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.

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.

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.

In order to fully understand the adaptive strategies of young leaves in performing photosynthesis under high irradiance, leaf orientation, chloroplast pigments, gas exchange, as well as chlorophyll a fluorescence kinetics were explored in soybean plants. The chlorophyll content and photosynthesis in young leaves were much lower than that in fully expanded leaves. Both young and fully expanded leaves exhibited down-regulation of the maximum quantum yield (FV/FM) at noon in their natural position, no more serious down-regulation being observed in young leaves. However, when restraining leaf movement and vertically exposing the leaves to 1200 µM01 m-2 s-1 irradiance, more pronounced down-regulation of Fv/FM was observed in young leaves; and the actual photosystem II (φPSII) efficiency (φPSII) drastically decreased with the significant enhancement of non-photochemical quenching (NPQ) and `High energy' quenching (qE) in young leaves. Under irradiance of 1200 µmol m 2 s-1, photorespiration (Pr) in young leaves measured by gas exchange were obviously lower, whereas the ratio of photorespiration/gross photosynthetic rate (Pr/Pg) were higher than that in fully expanded leaves. Compared with fully expanded leaves, young leaves exhibited higher xanthophyll pool and a much higher level of de-epoxidation components when exposure to high irradiance. During leaf development, the petiole angle gradually increased all the way. Especially, the midrib angle decreased with the increasing of irradiance in young leaves; however, no distinct changes were observed in mature leaves. The changes of leaf orientation greatly reduced the irradiance on young leaf surface under natural positions. In this study, we suggested that the co-operation of leaf angle, photorespiration and thermal dissipation depending on xanthophyll cycle could successfully prevent young leaves against high irradiance in field.

By analysis of gas exchange and chlorophyll fluorescence, the effects of NaCl treatment and supplemental CaCl2 on photosynthesis, photosystem II (PSII) photochemistry and photoinhibition were investigated in Rumex leaves. Photosynthesis in Rumex leaves was strongly inhibited by 200mM NaCl treatment. Such inhibition of photosynthesis was ameliorated by CaCl2 supplement. Neither NaCl treatment nor CaCl2 supplement had any significant effects on the PSII primary photochemical reaction in dark-adapted leaves. In light-adapted leaves, however, 200mM NaCl2 treatment significantly decreased photochemical quenching (qp), efficiency of excitation energy capture by open PSII reaction centers (FV’/FM’) and quantum yield of PSII electron transport (φPSII). These decreases in qp, FV’/FM’ and VPSII were mitigated by CaCl2 supplement with the maximum of its effect appearing at a concentration of 8 mM CaCl2. A similar mitigating effect was shown in 200 mM NaCl-treated Rumex leaves when susceptibility of φPSII to photo inhibition was determined under high irradiance. It is suggested that the mitigation of photo inhibition in NaCl-treated leaves is because of the amelioration of inhibition of photosynthesis.

Influences of different nitrogen applications on photosynthesis and utilization of excitation energy were explored by comparing two field-grown wheat (Triticum aestivum L.) cultivars with high or low grain protein content [High protein cultivar ‘8901’ (HC) and low protein cultivar ‘1391’ (LC)]. High nitrogen application significantly decreased both CO2 assimilation and photorespiration in both cultivars during the early stages after anthesis. However, the actual photosystem II (PSII) efficiency (φPSII) was not significantly different between high, moderate and low nitrogen applications in the HC. As a result, the ratio of VPSII to the quantum yield of carbon metabolism (φCO2) measured under non-photorespiratory conditions in the HC was higher under high nitrogen application than under low or medium nitrogen application. The grain protein content of the HC was also increased by high nitrogen application. In contrast, high nitrogen application decreased the actual PSII efficiency in the flag leaves of the LC in the early stages after anthesis and different nitrogen applications did not significantly alter the φPSII/φCO2 ratio or grain protein content in the LC. No significant difference was detected in the activity of superoxide dismutase or ascrobate peroxidase between different nitrogen treatments in either cultivar throughout the entire experimental period. These results indicate that more excitation energy is partitioned to nitrogen metabolism in the flag leaves of the HC under high nitrogen application whereas the partitioning of excitation energy in the LC was not affected by nitrogen application.

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.

Gas exchange, chlorophyll a fluorescence kinetics, chloroplast pigments and antioxidant enzymes were investigated to explore the development of photo protective mechanisms in soybean leaves from emergence to full expansion under field conditions. During leaf development, photosynthesis (Pn) gradually increased. Although the maximum quantum yield of PSII photochemistry (Fv/Fm) was quite high at the initial stages of leaf development, it was appreciably lower than that in fully expanded leaves. During daily courses, reversible decrease in Fv/Fm was noted at various stages, implying that no photodamage occurred. When exposed to irradiance, a substantial elevation in the actual PSII efficiency (φPSII) together with a remarkable decrease in non-photochemical quenching (NPQ) was observed with the process of leaf development. In this study, we observed that newly initiating leaves possessed relative larger xanthophyll pool size (V + A + Z)/Chl, but it began to decrease with leaf expanding until fully expanded. Further experiments revealed that the decline of relative xanthophyll cycle pool size during leaf expanding was owing to the fact that the chlorophyll contents per area increased faster than that of the xanthophyll cycle pigments. In addition, activities of the antioxidative enzymes, such as SOD, APX, CAT, POD and GR, were also enhanced at the early stages of leaf expansion. We suggested that photoprotective mechanisms, including xanthophyll cycle and antioxidant enzymic system, have developed firstly as soon as leaf emerged. It was the timely development of the mechanisms in young leaves that dissipate excess excitation energy protecting leaves against photodamage at the early stages of leaf development. The relative leaf water content increased with the process of leaf development and exhibited signicant negative linear correlation with NPQ, xanthophyll cycle pigments and antioxidant enzyme activities in soybean leaves. We proposed that the changes in leaf turgor might be involved in triggering the activation of photoprotective mechanisms together with high irradiance at the initiating stages of leaf expanding.

Changes in the activities of enzymes involved in scavenging active oxygen species were followed after exposing bean seedling leaves (Phaseolus vulgaris L.) to various cross stresses of irradiance and temperature. The activities of superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (AsAPOD, EC 1.11.1.11) increased to different extent with prolonged irradiation of the leaves, and were stimulated by high temperature (HT). The activity of catalase (CAT, 1.11.1.6) decreased when exposed to strong irradiance (HI), and the decrease was further exacerbated when HI was combined with HT. CAT activity was more sensitive to HT than to HI. Ascorbate (AsA) content slightly decreased and then increased during the treatment of HI, but decreased under the cross stress of HI and HT. On the contrary, glutathione (GSH) content increased all the time during various treatments of irradiance and temperature. The increase in the combined stress was even more pronounced. Irradiance is the major reason in triggering the operation of xanthophyll cycle, which was difficult to be started by HT. The antioxidant systems tended to be inactivated with prolonged exposure to the cross stress of HI and HT. The de-expoxidated state of xanthophyll cycle, however, was increasing all the time, which indicated that the zeaxanthin-dependent thermal dissipation was one major energy dissipation pathway during the cross stress of HI and HT.