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.

PROTECTIVE ROLES OF D1 PROTEIN TURNOVER AND THE XANTHOPHYLL CYCLE IN TOMATO(SOLANUM LYCOPERSICUM)UNDER SUB-HIGH TEMPERATURE AND HIGH LIGHT STRESS

D1 protein turnover and the xanthophyll cycle(XC)are important photo-protective mechanisms in plants that operate under adverse conditions.Here,streptomycin sulfate(SM)and dithiothreitol(DTT)were used in tomato plants as inhibitors of D1 protein turnover and XC to elucidate their photoprotec-tive impacts under sub-high temperature and high light conditions(HH,35°C,1000 limol m 2.S1).SM and DTT treatments significantly reduced the net photosynthetic rate,apparent quantum efficiency,maximum photochemical efficiency,and potential activity of photosystem II,leading to photoinhibition and a decline in plant biomass under HH.The increase in reactive oxygen species levels resulted in thylakoid membrane lipid peroxidation.In addition,there were increased non-photochemical quenching and decreased chlorophyll pigments in SM and DTT application,causing an inhibition of D1 protein production at both transcriptional and translational levels.Overall,inhibition of D1 turnover caused greater photoinhibition than XC inhibition.Additionally,the recovery levels of most photosynthesis indicators in DTT-treated plants were higher than in SM-treated plants.These findings support the view that D1 turnover has a more important role than XC in photoprotection in tomato under HH conditions.

Disruption of LEAF LESION MIMIC 4 affects ABA synthesis and ROS accumulation in rice

Lesion mimic mutants(LMMs) are advantageous materials for studying programmed cell death(PCD).Although some rice LMM genes have been cloned, the diversity of functions of these genes indicates that the mechanism of cell death regulation in LMMs needs further study. In this study, we identified a rice light-dependent leaf lesion mimic mutant 4(llm4) that showed abnormal chloroplast structure, photoinhibition, reduced photosynthetic protein levels, massive accumulation of reactive oxygen species(ROS), and PCD. Map-based cloning and complementation testing revealed that LLM4 encodes zeaxanthin epoxidase(ZEP), an enzyme involved in the xanthophyll cycle, which functions in plant photoprotection,ROS scavenging, and carotenoid and abscisic acid(ABA) biosynthesis. The ABA content was decreased,and the contents of 24 carotenoids differed between the llm4 mutant and the wild type(WT). The llm4mutant showed reduced dormancy and greater sensitive to ABA than the WT. We concluded that the mutation of LLM4 resulted in the failure of xanthophyll cycle, in turn causing ROS accumulation. The excessive ROS accumulation damaged chloroplast structure and induced PCD, leading eventually to the formation of lesion mimics.

Photosynthetic Characteristics of a Super High Yield Cultivar of Winter Wheat During Late Growth Period

The mechanism of high yield of winter wheat in the field at late growth period was investigated by measuring the photosynthetic characteristics of photosystem Ⅱ （PSⅡ） and xanthophylls cycle, which could provide physiological reference for breeding. Weimai 8 （W8）, a super high yield cultivar, and Lumai 14 （L14）, a control cultivar were object. The photosynthetic rate （Pn）, parameters of chlorophyll fluorescence and chlorophyll content were measured. The Pn, maximum photochemical efficiency of PSII （Fv/Fm）, quantum yield of PSII electron transport （ΦPSⅡ）, efficiency of excitation energy capture by open PSII reaction centers （Fv＇/Fm＇）, and photochemical quenching coefficient （qP） were higher in Weimai 8 compared to that in Lumai 14, a commercial high yield cultivar. Furthermore, Weirnai 8 showed a lower non- photochemical quenching coefficient and a lower de-epoxidized ratio of the xanthophyll cycle pigments than of Lumai 14 at late growth period. At mature stage, chlorophyll content of different leaves decreased both in Weimai 8 and Lumai 14. Chlorophyll content in flag, second and third leaf from the top of plant decreased more in Lumai 14 than in Weimai 8. These results suggested that Weimai 8 had more antenna pigments to absorb light energy, and had higher photosynthetic capability and photochemical efficiency of PSⅡ. The yield of Weimai 8 was also higher than that of Lumai 14.

