Homologs of plant PsbP and PsbQ proteins are necessary for regulation of photosystem ii activity in the cyanobacterium Synechocystis 6803

The mechanism of oxygen evolution by photosystem II (PSII) has remained highly conserved during the course of evolution from ancestral cyanobacteria to green plants. A cluster of manganese, calcium, and chloride ions, whose binding environment is optimized by PSII extrinsic proteins, catalyzes this water-splitting reaction. The accepted view is that in plants and green algae, the three extrinsic proteins are PsbO, PsbP, and PsbQ, whereas in cyanobacteria, they are PsbO, PsbV, and PsbU. Our previous proteomic analysis established the presence of a PsbQ homolog in the cyanobacterium Synechocystis 6803. The current study additionally demonstrates the presence of a PsbP homolog in cyanobacterial PSII. Both psbP and psbQ inactivation mutants exhibited reduced photoautotrophic growth as well as decreased water oxidation activity under CaCl(2)-depleted conditions. Moreover, purified PSII complexes from each mutant had significantly reduced activity. In cyanobacteria, one PsbQ is present per PSII complex, whereas PsbP is significantly substoichiometric. These findings indicate that both PsbP and PsbQ proteins are regulators that are necessary for the biogenesis of optimally active PSII in Synechocystis 6803. The new picture emerging from these data is that five extrinsic PSII proteins, PsbO, PsbP, PsbQ, PsbU, and PsbV, are present in cyanobacteria, two of which, PsbU and PsbV, have been lost during the evolution of green plants.     

Comparison of the electron transport properties of the psbo1 and psbo2 mutants of Arabidopsis thaliana

Genome sequence of Arabidopsis thaliana (Arabidopsis) revealed two psbO genes (At5g66570 and At3g50820) which encode two distinct PsbO isoforms: PsbO1 and PsbO2, respectively. To get insights into the function of the PsbO1 and PsbO2 isoforms in Arabidopsis we have performed systematic and comprehensive investigations of the whole photosynthetic electron transfer chain in the T-DNA insertion mutant lines, psbo1 and psbo2.The absence of the PsbO1 isoform and presence of only the PsbO2 isoform in the psbo1 mutant results in (i) malfunction of both the donor and acceptor sides of Photosystem (PS) II and (ii) high sensitivity of PSII centers to photodamage, thus implying the importance of the PsbO1 isoform for proper structure and function of PSII. The presence of only the PsbO2 isoform in the PSII centers has consequences not only to the function of PSII but also to the PSI/PSII ratio in thylakoids. These results in modification of the whole electron transfer chain with higher rate of cyclic electron transfer around PSI, faster induction of NPQ and a larger size of the PQ-pool compared to WT, being in line with apparently increased chlororespiration in the psbo1 mutant plants. The presence of only the PsbO1 isoform in the psbo2 mutant did not induce any significant differences in the performance of PSII under standard growth conditions as compared to WT. Nevertheless, under high light illumination, it seems that the presence of also the PsbO2 isoform becomes favourable for efficient repair of the PSII complex.     

The mitochondrial CMSII mutation of Nicotiana sylvestris impairs adjustment of photosynthetic carbon assimilation to higher growth irradiance

