| The one-year-old Myrica rubra were used to research the effects of high light stress on photosynthetic characteristics, chlorophyll fluorescence parameters. In order to find a possible approach or method in theory to improve plant photosynthetic capability through lowering photoinhibition on stress conditions, the effects of high light stress on the operation of photosynthetic apparatus and the different photoprotective mechanism induced by photoinhibition in leaves of Myrica rubra were studied. Some novel and significant results have been obtained as the followings:1. The diurnal variation of gas exchange parameters and chlorophyll fluorescence parameters in Myrica rubra leavesOn sunny day, the net photosynthetic rate (Pn) apparent quantum yield (AQY) and maximal photochemical efficiency of PSII (Fv/Fm) of Myrica rubra leaves decreased after exposed to midday high irradiance, this indicated that photoinhibition of photosynthesis already occurred in Myrica rubra leaves. The diurnal changes of Pn and AQY was two-peaked curve. It reached the maximum values at about 9:00, then declined gradually in the morning, and reached the lowest values at about 13:00 then increased with the decrease of incident light intensity and reached the second peak at about 15:00. But accompanying with the consistent decline of Pn and stomatal conductance (Gs) in the afternoon, the level of intercellular CO2 concentration (Ci) increased. This means that non-stomatal limitation was the main cause of the decrease of photosynthesis under high light stress. The diurnal change of Fv/Fm was the same as "V". In the morning, the Fv/Fm ratio kept on decreasing until it reached its minimum at 13:00, while the reverse change occurred in the later afternoon.2. the photoinhibition of photosynthesis in Myrica rubra leaves under simulated high lightUnder high light (1300umol-m-2 s-1) , Pn Gs decreased, whereas Ci was increased with increasing time of treatment in Myrica rubra leaves, and the chlorophyll fluorescence parameters Fm/Fo Fv/Fm Fm decreased along with the increase of time. Quantum yield of PSII photochemistry (F/Fm ) , linear electron transport rate (ETR) photochemical quenching coefficient (qP) and photochemical efficiency of PSII in the light (Fv' /Fm' ) increased within two hours, then decreased. On the contrary, minimal fluorescence intensity (Fo) and non-photochemical quenching coefficient (qN . NPQ) decreased within twohours, then increased. Fi-Fo and the slope of I-P in fluorescence induction kinetic curve declined. Recovery at low light for 10~20 hours, these parameters were recovered partly or completeness. The reduction of zeaxanthin content and enhancement of photoinhibition of photosynthesis after dithiothreitol (DTT) pretreatment suggested that xanthophyll cycle-dependent nonradiation energy dissipation was one of the mechanisms of photoinhibition. At high light stress, PSii / CO2 was elevated obviously. Accompanying with the consistent increase of time, the level of singlet oxygen (1O2) increased. After high light for 4 hours, qNm increased, while qNf and qNs declined.3. the mechanisms of photoprotection against photoinhibition in Myrica rubra leaves These results indicated that the photoinhibition was existed in Myrica rubra leaves under conditions in the absence of any environmental stress other than high light. Photoinhibition only induced by high light is reversible. During the early phase of high light stress, there were alternative electron pathways including water-water cycle in Myrica rubra leaves. On the other hand, during the latter phase of the treatment, PSH reaction center was destroyed by 1O2 and irreversibly inhibited, the proportion of opened photosystem II reaction center was decreased, the electron translate of QA- QB was limited, the number of electron used to carbon fixation reduced, the photo energy dissipated by heat was increased. The above results illustrated that xanthophyll cycle was the major protective mechanism to preventing from the deteriorated effects of stron...
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