Coordination of Cyclic Electron Flow and Water-Water Cycle Facilitates Photoprotection under Fluctuating Light and Temperature Stress in the Epiphytic Orchid Dendrobium officinale

Photosystem I (PSI) is the primary target of photoinhibition under fluctuating light (FL). Photosynthetic organisms employ alternative electron flows to protect PSI under FL. However, the understanding of the coordination of alternative electron flows under FL at temperature stresses is limited. To address this question, we measured the chlorophyll fluorescence, P700 redox state, and electrochromic shift signal in leaves of Dendrobium officinale exposed to FL at 42 degrees C, 25 degrees C, and 4 degrees C. Upon a sudden increase in illumination at 42 degrees C and 25 degrees C, the water-water cycle (WWC) consumed a significant fraction of the extra reducing power, and thus avoided an over-reduction of PSI. However, WWC was inactivated at 4 degrees C, leading to an over-reduction of PSI within the first seconds after light increased. Therefore, the role of WWC under FL is largely dependent on temperature conditions. After an abrupt increase in light intensity, cyclic electron flow (CEF) around PSI was stimulated at any temperature. Therefore, CEF and WWC showed different temperature responses under FL. Furthermore, the enhancement of CEF and WWC at 42 degrees C quickly generated a sufficient trans-thylakoid proton gradient (Delta pH). The inactivation of WWC at 4 degrees C was partially compensated for by an increased CEF activity. These findings indicate that CEF and WWC coordinate to protect PSI under FL at temperature stresses.

Automated summary

Photosystem I (PSI) is the main target of photoinhibition under fluctuating light (FL), and photosynthetic organisms use alternative electron flows to protect PSI under FL. The coordination of alternative electron flows under FL at different temperature stresses varies, with different responses observed in cyclic electron flow (CEF) and water-water cycle (WWC) at different temperatures. The findings indicate that CEF and WWC coordinate to protect PSI under FL at temperature stresses.