The CMSII mutant of Nicotiana sylvestris, which lacks a functional mitochondrial complex I, was used to investigate chloroplast-mitochondria interactions in light acclimation of photosynthetic carbon assimilation. CMSII and wild-type (WT) plants were grown at 80 micromol m(-2) s(-1) photosynthetic active radiation (PAR; 80) and 350 micromol m(-2) s(-1) PAR (350). Carbon assimilation at saturating PFD was markedly higher in WT 350 leaves as compared with WT 80 leaves, but was similar in CMS 80 and CMS 350 leaves, suggesting that the mutant is unable to adjust photosynthesis to higher growth irradiance. WT 350 leaves showed several general characteristic light acclimation responses [increases in leaf specific area (LSA), total chlorophyll content, and chlorophyll a/b ratio, and a higher light compensation point]. In contrast, a similar chlorophyll content and chlorophyll a/b ratio were measured for both CMS 80 and CMS 350 leaves, while LSA and the light compensation point acclimated as in the WT. The failure of CMSII to adjust photosynthesis to growth PFD did not result from lower quantum efficiency of PSII, lower whole-chain electron transport rates (ETRs), or lower ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) and sucrose phosphate synthase (SPS) capacities. Excess ETR not used for carbon assimilation was even higher in CMS 350 than in WT 350. Since photochemical fluorescence quenching and the initial activity of NADP malate dehydrogenase (NADP-MDH) were identical in WT 350 and CMS 350 leaves but the activation state of NADP-MDH was different, redox signals from primary ETR are not involved in the signal transduction of light acclimation, while a contribution of stromal redox state cannot be excluded. When mature plants were transferred between 350 and 80 conditions, the mutant showed acclimatory tendencies, although adjustments were not as rapid or as marked as in the WT, and the response of the initial activities of Rubisco and NADP-MDH was impaired or altered. Initial activities of Rubisco and SPS at limiting concentration were also affected in CMS 350 as compared with WT plants when compared at growth irradiance or after in situ activation at 1000 micromol m(-2) s(-1) PAR. The data demonstrate that chloroplast-mitochondria interactions are important in light acclimation, and modulation of the activation state of key photosynthetic enzymes could be an important mechanism in this cross-talk.     

The Arabidopsis PsbO2 protein regulates dephosphorylation and turnover of the photosystem II reaction centre D1 protein

The extrinsic photosystem II (PSII) protein of 33 kDa (PsbO), which stabilizes the water-oxidizing complex, is represented in Arabidopsis thaliana (Arabidopsis) by two isoforms. Two T-DNA insertion mutant lines deficient in either the PsbO1 or the PsbO2 protein were retarded in growth in comparison with the wild type, while differing from each other phenotypically. Both PsbO proteins were able to support the oxygen evolution activity of PSII, although PsbO2 was less efficient than PsbO1 under photoinhibitory conditions. Prolonged high light stress led to reduced growth and fitness of the mutant lacking PsbO2 as compared with the wild type and the mutant lacking PsbO1. During a short period of treatment of detached leaves or isolated thylakoids at high light levels, inactivation of PSII electron transport in the PsbO2-deficient mutant was slowed down, and the subsequent degradation of the D1 protein was totally inhibited. The steady-state levels of in vivo phosphorylation of the PSII reaction centre proteins D1 and D2 were specifically reduced in the mutant containing only PsbO2, in comparison with the mutant containing only PsbO1 or with wild-type plants. Phosphorylation of PSII proteins in vitro proceeded similarly in thylakoid membranes from both mutants and wild-type plants. However, dephosphorylation of the D1 protein occurred much faster in the thylakoids containing only PsbO2. We conclude that the function of PsbO1 in Arabidopsis is mostly in support of PSII activity, whereas the interaction of PsbO2 with PSII regulates the turnover of the D1 protein, increasing its accessibility to the phosphatases and proteases involved in its degradation.     

Isolation of a novel heterodimeric PSII complex via strep-tagged PsbO

The multi-subunit membrane protein complex photosystem II (PSII) catalyzes the light-driven oxidation of water and with this the initial step of photosynthetic electron transport in plants, algae, and cyanobacteria. Its biogenesis is coordinated by a network of auxiliary proteins that facilitate the stepwise assembly of individual subunits and cofactors, forming various intermediate complexes until fully functional mature PSII is present at the end of the process. In the current study, we purified PSII complexes from a mutant line of the thermophilic cyanobacterium Thermosynechococcus vestitus BP-1 in which the extrinsic subunit PsbO, characteristic for active PSII, was fused with an N-terminal Twin-Strep-tag. Three distinct PSII complexes were separated by ion-exchange chromatography after the initial affinity purification. Two complexes differ in their oligomeric state (monomeric and dimeric) but share the typical subunit composition of mature PSII. They are characterized by the very high oxygen evolving activity of approx. 6000 μmol O₂·(mg Chl·h)^?1. Analysis of the third (heterodimeric) PSII complex revealed lower oxygen evolving activity of approx. 3000 μmol O₂·(mg Chl·h)^?1 and a manganese content of 2.7 (±0.2) per reaction center compared to 3.7 (±0.2) of fully active PSII. Mass spectrometry and time-resolved fluorescence spectroscopy further indicated that PsbO is partially replaced by Psb27 in this PSII fraction, thus implying a role of this complex in PSII repair.     

