Effects Of Heat Stress On Photoprotection Mechanism Of Mazie And Soybean Leaves Acclimated To Different Light Regimes

The effects of heat stress(42℃, 24h)on photosynthesis, energy distribution and photodamage defence mechanism of maize and soybean leaves acclimated to different light regimes were studied. The influences of photorespiration on protection of photosynthetic organism and the distribution of excess excitation energy in soybean leaves before and after heat stress were analyzed. The possible reasons of the different responses to heat stress in both maize and soybean leaves were discussed. The main results are as follows: 1. The decreases of photosynthesis in sun-acclimated maize and soybean leaves after heat stress were greater than that in the shaded ones. The decrease was caused by non-stomatal restriction. 2. After heat stress, the PSII reaction centers of sun-acclimated maize and soybean leaves were severely inactivated, and the use of captured excitation energy via photochemical reaction was significantly reduced. In contrast, the inactivation of the PSII reaction center, the reduction of energy distribution to photochemistry in shade-acclimated maize and soybean leaves were less affected. 3. After heat stress, the electron transport was badly inhibited in sun-acclimated maize and soybean leaves. The electron transport from QA to QB was reduced, resulting in the largely accumulated QA－. The accumulation of QA－ might be caused by the lost or the disintegration of the QB protein, it might also correlate to the changes of the conformation of the D1/D2 protein. The loss of the fast PQ pool also contributed to the decrease of the receptor side electron transport in the sun-acclimated maize and soybean leaves. In contrast, the receptor side electron transport of the shade-acclimated maize and soybean leaves was slightly affected. The different responses of the electron transport in maize and soybean leaves acclimated to different light regimes might be related to the degree of thylakoid stacking and the destroy of the membrane caused by heat stress.Heat stress induced the increase of the thermal energy dissipation,4. including the part of the xanthophyll-cycle-independent ones, in both sun and shade plants. The xanthophyll-cycle-dependent energy dissipation was increased in shade-acclimated maize leaves after heat stress, probably due to the increase of the diffusion of the xanthophylls.5. After heat stress, the absolute value of photorespiration was markedly decreased while the ratio of the photorespiration to mass photosynthetic rate was slightly increased in sun-acclimated soybean leaves. It's photorespiration had little effect on theφPSII, Fv'/Fm', qP and NPQ. All this indicated that photorespiration might play a minor role in photoprotection in sun plants after heat stress, wheareas in the shade-acclimated soybean leaves, photorespiration might perform an important function in the maintain of the electron transport and the distribution of the excess light energy in the shade-acclimated soybean leaves after heat stress.6. The content of pigments in both sun- and shade-acclimated maize and soybean leaves were changed after heat stress. The Chl a/b ratio and the content of the carotenoids were increased markedly in the shaded leaves. The increase of the content of the carotenoids played an important role in the detoxifying of the active oxygen and the protection of the membrane. 7. The membrane of the sun-acclimated maize and soybean leaves were notably damaged, as the leakage of the electrolyte and the overoxidation of membrane lipids were more severe than that in the shade ones.8. The activities of PSII reaction center in sun-acclimated soybean leaves were mostly affected compared to the other plants, while the shade-acclimated soybean leaves were the least affected, due probably to the increase of the photorespiration and the content of carotenoids after heat stress.