Photoinhibition of O2 evolution and reactions leading to millisecond-delayed light emission (ms-DLE) of chlorophyll by illumination of leaves with excess white light were investigated in wheat seedlings greened for different times in a special chamber with constant conditions (20 degrees C; CO2 and humidity). A sharp reduction in initial and steady state rates of O2 evolution and in the intensity of different components of ms-DLE under excess light on the stage of lag-phase of chlorophyll biosynthesis (4-6h of greening) were observed. An increasing stability of the oxygen-evolving process and ms-DLE of chlorophyll during formation of the thylakoid membrane photosystems (12-24 h of greening) was shown. Rifampicin did not influence the stability of oxygen evolution whereas cycloheximide led to the intensification of photoinhibition of the initial and steady-state rates of oxygen evolution under the inhibitory light action. The early stages of photosystems formation during short time of greening of etiolated seedlings were more sensitive to the action of inhibitory light, possibly due to a weak interaction of the oxygen-evolving system components and connection with reaction centers of Photosystem II.

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Action sites of low positive temperature together with high light intensities in electron transport reactions of photosystem II (PSII) evaluated by ms range delayed fluorescence (ms-DLE) of chlorophyll a content in the maize and barley seedlings are presented. The main targets for these stress factors action were shown to be Yz and Mn4OxCa-cluster on the donor site of PSII in the case of simultaneous action of different temperatures and high light illumination and between QA and QB on the acceptor site of PSII in the case of low positive temperature influence only.