plants suffer from abiotic stresses, cyclic electron flow (CEF) is induced for photo-protection. Putrescine (Put), a primary polyamine in chloroplasts, plays a critical role in stress tolerance. However, the relationship between CEF and Put in chloroplasts for photo-protection is unknown. In this study, we investigated the role of Put-induced CEF for salt tolerance in cucumber plants (Cucumis sativus L). Treatment with 90mM NaCl and/or Put did not influence the maximum photochemical efficiency of PSII (Fv/Fm), but the photoactivity of PSI was severely inhibited by NaCl. Salt stress induced a high level of CEF; moreover, plants treated with both NaCl and Put exhibited much higher CEF activity and ATP accumulation than those exhibited by single-salt-treated plants to provide an adequate ATP/NADPH ratio for plant growth. Furthermore, Put decreased the trans-membrane proton gradient (ΔpH), which was accompanied by reduced pH-dependent non-photochemical quenching (NPQ) and an increased the effective quantum yield of PSII (Y(II)). The ratio of NADP+/NADPH increased significantly with Put in salt-stressed leaves compared with the ratio in leaves treated with NaCl, indicating that Put relieved over-reduction pressure at the PSI acceptor side caused by salt stress. Collectively, our results suggest that exogenous Put creates an excellent condition for CEF promotion: a large amount of pmf is predominantly stored as Δψ, resulting in moderate lumen pH and low NPQ, while maintaining high rates of ATP synthesis (high pmf).

Analyzing both the fast and the slow phases of chlorophyll a fluorescence and P700 absorbance changes in dark-adapted and preilluminated pea leaves using a Thylakoid Membrane model.

The dark-to-light transitions enable energization of the thylakoid membrane (TM), which is reflected in fast and slow (OJIPSMT or OABCDE) stages of fluorescence induction (FI) and P700 oxidoreduction changes (ΔA810). A Thylakoid Membrane model (T-M model), in which special emphasis has been placed on ferredoxin-NADP+-oxidoreductase (FNR) activation and energy-dependent qE quenching, was applied for quantifying the kinetics of FI and ΔA810. Pea leaves were kept in darkness for 15min and then the FI and ΔA810 signals were measured upon actinic illumination, applied either directly or after a 10-s light pulse coupled with a subsequent 10-s dark interval. On the time scale from 40 µs to 30s, the parallel T-M model fittings to both FI and ΔA810 signals were obtained. The parameters of FNR activation and the buildup of qE quenching were found to differ for dark-adapted and preilluminated leaves. At the onset of actinic light, photosystem II (PSII) acceptors were oxidized (neutral) after dark adaptation, while the redox states with closed and/or semiquinone QA(-)QB(-) forms were supposedly generated after preillumination, and did not relax within the 10s dark interval. In qE simulations, a pH-dependent Hill relationship was used. The rate constant of heat losses in PSII antenna kD(t) was found to increase from the basic value kDconst, at the onset of illumination, to its maximal level kDvar due to lumenal acidification. In dark-adapted leaves, a low value of kDconst of ∼2×106s-1 was found. Simulations on the microsecond to 30s time scale revealed that the slow P-S-M-T phases of the fluorescence induction were sensitive to light-induced FNR activation and high-energy qE quenching. Thus, the corresponding time-dependent rate constants kD(t) and kFNR(t) change substantially upon the release of electron transport on the acceptor side of PSI and during the NPQ development. The transitions between the cyclic and linear electron transport modes have also been quantified in this paper.

High ammonium supply impairs photosynthetic efficiency in rice exposed to excess light.

