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Effects of Cold-hardening on Photosynthetic Performance and Chilling-induced Photoinhibition in Sweet Pepper Leaves

LIU Peng 3 ZHAO Shi-Jie 1 MENG Qing-Wei 1* WEI You-Ying 2 ZOU Qi 1 ( 1College of Life Science, 2 College of Horticulture, Shandong Agricultural University, Taian 271018; 3Shandong Institute of Pomology, Taian 271000)  
Two genotypes of Capsicum annuum L. were cold-hardened at sub-optimal temperature (14℃) and an irradiance of 250 μmol m -2 s -1 for 5 d. The responses of photosynthetic characteristics and particularly the chlorophyll fluorescence parameters to low temperature in cold-hardened leaves (HL) and unhardened leaves (UHL) were investigated with a CIRAS-1 photosynthetic analyzer and a FMS2 fluorometer under controlled light intensity and temperature. Along with the decreasing temperature, the carboxylation efficiency, the photosynthetic rate of light saturation and the saturating light intensity of CO 2 fixation in sweet pepper leaves decreased, irrespective of varietal difference and cold-hardening. However, the apparent quantum yield of photosynthesis was less affected by the decrease in temperature, indicating that low temperature had less influence on the process of light reaction than carbon assimilation. The enhancement of light energy utilization due to strengthened carbon dioxide fixation by elevated CO 2 concentrations was able to relieve the chilling-induced photoinhibition. Nevertheless, as temperature was lowered to 8℃, the relief effects of enhancing CO 2 concentration became less significant. The photochemical efficiency of photosystem II (Ф PSII),photochemical quenching of fluorescence (q P) and the efficiency of excitation energy capture by open photosystem II reaction centres (Fv′/Fm′) were lower in HL than in UHL at 25℃, suggesting that photoinhibition occurred during cold-hardening. But cold-hardened leaves could maintain higher values of Ф PSII, q P and Fv′/Fm′ under low temperature, implying that they faced lesser excitation energy pressure. Cold hardening enhanced the resistance to chilling-induced photoinhibition of both two cultivars. However, the effects of cold hardening were more pronounced in cold-tolerant cultivar.
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