Leaf chloroplast ultrastructure and photosynthetic properties of a chlorophyll-deficient mutant of rice

Leaf chloroplast ultrastructure and photosynthetic properties of a natural, yellow-green leaf mutant (ygl1) of rice were characterized. Our results showed that chloroplast development was significantly delayed in the mutant leaves compared with the wild-type rice (WT). As leaves matured, more grana stacks formed concurrently with increasing leaf chlorophyll (Chl) content. Except for the lower intercellular CO₂ concentration, the ygl1 plants had a higher leaf net photosynthetic rate, stomatal conductance, and transpiration rate than those of the WT plants. Under equal amounts of Chl, the excitation energy of PSI and PSII was much stronger in the mutant than that in the WT. The ygl1 plants showed higher nonphotochemical quenching and lower photochemical quenching. They also exhibited higher actual photochemical efficiency of PSII with a higher electron transport rate. Under the light of 200 μmol(photon) m^?2 s^?1, the ygl1 mutant showed lesser deepoxidation of violaxanthin in the xanthophyll cycle than WT, but it increased substantially under strong light conditions. In conclusion, the photosynthetic machinery of the ygl1 remained stable during leaf development. The plants were less sensitive to photoinhibition compared with WT due to the active xanthophyll cycle. The ygl1 plants were efficient in both light harvesting and conversion of solar energy.     

Role of the reversible xanthophyll cycle in the photosystem II damage and repair cycle in Dunaliella salina

The Dunaliella salina photosynthetic apparatus organization and function was investigated in wild type (WT) and a mutant (zea1) lacking all beta,beta-epoxycarotenoids derived from zeaxanthin (Z). The zea1 mutant lacked antheraxanthin, violaxanthin, and neoxanthin from its thylakoid membranes but constitutively accumulated Z instead. It also lacked the so-called xanthophyll cycle, which, upon irradiance stress, reversibly converts violaxanthin to Z via a de-epoxidation reaction. Despite the pronounced difference observed in the composition of beta,beta-epoxycarotenoids between WT and zea1, no discernible difference could be observed between the two strains in terms of growth, photosynthesis, organization of the photosynthetic apparatus, photo-acclimation, sensitivity to photodamage, or recovery from photo-inhibition. WT and zea1 were probed for the above parameters over a broad range of growth irradiance and upon light shift experiments (low light to high light shift and vice versa). A constitutive accumulation of Z in the zea1 strain did not affect the acclimation of the photosynthetic apparatus to irradiance, as evidenced by indistinguishable irradiance-dependent adjustments in the chlorophyll antenna size and photosystem content of WT and zea1 strain. In addition, a constitutive accumulation of Z in the zea1 strain did not affect rates of photodamage or the recovery of the photosynthetic apparatus from photo-inhibition. However, Z in the WT accumulated in parallel with the accumulation of photodamaged PSII centers in the chloroplast thylakoids and decayed in tandem with a chloroplast recovery from photo-inhibition. These results suggest a role for Z in the protection of photodamaged and disassembled PSII reaction centers, apparently needed while PSII is in the process of degradation and replacement of the D1/32-kD reaction center protein.     

Stoichiometry of Photosystem I, Photosystem II, and Phycobilisomes in the Red Alga Porphyridium cruentum as a Function of Growth Irradiance