Mechanisms involving ammonium toxicity, excess light, and photosynthesis are scarcely known in plants. We tested the hypothesis that high NH4+ supply in presence of high light decreases photosynthetic efficiency of rice plants, an allegedly tolerant species. Mature rice plants were previously supplied with 10mM NH4+ or 10mM NO3- and subsequently exposed to 400µmolm-2s-1 (moderate light-ML) or 2000µmolm-2s-1 (high light-HL) for 8h. HL greatly stimulated NH4+ accumulation in roots and in a minor extent in leaves. These plants displayed significant delay in D1 protein recovery in the dark, compared to nitrate-supplied plants. These responses were related to reduction of both PSII and PSI quantum efficiencies and induction of non-photochemical quenching. These changes were also associated with higher limitation in the donor side and lower restriction in the acceptor side of PSI. This later response was closely related to prominent decrease in stomatal conductance and net CO2 assimilation that could have strongly affected the energy balance in chloroplast, favoring ATP accumulation and NPQ induction. In parallel, NH4+ induced a strong increase in the electron flux to photorespiration and, inversely, it decreased the flux to Rubisco carboxylation. Overall, ammonium supply negatively interacts with excess light, possibly by enhancing ammonium transport towards leaves, causing negative effects on some photosynthetic steps. We propose that high ammonium supply to rice combined with excess light is capable to induce strong delay in D1 protein turnover and restriction in stomatal conductance, which might have contributed to generalized disturbances on photosynthetic efficiency.

Effect of Elevated CO2 Concentration On Four <i>Populus</i> by the Fast Fluorescence Rise OJIP

Increased atmospheric carbon dioxide (CO 2 ) can influence the stability of ecosystems in the world. The chlorophyll fluorescence technique was considered as an effective tool to evaluate the photosynthetic levels on many plant species. In this study, we analyzed the state of PSII on four kinds of populus ( Populus L.) ( populus X, Pop ulusdeltoides × cathayana , Poplus alba ′Berdinensis ′ L, Populus euramerican ′N3016′ × Populus ussuriensis ) under high CO 2 condition. The results show that the PSII performance was negatively influenced by CO 2 stress. High CO 2 stress resulted in down-regulation of Fm, φPo (=Fv/Fm), ψEo, φEo and PItotal in four kinds of populus . And a significant decrease in the P-step level of the fluorescence transients OJIP curves of four populus species after 7 days of treatment was observed. Therefore, a fast decrease of the P-step level indicated there was main change to fluorescence transients. These results indicated that changes of PSII attribute to the degradation of antenna pigment and inhibition of the electron transport at the acceptor side of PSII.

Increased CO2 concentrations increasing water use efficiency and improvement PSII function of mulberry seedling leaves under drought stress

ABSTRACT In this paper, the effects of different CO2 concentrations (400 and 800 μmol·mol−1) on photosynthetic gas exchange and chlorophyll fluorescence of mulberry (Morus alba L.) seedling leaves under drought stress were studied using an artificial climate chamber. The results showed that under non-drought conditions, the stomatal concentrations (Gs ) and transpiratration rate (Tr ) of mulberry seedling leaves decreased slightly with increased CO2 concentrations, but the net photosynthetic rate (P n) increased significantly. Under mild drought stress (5 days of drought), higher CO2 concentrations significantly affected the photosynthetic gas exchange of mulberry seedling leaves, but had little effect on chlorophyll fluorescence. Under severe drought stress (10 das of drought), increased CO2 concentrations not only alleviated drought stress by increasing the WUE of mulberry seedling leaves, but also significantly increased its PSII photochemical activity, which promoted electron transfer on the PSII acceptor side. In conclusion, increased CO2 concentrations could raise the WUE of mulberry seedling leaves under normal water conditions and drought stress, and this effect was more significant under drought stress. Under severe drought stress, increased CO2 concentrations improved the drought resistance of mulberry seedlings by improving their PSII function.

Exogenous melatonin improves salt tolerance in tomato by regulating photosynthetic electron flux and the ascorbate–glutathione cycle

ABSTRACT Melatonin (MT) can protect plants against abiotic stress. In order to explore whether melatonin can improve photosynthetic function under NaCl stress, Solanum lycopersicum L. cv. Liaoyuanduoli were exposed to 150 mmol L−1 NaCl stress with or without pretreatment with 150 μmol L−1 melatonin. The results showed that NaCl stress can lead to reduced chlorophyll content, lower photosynthetic function, increased reaction oxygen species (ROS) levels, and decreased PSII activity. These changes were mainly due to the reduction in oxygen-evolving complex (OEC) activity on the donor side of PSII and the blockage of electron transfer from QA to QB on receptor side of PSII. The donor side of PSII was more sensitive to NaCl stress relative to the receptor side of PSII. Interestingly, application of MT enhanced tomato NaCl tolerance. MT reduced the production of ROS by balancing the distribution of photosynthetic electron flux, facilitated the repair of PSII by maintaining the abundance of Psb O and D1, and promoting the ability of the donor and acceptor sides of PSII to deliver electrons. MT also enhanced the scavenging ability of ROS by stimulating the activity of enzymes involved in the AsA-GSH cycle.

