Abstract:

Two tropical seedlings, Calophyllum polyanthum, the canopy species, and Barringtonia macrostachya, the middle layer species, growing in man-made shade conditions (8%, 25% of full sunlight) were investigated through the whole fog season between October 2000 and December 2001 by monitoring the photosynthetic characteristics, antioxidant enzyme activities of SOD, CAT and APX, and MDA content as temperature fell in Xishuangbanna, SW China. Leaves were sampled at approximately 30 days interval. The net photosynthetic rate and chlorophyll content of two species declined markedly in late December compared with those in October. The photochemical efficiency in dawn was not different statistically in fog season except for C. polyanthum growing in moderate light conditions, indicating that PSII was not damaged by photoinhibition under chilling due to the protective responses of the photosynthetic system to the stress. SOD and APX activities in leaves of both species increased from October to early December, and MDA content did not change greatly during this period. While in late December, SOD and APX activities in leaves of plants growing in moderate light declined, and MDA content increased. But CAT activity fluctuated through the whole fog season. APX was important scavengers for this study. A strong positive correlation occurred between APX acitivities and lowest ambient temperatures, suggesting that this enzyme was temperature-regulated, and the decline of CAT activity did not mean the decline of scavenging oxygen enzymes. According to the data of net photosynthetic parameters, enzymatic activities and MDA content, we concluded that the canopy species, C. polyanthum, was affected more significantly by low temperature than the middle layer species, B. macrostachya, and the seedlings growing in moderate light were affected more significantly by low temperature than those growing in low light.The results provided evidence for the ability of the tropical rainforest seedlings to activate enzymatic defense mechanisms to limit the production of free radicals to protect membrane integrity under chilling stress. The protective role of the decline of the chlorophyll was also discussed.

Key words: Tropical tree species, Seedlings, Shading, Temperature fall, Antioxidant enzyme, Lipid peroxidation, Photochemical efficiency