Exogenous Calcium Alleviates Low Night Temperature Stress on the Photosynthetic Apparatus of Tomato Leaves

The effect of exogenous CaCl(2) on photosystem I and II (PSI and PSII) activities, cyclic electron flow (CEF), and proton motive force of tomato leaves under low night temperature (LNT) was investigated. LNT stress decreased the net photosynthetic rate (Pn), effective quantum yield of PSII [Y(II)], and photochemical quenching (qP), whereas CaCl(2) pretreatment improved Pn, Y(II), and qP under LNT stress. LNT stress significantly increased the non-regulatory quantum yield of energy dissipation [Y(NO)], whereas CaCl(2) alleviated this increase. Exogenous Ca(2+) enhanced stimulation of CEF by LNT stress. Inhibition of oxidized PQ pools caused by LNT stress was alleviated by CaCl(2) pretreatment. LNT stress reduced zeaxanthin formation and ATPase activity, but CaCl(2) pretreatment reversed both of these effects. LNT stress caused excess formation of a proton gradient across the thylakoid membrane, whereas CaCl(2) pretreatment decreased the said factor under LNT. Thus, our results showed that photoinhibition of LNT-stressed plants could be alleviated by CaCl(2) pretreatment. Our findings further revealed that this alleviation was mediated in part by improvements in carbon fixation capacity, PQ pools, linear and cyclic electron transports, xanthophyll cycles, and ATPase activity.