2-Cys Prx plays a critical role in scavenging H2O2 and protecting photosynthetic function in leaves of tobacco seedlings under drought stress

ABSTRACT Drought stress led to a decrease in PSII photochemical activity in tobacco leaves and the blockage of electron transfer, especially from Q A to Q B on PSII acceptor side. In addition, drought stress caused a dramatic increase in H2O2 and MDA contents. However, the overexpression of 2-Cys Prx significantly reduced the H2O2 accumulation in tobacco seedlings and alleviated the degree of oxidative damage under drought stress. It also improved stomatal limitation and net photosynthetic rate of tobacco seedlings and reduced its PSII photoinhibition under drought stress which promoted the ability of PSII electron transfer.

Rootstock Alleviates Salt Stress in Grafted Mulberry Seedlings: Physiological and PSII Function Responses.

This study investigated the effect of NaCl stress on Na+ and K+ absorption and transport by roots, nitrogen and phosphorus content in leaves, PSII photochemical activity and reactive oxygen species (ROS) in leaves of mulberry own-root seedlings and grafted seedlings. To determine the response, own-root seedlings of a high yielding mulberry cultivar, Tieba mulberry (Morus alba L.), and the grafted seedlings, obtained by using Qinglong mulberry with high salt tolerance as rootstock and Tieba mulberry as scion, were used. The Na+ content in roots and leaves of grafted seedlings was significantly lower than that in own-root seedlings under salt stress; while K+ content in roots and leaves of grafted seedlings was significantly higher than that in own-root seedlings. The root activity in grafted seedlings was significantly higher than that in own-root seedlings, as well as the content of nitrogen, phosphorous and water. PSII photochemical activity in leaves of grafted seedlings was less significantly affected by salt stress compared to own-root seedlings. The electron transport at the acceptor side of PSII from QA to QB was less affected by salt stress, which resulted in a significantly lower ROS content in leaves of grafted seedlings than that of own-root seedlings. Therefore, grafting high-yielding and good-quality Tieba mulberry with salt tolerant Qinglong mulberry as rootstock showed a relatively high salt tolerance. This may be because (1) the root system of rootstock presented high Na+ resistance and has selective absorption capacity for Na+ and K+ (2) the root system of rootstock prevented excess Na+ from being transported to aerial parts in order to reduce adverse effects of Na+ (3) the root system of rootstock had enhanced root activity under salt stress, which accelerated water and nutrient absorption (4) the leaves of grafted seedlings had higher PSII photochemical activity and electron transport rate compared with those of own-root seedlings under salt stress, which effectively reduced ROS burst mediated by photosynthesis and reduced oxidative damage.

Drought-induced changes in photosynthetic electron transport in maize probed by prompt fluorescence, delayed fluorescence, P700 and cyclic electron flow signals

