| Potted plants of two citrus varieties differing in high temperature resistivity (Citrus unshiu Marc. cv. zaogongchuan and Citrus sinensis Osbeck. cv. pcngna) were subjected to 25 1 (control) , 38 2 (high temperature stress), 800-1000 mol m-2 s-1 PPFD and 10 hours photoperiod during 15 days and recovered during 10 days at 25 1 at the same light regime, and were preconditioned at 25 1 for 10 days before the high temperature stress. At the recovery phase, we sprayed 0.025 mol/L AsA over the leaves of materials to look into the recovery effect of ascorbic acid (AsA) after high temperature stress. The effects of high temperature stress on the operation of photosynthetic apparatus and the protective mechanism in leaves of citrus plant were studied. The primary results are as follows:Compared with control, net photosynthetic rate (Pn), stomatal conductance (Gs) and intercellular CO2 concentration (Ci) declined, while stomatal limitation L increased under short-term high temperature stress of 5 days. Pn and Gs were further decreased, whereas Ci was increased and stomatal limitation L was decreased with time of high temperature stress is longer (more than 5 days). At the same time, apparent photosynthetic quantum yield (AQY), carboxylation efficiency (CE), light-saturating photosynthetic rate, CO2-saturating photosynthetic rate, photochemical efficiency (Fv/Fm), photochemical quenching (qP), quantum yield of PS 11 photochemistry ( pSII) and maximum quantum yield of CO2 assimilation ( CO2) as well as the slope of O-P and I-P in fluorescence induction kinetic curve declined. On the other hand, CO2 compensated point ex respiration in day ( ), respiration rate in day(Rd), minimal fluorescence intensity (Fo) and no- photochemical quenching (qN ) increased under high temperature stress. Recovery at 25 1 for 10 days, the date hereinbefore were recovered partly or completeness. This means that it was the stomatal limitation caused the decrease of photosynthesis under short-term high temperature stress and non-stomatal limitation was the main cause of the decrease of photosynthesis under long-term high temperature stress. In Citrus plant high temperature treatment, it were the decrease of activity of RuBPase and the regenerative rate of RuBP influence the photosynthetic carbon assimilation; photosystem reaction center was reversibly inactivated or irreversibly inhibited, the proportion of opened photosystem reaction was decreased, the electron translate of QA -QB was limited, the number of electron used to carbonfixation reduced, the photo energy dissipated by heat was increased; the photoinhibition induced by high temperature was exist. All this non-stomatal factors limited photosynthesis. ?At high temperature stress, O PSII/& co2 was elevated obviously, the rate of (V generated were increased, and it were correlated markedly with the decline of Chi a+b, Chi a and Chi b, correlated markedly with the increase of content of H2O2 and Malondialdehyde (MDA), and it have the remarkable correlation with the increase of activity of superoxide dismutase (SOD), Catalase (CAT), ascorbic acid peroxidase (AsA-POD), Guaiacol peroxidase (G-POD) and dehydroascorbate reductase (DHAR); the Pn and photorespiration rate (Pr) were declined, but Pr/Pn was raised; the plants sprayed with AsA have the more recovery compared with the plants sprayed with water in recovery phase. This means high temperature stress enhanced the photooxidation and increased the number of active oxygen, enhanced the lipid peroxidation in citrus plant, it was the important damage that high temperature to photosynthetic apparatus; the water-water cycle depended on SOD, CAT, AsA-POD, G-POD and DHAR and more photo electrons flowing to photorespiration were important to lighten the photooxidation in photosynthetic apparatus. Used of exogenous active oxygen scavengers can enhanced the ability to clear the active oxygens in citrus plant, quicken the recovery of pholosynthetic apparatus.Compared the two citrus varieties, we found that under high tem...
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