Cells of the red alga Porphyridium cruentum (ATCC 50161) exposed to increasing growth irradiance exhibited up to a three-fold reduction in photosystems I and II (PSI and PSII) and phycobilisomes but little change in the relative numbers of these components. Batch cultures of P. cruentum were grown under four photon flux densities of continuous white light; 6 (low light, LL), 35 (medium light, ML), 180 (high light, HL), and 280 (very high light, VHL) microeinsteins per square meter per second and sampled in the exponential phase of growth. Ratios of PSII to PSI ranged between 0.43 and 0.54. About three PSII centers per phycobilisome were found, regardless of growth irradiance. The phycoerythrin content of phycobilisomes decreased by about 25% for HL and VHL compared to LL and ML cultures. The unit sizes of PSI (chlorophyll/P₇₀₀) and PSII (chlorophyll/Q_A) decreased by about 20% with increase in photon flux density from 6 to 280 microeinsteins per square meter per second. A threefold reduction in cell content of chlorophyll at the higher photon flux densities was accompanied by a twofold reduction in β-carotene, and a drastic reduction in thylakoid membrane area. Cell content of zeaxanthin, the major carotenoid in P. cruentum, did not vary with growth irradiance, suggesting a role other than light-harvesting. HL cultures had a growth rate twice that of ML, eight times that of LL, and slightly greater than that of VHL cultures. Cell volume increased threefold from LL to VHL, but volume of the single chloroplast did not change. From this study it is evident that a relatively fixed stoichiometry of PSI, PSII, and phycobilisomes is maintained in the photosynthetic apparatus of this red alga over a wide range of growth irradiance.     

Response of the Photosynthetic Apparatus in Dunaliella salina (Green Algae) to Irradiance Stress

The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem II (PSII) was PSII_α(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSII_β centers (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII_γ. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyll-protein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.     

Ambient UV-B radiation reduces PSII performance and net photosynthesis in high Arctic Salix arctica

Ambient ultraviolet-B (UV-B) radiation potentially impacts the photosynthetic performance of high Arctic plants. We conducted an UV-B exclusion experiment in a dwarf shrub heath in NE Greenland (74°N), with open control, filter control, UV-B filtering and UV-AB filtering, all in combination with leaf angle control. Two sites with natural leaf positions had ground angles of 0° (‘level site’) and 45° (‘sloping site’), while at a third site the leaves were fixed in an angle of 45° to homogenize the irradiance dose (‘fixed leaf angle site’). The photosynthetic performance of the leaves was characterized by simultaneous gas exchange and chlorophyll fluorescence measurements and the PSII performance through the growing season was investigated with fluorescence measurements. Leaf harvest towards the end of the growing season was done to determine the specific leaf area and the content of carbon, nitrogen and UV-B absorbing compounds. Compared to a 60% reduced UV-B irradiance, the ambient solar UV-B reduced net photosynthesis in Salix arctica leaves fixed in the 45° position which exposed leaves to maximum natural irradiance. Also a reduced Calvin Cycle capacity was found, i.e. the maximum rate of electron transport (J_max) and the maximum carboxylation rate of Rubisco (V_cmax), and the PSII performance showed a decreased quantum yield and increased energy dissipation. A parallel response pattern and reduced PSII performance at all three sites indicate that these responses take place in all leaves across position in the vegetation. These findings add to the evidence that the ambient solar UV-B currently is a significant stress factor for plants in high Arctic Greenland.     

Induction characteristics and response of photosynthetic quantum conversion to changes in irradiance in mulberry plants

A study was conducted, using rapid time course of chlorophyll (Chl) fluorescence parameters, and light-response curves of Chl fluorescence parameters, to determine the induction requirements and response of photosystem II (PSII) photochemistry and non-photochemical reactions after changes in irradiance in greenhouse mulberry plants. The induction of PSII photochemistry rapidly approached to steady state after leaves were treated from darkness to low irradiance (LI). When irradiance of leaves changed from darkness to high irradiance (HI), a biphasic induction was observed. A slight photoinhibition occurred in the leaves exposed to sunlight coming to the greenhouse, whereas a chronic photoinhibition occurred in the leaves fully exposed to sunlight outside the greenhouse. The chronic photoinhibition was demonstrated by sustained reduction of maximal quantum yield of PSII photochemistry (Fv/Fm). Moreover, the leaves of mulberry plants in greenhouse were sensitive to abrupt changes in irradiance and the sensitivity of leaves suffered in a short-term (1h) high light treatment was reduced, based on the changes in photosynthetic quantum conversion. These results demonstrated an inducible response of photosynthetic quantum conversion to changes in irradiance in mulberry.     