The effect of drought on the prompt chlorophyll a fluorescence (PF) transient (OJIP), delayed chlorophyll a fluorescence (DF), modulated 820-nm reflection (MR), energy conversion efficiencies in photosystems (PS) I and II, and cyclic electron flow (CEF) activity in two maize hybrids with contrasting drought tolerance was investigated. Our aim was to identify the target site of drought stress on the photosynthetic electron transport chain and investigate the relevance of the CEF pathway to the drought tolerance of maize plants. The OJIP analysis showed that drought stress, depending on its duration, decreased FP, increased FJ, and induced a pronounced K-band and a positive L-band. Moreover, OJIP parameters, including PIABS, RC/CSO, TRO/ABS, and ETO/TRO, were significantly reduced. The DF analysis showed that the values of I1 and I2 in the induction curve and L1 and L2 derived from the decay curve decreased progressively with the duration of drought stress. The MR analysis showed that drought stress inactivated both the fast decrease and slow increase phases of the MR transient, resulting in a gradual decrease in both VPSI and VPSII-PSI. The energy conversion analysis showed that drought stress decreased the PSI photochemical quantum yield Y(I) and PSII photochemical quantum yield Y(II). Compared to the tolerant hybrid, the drought-induced changes in the sensitive hybrid were stronger and appeared at an earlier treatment stage. The CEF activity analysis showed that the CEF pathway under drought stress operated for a longer time in the tolerant hybrid than that in the sensitive hybrid. The above results indicate that drought stress damaged the donor and acceptor sides of PSII, the PSII reaction center and the acceptor side of PSI and decreased the efficiency of both PSI and PSII and the capacity of electron transfer. The CEF pathway might play an important role in the tolerance of the maize photosynthetic electron transport chain to drought stress.

Phenotyping of isogenic chlorophyll-less bread and durum wheat mutant lines in relation to photoprotection and photosynthetic capacity.

In our experiments, we examined high light responses and photosynthetic capacity of chlorophyll-less isogenic mutant lines of hexaploid bread wheat (Triticum aestivum L.) and tetraploid durum wheat (Triticum durum L.) in comparison to parental lines representing the wild type (WT), in two growth phases and two environments. In young plants, we observed a typical yellow-green phenotype with low chlorophyll content, significantly lower CO2 assimilation rate, elevated chlorophyll a-to-b ratio and insufficient regulation of linear electron transport. In the mutants grown in a moderate light in the growth chamber, a typical "chlorina" phenotype almost disappeared or, at least, was significantly alleviated in later growth stages, including the values of CO2 assimilation and the majority of the measured parameters related to photoprotective responses. On the other hand, in the case of the mutant lines grown in direct sunlight and fluctuating environment, the chlorophyll-less phenotype was evident also in latter growth phases. The chlorophyll-less phenotype was more severe in the durum wheat mutant lines compared to the bread wheat. For example, the durum wheat mutant lines grown outdoors expressed lower flexibility of photoprotective responses, including lower non-photochemical quenching and low rate of cyclic electron flow compared to WT or bread wheat mutants. Based on the analyses, we have identified a set of parameters providing information on the specific photosynthetic traits typical for the chlorophyll-less phenotype. Thus, the proposed way of phenotyping may serve for efficient selection of mutant genotypes for future research or screening activities. As a general result, we observed that the decrease of the chlorophyll content due to mutation was always associated with improper regulation of linear electron transport and a limited ability to prevent over-reduction of PSI acceptor side, regardless of the genotype, environment, and growth stage. This can partly explain why the low chlorophyll mutants were not successful in the evolution of higher plants, despite the photosynthetic capacity observed is high enough and they are fully competitive with wild-type plants in non-fluctuating controlled environment.

Cold Tolerance of Photosynthetic Electron Transport System Is Enhanced in Wheat Plants Grown Under Elevated CO2.

The effects of CO2 elevation on sensitivity of photosynthetic electron transport system of wheat in relation to low temperature stress are unclear. The performance of photosynthetic electron transport system and antioxidant system in chloroplasts was investigated in a temperature sensitive wheat cultivar Lianmai6 grown under the combination of low temperature (2 days at 2/−1°C in the day/night) and CO2 elevation (800 μmol l−1). It was found that CO2 elevation increased the efficiency of photosynthetic electron transport in wheat exposed to low temperature stress, which was related to the enhanced maximum quantum yield for electron transport beyond QA and the increased quantum yield for reduction of end electron acceptors at the PSI acceptor side in plants under elevated CO2. Also, under low temperature, the activities of ATPases, ascorbate peroxidase, and catalase in chloroplasts were enhanced in wheat under elevated CO2. It suggested that the cold tolerance of photosynthetic electron transport system is enhanced by CO2 elevation.

