Exogenous calcium alleviates low night temperature stress on the photosynthetic apparatus of tomato leaves.

Guoxian Zhang,Tianlai Li,Zhaojuan Meng,Yang Ni,Tao Lu,Yufeng Liu

doi:10.1371/journal.pone.0097322

Abstract

The effect of exogenous CaCl2 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 CaCl2 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 CaCl2 alleviated this increase. Exogenous Ca2+ enhanced stimulation of CEF by LNT stress. Inhibition of oxidized PQ pools caused by LNT stress was alleviated by CaCl2 pretreatment. LNT stress reduced zeaxanthin formation and ATPase activity, but CaCl2 pretreatment reversed both of these effects. LNT stress caused excess formation of a proton gradient across the thylakoid membrane, whereas CaCl2 pretreatment decreased the said factor under LNT. Thus, our results showed that photoinhibition of LNT-stressed plants could be alleviated by CaCl2 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.

Highlights

Tomato (Lycopersicon esculentum Mill.), an important vegetable crop cultivated in Northeastern China, often encounters low night temperature (LNT) stress followed by warm sunny days
photosynthetic rate (Pn) increased markedly with CaCl2 pretreatment but decreased with ethylene glycol tetraacetic acid (EGTA) pretreatment (Fig. 1). These results indicate that the sensitivity of tomato leaves to high light was increased significantly by LNT stress, while CaCl2 pretreatment decreased it
Our results show that the ratio DpH/DYwas significantly higher under LNT and this change was accompanied by lower photosystem II (PSII) activity and ATP synthesis compared to the control (Fig. 8B, Fig. 3A and Fig. 7B, respectively)

Summary

Introduction

Tomato (Lycopersicon esculentum Mill.), an important vegetable crop cultivated in Northeastern China, often encounters low night temperature (LNT) stress followed by warm sunny days. Our previous studies showed that carbon fixation capacity, linear electron transport, and active oxygen-scavenging enzymes is inhibited by LNT stress followed by growth light [1,2]. Excess ROS accumulation causes peroxidation of thylakoid membrane lipids, degradation of D1 protein, and photoinhibition of PSII and PSI [13,14,15,16]. Non-photochemical quenching (NPQ) mechanism and alternative electron transport pathways can dissipate excessive excitation pressure accumulated in PSII reaction centers without causing adverse effects. Aside from NPQ, cyclic electron flow (CEF) is another major protection mechanism for PSII under excessive light conditions because it generates extra proton gradient (DpH) across the thylakoid membrane [18,19,20,21]. Researchers have strived for years to fully understand the mechanism of photoinhibition and photoprotection and to find a proper method to alleviate photoinhibition

Methods

Results

Conclusion