Adaptation mechanism of plant leaves to photooxidative stresses

1. 蓝藻的循环电子流：Synechocystis PCC 6803 （WT）显示P700的低氧化水平，但其NAD(P)H脱氢酶缺陷突变体在远红光（FR）下显示P700的完全氧化，表明来自呼吸底物的电子通过NAD(P)H脱氢酶介导的光合系统间链减少了P700^+。FR照射后P700^+的减少主要是由于呼吸供体的电子流（t_<1/2>= 0.3 s），而在DCMU存在下的50 ms脉冲光后，PSI产生的NAD(P)H的电子流（t_<1/2>= 72 ms）导致P700^+的减少。这在突变细胞中也被抑制。因此，NAD(P)H脱氢酶似乎是NAD(P)H介导的循环流动和呼吸流动的主要进入系统间链的位点。C4植物的循环电子流：通过光化照射和50ms脉冲光照射后远红光对P700再氧化的时间过程，估计了C3和C4植物在光化照射期间捐赠给P700^+并储存在基质中的电子池大小。光化照射后叶绿素荧光的升高进一步维持基质中的C4电子，而光化照射后的远红光和50ms脉冲光使叶绿素荧光猝灭。C4植物的池一般比C3植物大得多。光化照射后叶绿素荧光的上升进一步维持了基质的电子捐赠，而光化照射被远红光猝灭。光化光照下积累的还原物，可能是苹果酸盐的NADPH，可以通过C4植物PSII缺陷束鞘叶绿体的质体醌池捐献给P700^+。

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.