Photoinduction of electron transport on the acceptor side of PSI in Synechocystis PCC 6803 mutant deficient in flavodiiron proteins Flv1 and Flv3

After transferring the dark-acclimated cyanobacteria to light, flavodiiron proteins Flv1/Flv3 serve as a main electron acceptor for PSI within the first seconds because Calvin cycle enzymes are inactive in the dark. Synechocystis PCC 6803 mutant Δflv1/Δflv3 devoid of Flv1 and Flv3 retained the PSI chlorophyll P700 in the reduced state over 10 s (Helman et al., 2003; Allahverdiyeva et al., 2013). Study of P700 oxidoreduction transients in dark-acclimated Δflv1/Δflv3 mutant under the action of successive white light pulses separated by dark intervals of various durations indicated that the delayed oxidation of P700 was determined by light activation of electron transport on the acceptor side of PSI. We show that the light-induced redox transients of chlorophyll P700 in dark-acclimated Δflv1/Δflv3 proceed within 2 min, as opposed to 1–3 s in the wild type, and comprise a series of kinetic stages. The release of rate-limiting steps was eliminated by iodoacetamide, an inhibitor of Calvin cycle enzymes. Conversely, the creation with methyl viologen of a bypass electron flow to O2 accelerated P700 oxidation and made its extent comparable to that in the wild-type cells. The lack of major sinks for linear electron flow in iodoacetamide-treated Δflv1/Δflv3 mutant, in which O2- and CO2-dependent electron flows were impaired, facilitated cyclic electron flow, which was evident from the decreased steady-state oxidation of P700 and from rapid dark reduction of P700 during and after illumination with far-red light. The results show that the photosynthetic induction in wild-type Synechocystis PCC 6803 is largely hidden due to the flavodiiron proteins whose operation circumvents the rate-limiting electron transport steps controlled by Calvin cycle reactions.

Antimycin A inhibits cytochrome b559-mediated cyclic electron flow within photosystem II

The light reactions of photosynthesis are known to comprise both linear and cyclic electron flow in order to convert light energy into chemical energy in the form of NADPH and ATP. Antimycin A (AA) has been proposed as an inhibitor of ferredoxin-dependent cyclic electron flow around photosystem I (CEF-PSI) in photosynthesis research. However, its precise inhibitory mechanism and target site had not been elucidated yet. Here we show that AA inhibits the cyclic (alternative) electron flow via cytochrome b559 (Cyt b559) within photosystem II (CEF-PSII). When AA was applied to thylakoid membranes isolated from spinach leaves, the high potential form of Cyt b559, which was reduced in the dark, was transformed into the lower potential forms and readily oxidized by molecular oxygen. In the absence of AA, the reduced Cyt b559 was oxidized by P680+ upon light illumination and re-reduced in the dark, mainly by the electron from the QB site on the acceptor side of PSII. In contrast, AA suppressed the oxidation of Cyt b559 and induced its reduction under the illumination. This inhibition of Cyt b559 oxidation by AA enhanced photoinhibition of PSII. Based on the above results, we propose caution regarding the use of AA for evaluating CEF-PSI per se and concurrently propose that AA provides for new insights into, and interpretations of, the physiological importance of Cyt b559, rather than that of CEF-PSI in photosynthetic organisms.

Response of photosynthetic performance, water relations and osmotic adjustment to salinity acclimation in two wheat cultivars

Experiment was conducted to identify the impacts of the salinity acclimation process on the photosynthetic efficiency, osmotic adjustment, membrane integrity, and yield components in two wheat cultivars differing in their salinity tolerance. The design of the experiment was factorial randomized block, where genotype is factor 1 and acclimation treatments represent factor 2. Genotypes were grown from emergence to 30 days after sowing (DAS) by irrigating with tap water [electrical conductivity (EC) of 0.776 dS m−1]. Thereafter, both the genotypes were divided into two groups and exposed to either irrigation with sublethal level of salinity EC of 2.09 or 3.76 dS m−1 for 21 days. At booting stage (65 DAS), both groups were subjected to lethal level of salinity stress EC of 12 dS m−1 for 21 days, followed by irrigation with tap water till maturity. Non-acclimated plants were irrigated with tap water from emergence to 65 days, then directly irrigated with lethal level of salinity for 21 days, followed by irrigation with tap water till maturity. The control plants were continuously irrigated with tap water from emergence until maturity. The non-acclimated plants had decreased electron transport rates at the donor and acceptor side of PSII and PSI in Giza 168, and decreased electron transport rates at PSII acceptor side in Sakha 8 compared to control plants. In both genotypes, the non-acclimated plants had decreased chlorophyll a, b, carotenoid, proline and total soluble sugar concentration, relative water content, membrane stability index, yield and yield components compared with acclimated plants. While, osmotic potential and lipid peroxidation showed an opposite trend. Overall, acclimation treatment (EC of 2.09 dS m−1) during vegetative stage alleviated the inhibitory effects of lethal level of salinity stress at booting stage through enhanced photosynthetic efficiency and osmotic adjustment, resulting in increased membrane integrity, biomass production and grain yield than in non-acclimated plants.

