The effects of salt stress on photosynthesis, Mehler-peroxidase reaction (MPR) and the susceptibility of PS II to photoinhibition were investigated in Rurnex K-1 leaves. Salt stress resulted in dramatic decrease in photosynthesis, but had no significant effect on maximal photochemistry of PSII (Fv/Fm). During photosynthetic induction, a considerable electron flow was transported to oxygen in MPR both in control and salt-stressed leaves. Under steady state photosynthesis, enhanced electron flow to oxygen in MPR was observed only in salt-stressed leaves. The enhanced MPR in salt-stressed leave was accompanied by enhanced activities of scavenging enzymes, i. e. superoxidase dismutase (SOD) and ascorbate peroxidase (APX). In the presence of saturating CO2 decreasing oxygen concentration from 21% to 2% did not affect the susceptibility to photoinhibition in control leaves, but largely increased the susceptibility to photoinhibition in salt-stressed leaves. Based on these results, it is concluded that the enhanced MPR in salt-stressed Rurnex leaves serves as a sink to drain the excess electrons off the electron chain and thus mitigates photoinhibition.

A study was conducted, using chlorophyll fluorescence, rapid fluorescence induction kinetics, and polyphasic fluorescence transients, to determine the effect of salt treatment and heat stress on PSII photochemistry in Rumex leaves. Salt treatment was accomplished by adding NaCl solutions of different concentrations ranging from 50 to 200 mmol/L. Heat stress was induced by exposing the plant leaves to temperatures ranging from 29 to 47 C. The control plants were grown without NaCl treatment. The data acquired in this study showed that NaCl treatment alone had no effect on the maximal photochemistry of PSII or the polyphasic rise of chlorophyll fluorescence. However, the NaCl treatment modified heat stress on PSII photochemistry in Rumex leaves, which was manifested by a lesser heat-induced decrease in photochemical quenching (qP), efficiency of excitation energy capture by open PSII reaction centers (Fv′/Fm′), and quantum yield of PSII electron transport (φPSII). The data also showed that NaCl treatment compromised the impact of heat stress on the capacity of transferring electrons from QA-to QB. Furthermore, the NaCl treatment promoted heat resistance of O2-evolving complex (OEC). In summary, NaCl treatment enhanced the thermostability of PSII.

By measurement of gas exchange and chlorophyll fluorescence, the effects of salt shock on photosynthesis and the mechanisms to protect photosynthetic machinery against photodamage during salt shock were investigated in leaves of Rumex seedlings. Salt shock induced significant decrease in photosynthesis both in 21 and 2 % O2. In 21 % O2, quantumyield of photosystem 2 (PS2) electron transport (ФPS2) decreased slightly and qP remained constant, suggesting that the excitation pressure on PS2 did not increase during salt shock. In 2 % O2, however, both ФPS2 and qP decreased significantly, suggesting that the excitation pressure on PS2 increased during salt shock. NPQ increased slightly in 21 % O2 whereas it increased significantly in 2 % O2. The data demonstrated that during salt shock a considerable electron flow was allocated to oxygen reduction in the Mehler-peroxidase reaction (MPR). Under high irradiance and in the presence of saturating CO2, the susceptibility of PS2 to photoinhibition in salt-shocked leaves was increased when the electron flow to oxygen in MPR was inhibited in 2 % O2. Hence, MPR is important in photoprotection of Rumex seedlings during salt shock.

Excitation energy dissipation, including the xanthophyll cycle, during senescence in wheat flag leaves grown in the field was investigated at midday and in the morning. With progress of senescence, photosynthesis (Pn) and actual PSII photochemical efficiency (φPSII) decreased markedly at midday. The decrease in extent of Pn was greater than that of DPSII. However, there was no significant decline in Pn and CPSII observed in the morning, except in leaves 60 days after anthesis. The kinetics of xanthophyll cycle activity, thermal dissipation (NPQ), and of observed at midday during senescence exhibited two distinct phases. The first phase was characterized by an increase of xanthophyll cycle activity, NPQ, and of during the first 45 days after anthesis. The second phase took place 45 days after anthesis, characterized by a dramatic decline in the above parameters. However, the qI, observed both at midday and in the morning, always increased along with senescence. A larger proportion of NPQ insensitive to DTT (an inhibitor of the de-epoxidation of V to Z) was also observed in severely senescent leaves. In the morning, only severely senescent leaves showed higher xanthophyll cycle activity, NPQ, qf, and ql. It was demonstrated that, at the beginning of senescence or under low light, wheat leaves were able to dissipate excess light energy via NPQ, depending on the xanthophyll cycle. However, the xanthophyll cycle was insufficient to protect leaves against photodamage under high light, when leaves became severely senescent. The ratio of (Fj Fo)/(Fp Fo) increased gradually during the first 45 days after anthesis, but dramatically increased 45 days after anthesis. We propose that another photoprotection mechanism might exist around reaction centres, activated in severely senescent leaves to protect leaves from photodamage.

Chlorophyll fluorescence kinetics was used to investigate the effect of 1,4-dithiothreitol (DTT) on the distribution of excitation energy between photosystem 1 (PS1) and photosystem 2 (PS2) in soybean leaves under high irradiance (HI). The maximum PS2 quantum yield (Fv/Fm) was hardly affected by the presence of DTT, however, photon-saturated photosynthesis was depressed distinctly. Photochemical efficiency of open PS2 reaction centres during irradiation (Fv’/Fm’) was enhanced by about 30–40 % by DTT treatment, whereas photochemical quenching (qP) was depressed by about 40 % under HI. DTT treatment caused a 30 % decrease in allocation of excitation energy to PS1 under HI and a 20 % increase to PS2. An obvious shift in the balance of excitation energy distribution between photosystems was observed in DTT-treated leaves. Though high excitation pressure (1-qP) resulted from DTT treatment, nonphotochemical quenching (qN) was lower. DTT completely inhibited the formation of zeaxanthin and also distinctly depressed the state transition (qT). The shift in the balance of excitation distribution between the two photo systems induced by DTT was mainly due to the enhancement of excitation energy capture by PS2 antenna and the inhibition of state transition. It might be the shift in the balance between the two photo systems that mainly induced the depression of photosynthesis. Thus, to keep high efficiency of use of absorbed photon energy, it is necessary to maintain the balance of excitation distribution between PS2 and PS1.