Kojic Acid Oxidase

Photosystem II (PSII), which catalyzes photosynthetic water oxidation, is composed of more than 20 subunits, including membrane-intrinsic and -extrinsic proteins. The extrinsic proteins of PSII shield the catalytic Mn₄CaO₅ cluster from exogenous reductants and serve to optimize oxygen evolution at physiological ionic conditions. These proteins include PsbO, found in all oxygenic organisms, PsbP and PsbQ, specific to higher plants and green algae, and PsbU, PsbV, CyanoQ, and CyanoP in cyanobacteria. Furthermore, red algal PSII has PsbQ′ in addition to PsbO, PsbV, and PsbU, and diatoms have Psb31 in supplement to red algal-type extrinsic proteins, exemplifying the functional divergence of these proteins during evolution. This review provides an updated summary of recent findings on PSII extrinsic proteins and discusses their binding, function, and evolution within various photosynthetic organisms.     

Photosynthetic acclimation of Tradescantia albiflora to growth irradiance: Lack of adjustment of light-harvesting components and its consequences

The photosynthetic acclimation of Tradescantia albiflora (Kunth), a trailing ground species naturally occurring in the deep shade of rainforests, was studied in relation to growth irradiance (glasshouse; direct light and 1 to 4 layers of shade cloth, giving 100 to 1.4% relative growth irradiance). Contrary to other irradiance studies of higher plants grown in natural habitats or controlled light environments, the chlorophyll a/b ratios of Tradescantia leaves were low (∼2.2) and constant. Acclimation to growth irradiance caused no changes in the relative amounts of specific Chl-proteins or the numbers of photosystem I (PSI) and PSII reaction centres on a chlorophyll basis, indicating that the light-harvesting antenna sizes of PSII and PSI, as well as the photosystem stoichiometry, were independent of growth irradiance. However, the amount of cytochrome f and ATP synthase on a chlorophyll basis increased with increasing the relative growth irradiance from 1.4 to 35%, showing acclimation of electron transport and photophosphorylation capacity. The photosynthetic capacity and ribulose 1, 5-bisphosphate carboxylase (EC 4.1.1.39) activity also increased with increase of the growth irradiance to 35%. Beyond that, the inflexible PSII/PSI stoichiometry and shade-type photosystem II/light-harvesting units in Tradescaniia are a disadvantage for long-term exposure to high irradiance since the leaves are more prone to photoinhibition.     

Effects on Photosystem II Function,Photoinhibition, and Plant Performance of the Spontaneous Mutation of Serine-264 in the Photosystem II Reaction Center D1 Protein in Triazine-Resistant Brassica napus L

Wild-type and an atrazine-resistant biotype of Brassica napus, in which a glycine is substituted for the serine-264 of the D1protein, were grown over a wide range of constant irradiances in a growth cabinet. In the absence of serine-264, the function of photosystem II (PSII) was changed as reflected by changes in chlorophyll fluorescence parameters and in photosynthetic oxygen-evolving activity. The photochemical quenching coefficient was lower, showing that a larger proportion of the primary quinone acceptor is reduced at all irradiances. At low actinic irradiances, the nonphotochemical quenching coefficient was higher, showing a greater tendency for heat emission. Decreased rates of light-limited photosynthesis (quantum yield) and lower oxygen yields per single-turnover flash were also observed. These changes were observed even when the plants had been grown under low irradiances, indicating that the changes in PSII function are direct and not consequences of photoinhibition. In spite of the lowered PSII efficiency under light-limiting conditions, the light-saturated photosynthesis rate of the atrazine-resistant mutant was similar to that of the wild type. An enhanced susceptibility to photoinhibition was observed for the atrazine-resistant biotype compared to the wild type when plants were grown under high and intermediate, but not low, irradiance. We conclude that the replacement of serine by glycine in the D1 protein has a direct effect on PSII function, which in turn causes increased photoinhibitory damage and increased rates of turnover of the D1 protein. Both the intrinsic lowering of light-limited photosynthetic efficiency and the increased sensitivity to photoinhibition probably contribute to reduced crop yields in the field, to different extents, depending on growth conditions.     