The multiplicity of roles for (bi)carbonate in photosystem II operation in the hypercarbonate-requiring cyanobacterium Arthrospira maxima

Arthrospira maxima is unique among cyanobacteria, growing at alkaline pH (<11) in concentrated (bi)carbonate (1.2 M saturated) and lacking carbonic anhydrases. We investigated dissolved inorganic carbon (DIC) roles within PSII of A. maxima cells oximetrically and fluorometrically, monitoring the light reactions on the donor and acceptor sides of PSII. We developed new methods for removing DIC based on a (bi)carbonate chelator and magnesium for (bi)carbonate ionpairing. We established relative affinities of three sites: the water-oxidizing complex (WOC), non-heme iron/QA–, and solvent-accessible arginines throughout PSII. Full reversibility is achieved but (bi)carbonate uptake requires light. DIC depletion at the non-heme iron site and solvent-accessible arginines greatly reduces the yield of O2 due to O2 uptake, but accelerates the PSII–WOC cycle, specifically the S2→S3 and S3→S0 transitions. DIC removal from the WOC site abolishes water oxidation and appears to influence free energy stabilization of the WOC from a site between CP43-R357 and Ca2+.

Photooxidation and photoreduction of exogenous cytochrome c by photosystem II preparations after various modifications of the water-oxidizing complex

The redox interaction of exogenous cytochrome c550 (Cyt) with PSII isolated from spinach was studied. Illumination of PSII particles in the presence of Cyt led to: (1) Cyt photooxidation by PSII reaction center (demonstrated at the first time), (2) Cyt photoreduction via O2−• photoproduced on the acceptor side of PSII, and (3) Cyt photoreduction by reduced electron carriers of PSII. A step-by-step removal of components of water-oxidizing complex was accompanied by the appearance of Cyt photooxidation, an increase in the superoxide dismutase (SOD)-dependent Cyt photoreduction (related to O2−• formation), and a decrease in the SOD-independent Cyt photoreduction. Re-addition of PsbO protein diminished the Cyt-induced restoration of electron transfer in PSII. Addition of diuron led to inhibition of these photoprocesses, while exogenous Mn2+ inhibited only the Cyt c photooxidation. The results can be important for correct measurements of O2−• photoproduction in PSII and for elucidation of the role of cytochrome c550 in cyanobacterial PSII.

Altitude of origin influences the responses of PSII photochemistry to heat waves in European beech (Fagus sylvatica L.)

The photosynthetic responses to the combined effect of drought and heat stress were studied in leaves of 19-years-old European beech (Fagus sylvatica L.) trees originating from five provenances in Central Europe differing by altitude (55–1250 m), and grown in the same experimental plot. The measurements were conducted at the beginning of heat waves, under two different conditions: during a dry period (middle of July) and after recovery in wetter period, at the beginning of August. The decreases of stomatal conductance (gs) and net photosynthesis rate (ACO2) during drought and heat wave stress were very similar in all provenances. However, we observed distinct response of PSII photochemistry on combined drought and high temperature stress, well associated with altitude of origin of the beech provenances. Measurements of pulse amplitude modulated (PAM) fluorescence identified maintenance of a high electron transport rate in beech provenances from high altitudes under drought and heat wave conditions, associated with some decrease of excitation pressure on PSII. This can be explained by enhanced capacity of alternative electron sinks to utilize the excess of electrons as a photoprotective mechanism. The analyses of fast chlorophyll fluorescence kinetics confirmed the differences in responses of PSII photochemistry between provenances originating from different altitudes. Compared with provenances at higher altitudes, we found more sensitive response (i.e. more limited electron transport at the PSII acceptor side and changes in the size of light harvesting complexes) to drought and heat stress, in those growing at low altitudes. Our results support the hypothesis that the provenances originating from higher altitudes possess enhanced phenotypic plasticity related to photoprotective responses, resulting from the long-term adaptation to marginal mountain conditions.

