The water-water cycle (WWC) has the potential to alleviate photoinhibition of photosystem I (PSI) in fluctuating light (FL) at room temperature and moderate heat stress. However, it is unclear whether WWC can function as a safety valve for PSI in FL at chilling temperature. In this study, we measured P700 redox state and chlorophyll fluorescence in FL at 25 °C and 4 °C in the high WWC activity plant Dendrobium officinale. At 25 °C, the operation of WWC contributed to the rapid re-oxidation of P700 upon dark-to-light transition. However, such rapid re-oxidation of P700 was not observed at 4 °C. Upon a sudden increase in light intensity, WWC rapidly consumed excess electrons in PSI and thus avoided an over-reduction of PSI at 25 °C. On the contrary, PSI was highly reduced within the first seconds after transition from low to high light at 4 °C. Therefore, in opposite to 25 °C, the WWC is not a major alternative sink in FL at chilling temperature. Upon transition from low to high light, cyclic electron transport was highly stimulated at 4 °C when compared with 25 °C. These results indicate that D. officinale enhances cyclic electron transport to partially compensate for the inactivation of WWC in FL at 4 °C.

水-水循环（WWC）具有减轻光系统I（PSI）在室温和中等热应力下的波动光（FL）中的光抑制的潜力。但是，尚不清楚WWC是否可以在冷却温度下用作FL中PSI的安全阀。在这项研究中，我们测量在FL P700氧化还原状态和叶绿素荧光在25℃下和在高WWC活性的植物4℃铁皮石斛。在25°C下，WWC的运行有助于从暗到亮过渡时P700的快速再氧化。但是，在4°C时未观察到P700的这种快速再氧化。当光强度突然增加时，WWC迅速消耗了PSI中的过量电子，因此避免了25°C下PSI的过度还原。相反，在4°C下从弱光过渡到强光后的最初几秒钟内，PSI显着降低。因此，与25°C相反，WWC在冷却温度下并不是FL中主要的替代接收器。从低光过渡到高光时，与25°C相比，在4°C时强烈刺激了循环电子传输。这些结果表明，D。officinale增强了循环电子传递，以部分补偿FL在4°C下WWC的失活。