Mutation of Residue Arginine18 of Cytochrome b559 α-Subunit and its Effects on Photosystem Ⅱ Activities in Chlamydomonas reinhardtii

It has been known that arginine is used as the basic amino acid in the ?subunit of cytochrome b559 (Cyt b559) except histidine. However, previous studies have focused on the function of histidine in the activities of photosystem (PS) Ⅱ and there are no reports regarding the structural and/or functional roles of arginine in PSII complexes. In the present study,two arginine18 (R18) mutants of Chlamydomonas reinhardtii were constructed using site-directed mutagenesis, in which R18 was replaced by glutamic acid (E) and glycine (G). The results show that the oxygen evolution of the PSII complex in the R18G and R18E mutants was approximately 60% of wild-type (WT) levels and that, after irradiation at high light intensity, oxygen evolution for the PSII of mutants was reduced to zero compared with 40% in WT cells. The efficiency of light capture by PSII (Fv/Fm) of R18G and R18E mutants was approximately 42%-46% that of WT cells. Furthermore, levels of the ?subunit of Cyt b559 and PsbO proteins were reduced in thylakoid membranes compared with WT. Overall, these data suggest that R18 plays a significant role in helping Cyt b559 maintain the structure of the PSII complex and its activity,although it is not directly bound to the heme group.     

Low temperature enhances photosynthetic down-regulation in French bean (Phaseolus vulgaris L.) plants

The mechanisms of photosynthetic adaptation to different combinations of temperature and irradiance during growth, and especially the consequences of exposure to high light (2000 micro mol m(-2) s(-1) PPFD) for 5 min, simulating natural sunflecks, was studied in bean plants (Phaseolus vulgaris L.). A protocol using only short (3 min) dark pre-treatment was introduced to maximize the amount of replication possible in studies of chlorophyll fluorescence. High light at low temperature (10 degrees C) significantly down-regulated photosynthetic electron transport capacity [as measured by the efficiency of photosystem II (PSII)], with the protective acclimation allowing the simulated sunflecks to be used more effectively for photosynthesis by plants grown in low light. The greater energy dissipation by thermal processes (lower F(v)'/F(m)' ratio) at low temperature was related to increased xanthophyll de-epoxidation and to the fact that photosynthetic carbon fixation was more limiting at low than at high temperatures. A key objective was to investigate the role of photorespiration in acclimation to irradiance and temperature by comparing the effect of normal (21 kPa) and low (1.5 kPa) O(2) concentrations. Low [O(2)] decreased F(v)/F(m) and the efficiency of PSII (Phi(PSII)), related to greater PSII down-regulation in cold pre-treated plants, but minimized further inhibition by the mild 'sunfleck' treatment used. Results support the hypothesis that photorespiration provides a 'safety-valve' for excess energy.     

The change of chlorophyll fluorescence parameters in winter barley during recovery after freezing shock and as affected by cold acclimation and irradiance

The change of chlorophyll fluorescence parameters in froze leaves of 3 leaf-age seedlings were examined using two winter barley cultivars (Chumai 1 and Mo 103) differing in cold tolerance to investigate physiological response to low temperature as affected by cold acclimation (under 3/1 degrees C, day/night for 5 days before freezing treatment) and irradiation size (high irradiance: 380+/-25 micromol m(-2)s(-1) and low irradiance: 60+/-25 micromol m(-2)s(-1)) during recovery. The results showed that non-lethal freezing shock (exposed to -8 degrees C for 18 h) did not obviously affect maximum quantum efficiency in photosystem II (PSII), but dramatically increased non-photochemical quenching and reduced effective quantum yield in PSII. Cold acclimation significantly improved stability of photosynthetic function of leaves after freezing stress through buffering excessive energy and alleviating photoinhibition during recovery, indicating it increased recovery ability of barley plants from freezing injury. High irradiance was quite harmful to the stability of PSII in barley plants during recovery from freezing injury. The electron transport rate of PSII varied with cold-acclimation, irradiance and genotype. Cold acclimation caused significant increase in electron transport rate of PSII for relatively tolerant cultivar Mo 103, but not for relatively sensitive cultivar Chumai 1. It can be concluded that some chlorophyll fluorescence parameters during recovery from freezing shock may be used as the indicators in identification and evaluation of cold tolerance in barley.     