Chlorophyll-a fluorescence evaluation of PEG-induced osmotic stress on PSII activity in Arabidopsis plants expressing SIP1

In this study, we evaluated the effect of osmotic stress on photosynthetic machinery of Arabidopsis plants expressing a gene encoding small basic intrinsic protein (SIP1) isolated from Solanum tuberosum. Intact leaves of SIP Arabidopsis plants were exposed to 15% polyethylene glycol (PEG) solution and fast Chlorophyll-a (Chl-a) fluorescence induction kinetics was measured. Photosynthetic parameters like ratio of variable and maximum fluorescence (FV/FM), absorbance of photons per active reaction center (ABS/RC), trapping of photons per active reaction center (TRo/RC), electron transport per active reaction center (ETo/RC), and performance index (PI) were measured. Furthermore, the energy pipeline model was deduced in response to PEG stress. The membrane model includes a visualization of the average “antenna size”, which follows the value of the ABS/RC. Analysis of SIP Arabidopsis plants under PEG stress through fast Chl-a fluorescence transient showed that the damage caused due to PEG is more prominent at the donor side rather than the acceptor side of PSII. Higher PI in SIP plants under PEG stress indicated a better vitality than control plants. Overall, these results indicate that constitutive expression of SIP1 in Arabidopsis plants induces significant changes in the photosynthetic machinery under PEG-induced osmotic stress.

Photosynthetic Response of Soybean Leaf to Wide Light-Fluctuation in Maize-Soybean Intercropping System.

In maize-soybean intercropping system, soybean plants will be affected by the wide light-fluctuation, which resulted from the shading by maize plants, as the shading of maize the light is not enough for soybean in the early morning and late afternoon, but at noon, the light is strong as the maize shading disappeared. The objective of this study is to evaluate the photosynthetic response of soybean leaf to the wide light-fluctuation. The data of diurnal variation of photosynthetic characters showed that the photosynthetic rate of intercropped soybean was weaker than that of monocropped soybean. The chlorophyll content, ratio of chlorophyll a/b, and AQE (apparent quantum efficiency) were increased and Rd (dark respiration rate) was decreased for the more efficient interception and absorption of light and carbon gain in intercropping. δRo (The efficiency/probability with which an electron from the intersystem electron carriers was transferred to reduce end electron acceptors at the PSI acceptor side) and φRo (the quantum yield for the reduction of the end electron acceptors at the PSI acceptor side) in intercropped soybean leaf were lower compared to those in monocropped one, which showed that the acceptor side of PSI might be inhibited, and also it was the main reason that soybean plants showed a low photosynthetic capacity in intercropping. ψEo (the efficiency/probability with an electron moves further than QA-) in monocropping and intercropping decreased 5.8, and 35.7%, respectively, while φEo (quantum yield for electron transport) decreased 27.7 and 45.3% under the high radiation at noon, which suggested that the acceptor side of PSII was inhibited, while the NPQ became higher. These were beneficial to dissipate excess excitation energy in time, and protect the photosynthetic apparatus against photo-damage. The higher performance index on the absorption basis (PIABS) and lower δRo, φRo, ψEo, and φEo of intercropped soybeans compared to monocropping under high radiation indicated that the electron transfer of intercropped soybean was inhibited more seriously and intercropped soybean adjusted the electron transport between PSII to PSI to adapt the light-fluctuation. Higher NPQ capacity of intercropped soybeans played a key role in keeping the leaf with a better physiological flexibility under the high radiation.