Effects of Cyclic Electron Flow Inhibitor(Antimycin A)on Photosystem Photoinhibition of Sweet Pepper Leaves upon Exposure to Chilling Stress Under Low Irradiance

In chloroplast, there were two pathways involved in the cyclic electron flow around photosystem 1 （PS 1）. One was the NADH dehydrogenase （NDH）-dependent flow and the other was the ferredoxin quinone reductase-dependent flow. It was proposed that the NDH-dependent cyclic electron flow around PSI was related to the xanthophyll cycle-dependent non-photochemical quenching （NPQ） at chilling temperature under low irradiance （CL）. The function of the chloroplastic cyclic electron flow around PS 1 was examined by comparing sweet pepper （Capsicum annuum L.） control with its antimycin A （AA）-fed leaves upon exposure to CL stress. During CL stress, the maximum photochemical efficiency of PS2 （Fv/Fm） decreased markedly in both controls and AA-fed leaves, and P700＋ was also lower in AA-fed leaves than in controls. These results implied that cyclic electron flow around PS 1 functioned to protect the photosynthetic apparatus from CL stress. Under such stress, NPQ and PS2-driven electron transport rate were different between AA-fed leaves and controls. The lower NPQ in AA-fed leaves might be related to an inefficient proton gradient across thylakoid membranes （ApH） because of inhibiting cyclic electron flow around PS 1 under CL stress.

Traits Related to Chilling-Induced Photoinhibition in Leaves of indica and japonica Rice (Oryza sativa)

Physiological indices related to PS II photochemical efficiency (Fv/Fm) and membrane lipid peroxidation were measured in leaves of indica rice cv Shanyou 63 and japonica rice 9516 at different temperatures and light intensities for four days. No obvious changes in Fv/Fm and MDA were observed in both indica and japonica rice at moderate temperature and medium PFD, implying neither photoinhibition nor photooxidation happened in these cases. In indica rice either at medium temperature with higher PFD or at lower temperature with medium PFD Fv/Fm dropped obviously with no changes in MDA contents, and photoinhibition appeared while photooxidation did not occur. However, D1 protein, Fv/Fm, (A+Z)/(A+Z+V), and SOD activities dropped, and O2 - production and MDA content increased accordingly, as well as both photoinhibition and photooxidation appeared in two rice varieties at lower temperature and higher PFD. Experiment with inhibitors at lower temperature and higher PFD showed that as compared with japonica rice the decrements appeared in D1 protein contents, SOD activities, and (A+Z)/(A+Z+V) ratios, the xanthophyll cycle and non-photochemical quench (qN) were inhibited in a more degree, as well as increments of MDA content were greater, thus exhibiting more distinct photoinhibition and photooxidation in indica rice. It is suggested that Fv/Fm and membrane lipid peroxidation product-MDA were the key indices to predict and diagnose photooxidation.

Photoinhibitive and Recovery Properties of Hybrid Rice Ⅱ You 129 under Field Conditions

Photoinhibitive properties of super-high-yielding hybrid rice Ⅱ you 129 and its adaptation mechanism to strong light stress were investigated by measuring the light-response curve, diurnal variations of net photosynthetic rate and chlorophyll fluorescence parameters of Ⅱ you 129 leaves and compared with Shanyou 63. Photoinhibition of rice flag leaves under field conditions mainly resulted from the increase of thermal dissipation, especially for thermal dissipation depended on the xanthophyll circle, but no destruction of photosynthetic apparatus occurred. Potentially super-high-yielding hybrid rice Ⅱ you 129 was more tolerant to photoinhibition than Shanyou 63, because it had higher light saturation intensity and maximum net photosynthetic rate; more active xanthophyll cycle, and more rapid recovery ability after photoinhibition.

High non-photochemical quenching of VPZ transgenic potato plants limits CO_(2) assimilation under high light conditions and reduces tuber yield under fluctuating light

Under natural conditions,photosynthesis has to be adjusted to fluctuating light intensities.Leaves exposed to high light dissipate excess light energy in form of heat at photosystem II(PSII)by a process called non-photochemical quenching(NPQ).Upon fast transition from light to shade,plants lose light energy by a relatively slow relaxation from photoprotection.Combined overexpression of violaxanthin de-epoxidase(VDE),PSII subunit S(PsbS)and zeaxanthin epoxidase(ZEP)in tobacco accelerates relaxation from photoprotection,and increases photosynthetic productivity.In Arabidopsis,expression of the same three genes(VPZ)resulted in a more rapid photoprotection but growth of the transgenic plants was impaired.Here we report on VPZ expressing potato plants grown under various light regimes.Similar to tobacco and Arabidopsis,induction and relaxation of NPQ was accelerated under all growth conditions tested,but did not cause an overall increased photosynthetic rate or growth of transgenic plants.Tuber yield of VPZ expressing plants was unaltered as compared to control plants under constant light conditions and even decreased under fluctuating light conditions.Under control conditions,levels of the phytohormone abscisic acid(ABA)were found to be elevated,indicating an increased violaxanthin availability in VPZ plants.However,the increased basal ABA levels did not improve drought tolerance of VPZ transgenic potato plants under greenhouse conditions.The failure to benefit from improved photoprotection is most likely caused by a reduced radiation use efficiency under high light conditions resulting from a too strong NPQ induction.Mitigating this negative effect in the future might help to improve photosynthetic performance in VPZ expressing potato plants.