High-nitrogen and low-irradiance can restrict energy utilization in photosynthesis of successional tree species in low subtropical forest

Responses of photosynthesis and the partition of energy utilization to high-nitrogen importation and high-light intensity in leaves of three dominant tree species of subtropical forest,including sun plant or early-successional species Schima superba,mesophyte or intermediate-successional species Canstanopsis hystrix,and shading-tolerant plant or late-successional species Cryptocarya concinna were studied by using the CO2 exchange system and chlorophyll fluorescence method.Our results showed that,regardless of plant species,net photosynthetic rate(Pn)was higher in high-nitrogen supply and high irradiance(HNHL)plants than in low-nitrogen supply and high irradiance(LNHL)plants,implying that low-nitrogen importation would limit Pn of plants grown under high irradiance.However, high-nitrogen supply and low irradiance(HNLL)plants had a lower Pn.Insignificant change of quantum yield(Fv′/Fm′)in opened PS II was found in leaves of HNHL,LNHL or HNLL plants of S.superba and C. hystrix,while a higher Fv′/Fm′occurred in HNHL plants of C.concinna in comparison with LNHL or HNLL plants.The HNHL plants of C.concinna also had a higher photochemical quantum yield(△F/Fm′) than LNHL or HNLL plants,however no similar responses were found in plants of S.superba and C. hystrix(P<0.05).In the irradiance range of 0―2000μmol photon·m -2·s -1,the fraction of energy consumed by photochemistry(φ PSII )was 18.2%in LNHL plants of S.superba which was higher than that in HNHL plants(P>0.05)and it was significantly higher than in HNLL plants(P<0.05).C.hystrix also had a similar response inφ PSII to nitrogen supply and irradiance.Regardless of species HNLL plants had a significantφ PSII and higher heat dissipation in light,and this effect was more severe in C.concinna than in S.superba or C.hystrix.The results may mean that high-nitrogen importation by nitrogen deposit and low irradiance caused by changing climate or air pollution would more severely restrict photosynthetic processes in the late-successional species C.concinna than in the early-successional species S.superba and intermediate-successional species C.hystrix.The continuous high-nitrogen precipitation in the future and the over cast mist or pollution smoke could induce late-successional species to degrade,however,early-successional species would be more adapted to competition for more resources to keep their dominance in ecosystems.In this sense,the zonal vegetation may accelerate degradation caused by high nitrogen precipitation and low irradiance,while the early-successional and mesophytic vegetations can remain longer.Thus,nitrogen precipitation may play an important role in plant community succession.     

Acclimation of winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>, cv. Yangmai 13) to low levels of solar irradiance

Winter wheat is a grass species widely planted in northern and central China, where the increase of aerosols, air pollutants and population density are causing significant reduction in solar irradiance. In order to investigate the adaptation of winter wheat (Triticum aestivum L., cv. Yangmai 13) to low irradiance conditions occurring in the downstream plain of the Yangtze River (China), plants were subjected to four solar irradiance treatments (100%, 60%, 40%, and 20% of environmental incident solar irradiance). Significant increases in chlorophyll (Chl) and xanthophyll (Xan) pigments, and decreases in Chl a/b and Xan/Chl ratios were observed in plants under low light. Light-response curves showed higher net photosynthetic rates (P _N) in fully irradiated plants, that also showed a higher light-compensation point. Shaded plants maintained high values of minimal fluorescence of dark-adapted state (F_o) and maximum quantum efficiency of PSII photochemistry (F_v/F_m) that assess a lower degree of photoinhibition under low light. Reduced irradiance caused decreases in effective quantum yield of PSII photochemistry (Φ_PSII), electron transport rate (ETR), and nonphotochemical quenching coefficient (q_N), and the promotion of excitation pressure of PSII (1 − q_P). The activities of the antioxidant enzymes superoxide dismutase and peroxidase were high under reduced light whereas no light-dependent changes in catalase activity were observed. Thiobarbituric acid reactive species content and electrolyte leakage decreased under shaded plants that showed a lower photooxidative damage. The results suggest that winter wheat cv. Yangmai 13 is able to maintain a high photosynthetic efficiency under reduced solar irradiance and acclimates well to shading tolerance. The photosynthetic and antioxidant responses of winter wheat to low light levels could be important for winter wheat cultivation and productivity.