Adaptation mechanism of plant leaves to photooxidative stresses

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1. Cyclic electron flow in cyanobacteria : Synechocystis PCC 6803 (WT)showed a low oxidation level of P700, but its NAD(P)H dehydrogenase-defective mutant showed a complete oxidation of P700, under a far-red light (FR), suggesting that electrons from respiratory substrates reduce P700^+ Via photosynthetic intersystem chain mediated by NAD(P)H dehydrogenase. The reduction of P700^+ after illumination of FR was mostly accounted for the electron flow from respiratory donors (t_<1/2> = 0.3 s), whereas, that after a 50-ms, saturating pulse light in the presence of DCMU by flow from the NAD(P)H photoproduced by PSI (t_<1/2>= 72 ms). This was inhibited also in the mutant cells. Thus NAD(P)H dehydyrogenase appears to be the site of main entry into the intersystem chain for the NAD(P)H-mediated cyclic flow as well as for the respiratory flow.2. Cyclic electron flow in C4 plants : The pool size of electrons from the stroma that were donated to P700^+ and stored during actinic illumination was estimated in C3 and C4 plants by the time courses of re-oxidation of P700 by far-red light after actinic illumination and after50-ms pulse light. C4 electrons from stroma was further sustained by the rise of chlorophyll fluorescence after actinic illumination which was quenched by far-red light after actinic illumination and after 50-ms pulse light. C4 plants generally showed much larger pool than C3 plants. The donation of electrons from stroma was further sustained by the rise of chlorophyll fluorescence after actinic illumination which was quenched by far-red light. The reducing equivalent, possibly NADPH from malate, accumulated during actinic illumination can be donated to P700^+ Via the plasto-quinone pool in PSII deficient bundle sheath chloroplasts in C4 plants.