Adaptation mechanism of plant leaves to photooxidative stresses

1.蓝细菌的循环电子流：集胞藻PCC 6803（WT）在远红光（FR）下表现出低水平的P700氧化，但其NAD（P）H脱氢酶缺陷型突变体表现出完全的P700氧化，这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还原P700^+。FR照射后P700^+的减少主要是由于来自呼吸供体的电子流（t_<1/2> = 0.3 s），而在DCMU存在下饱和脉冲光照射50 ms后，PSI光产生的NAD（P）H流（t_<1/2>= 72 ms）导致P700^+减少。这在突变细胞中也受到抑制。因此，NAD（P）H脱氢酶似乎是NAD（P）H介导的循环流以及呼吸流进入系统间链的主要位点. C4植物中的循环电子流：在C3和C4植物中，通过光化照射后和50 ms脉冲光后远红光对P700再氧化的时间进程，估算了在光化照射过程中，基质中贡献给P700^+并储存的电子池大小。C4电子从基质进一步持续的叶绿素荧光光化照明后，光化照明和50毫秒脉冲光后，远红光淬灭的上升。C4植物通常表现出比C3植物大得多的库。远红光猝灭的光化照射后叶绿素荧光的上升进一步维持了基质电子的捐赠。光化光照过程中积累的还原当量（可能是苹果酸盐中的NADPH）可以通过C4植物中PSII缺陷的维管束鞘叶绿体中的质体醌库提供给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.