Photoprotective Mechanisms And Regulation Of PSⅡ Photochemistry In Rumex Leaves Under NaCl Stress

Rumex, a hybrid of Rumex Patientia × R. tianschaious, was used in this study. Rumex is asalt-tolerant fodder crop with a high content of leaf protein, which is used in NorthwesternChina for the reclamation of saline soil where plants often experience high temperature duringthe summer. By measurements of photosynthetic gas exchange, modulated chlorophyllfluorescence, rapid fluorescence induction kinetics and polyphasic rise of fluorescence, weevaluated the photoprotective mechanisms and regulation of PSII photochemistry in Rumexleaves under NaCl stress. The characteristics of PSII photochemistry and thermostability inNaCl-stressed Rumex leaves were also examined. Furthermore, we examined the effects ofCa2+ supplement on photosynthesis and PSII photochemistry in Rumex leaves under NaClstress. The main results are as follows:1. Regulation of PSII photochemistry under NaCl stress NaCl stress had no effect on themaximal photochemistry of PSII (Fv/Fm), the rapid fluorescence induction kinetics or thepolyphasic rise of chlorophyll fluorescence, indicating that NaCl stress had no effect onprimary photochemistry of PSII. However, in the light–adapted leaves NaCl stress decreasedthe photochemical coefficient (qP), the efficiency of excitation energy capture by open PSIIreaction center (Fv'/Fm') and quantum yield of PSII electron transport (ФPSII) and increasednon-photochemical quenching (NPQ). This suggests that NaCl stress modified the PSIIphotochemistry in the light-adapted leaves and such modifications may be a mechanism todown-regulate the photosynthetic electron transport to match a decreased CO2 assimilation.In NaCl-stressed leaves, post-illumination rise of chlorophyll fluorescence was observed,which was absent in control leaves. This suggests that NaCl stress induced cyclic electrontransport around PSI. In addition, it was found that NaCl stress induced modification of 1Chen Hua-xin: Photoprotective Mechanisms and Regulation of PSII Photochemistry in Rumex Leaves under NaCl Stressenergy distribution between the two photosystems, with more light energy distributed to PSI.We speculated that NaCl stress increased PSI activities and that the light energy flowing fromPSII is used to drive PSI electron transport.2. Effects of NaCl stress on Mehler-peroxidase reaction (MPR) During photosyntheticinduction, it was observed that Jf (electron transport rate estimated by chlorophyllfluorescence) was much higher than Jg (electron transport rate estimated by gas exchange).The ratio of Jf/Jg was at its maximum at the start time of illumination both in control andsalt-stressed leaves. When steady state photosynthesis was approached, Jf and Jg almostconverged in control leaves whereas in salt-stressed leaves Jf was still much higher than Jg. Inthe presence of 2 % O2, the difference between Jf and Jg in salt-stressed leaves was largelydiminished, indicating that the alternative electron sink was restricted to Mehler-peroxidasereaction (MPR). Under steady state photosynthesis, ФPSII and Jf in control leaves wereinsensitive to O2 concentration. In 200 mmol/L NaCl-stressed leaves, however, both ФPSII andJf were sensitive to O2 concentration. These data suggests that under steady statephotosynthesis, a considerable electron flow to oxygen in the Mehler-peroxidase reactionoccurred in salt-stressed leaves. The enhanced MPR in salt-stressed leave was accompaniedby enhanced activities of scavenging enzymes (SOD and APX). In the presence of saturatingCO2, decreasing O2 concentration from 21 % to 2 % did not affect the susceptibility tophotoinhibition in control leaves, but largely increased the susceptibility to photoinhibition inNaCl-stressed leaves. It is concluded that the enhanced MPR in salt-stressed Rumex leavesserves as a sink to drain the excess electrons off the electron chain and thus mitigatesphotoinhibition.Salt shock induced significant decrease in photosynthesis both in 21 and 2...