遮荫下2种热带树苗叶片光合特性和抗氧化酶系统对自然降温的响应

选取 2种遮荫条件下 (8%、2 5 %全光照 )西双版纳地区热带雨林上层树种滇南红厚壳和中层树种玉蕊的幼苗 ,测定了雨季末自然降温过程中 2种植物叶片光合特性、抗氧化酶 (SOD、CAT、APX)活性、丙二醛 (MDA)含量的变化。研究结果表明 :低温使 2种植物单位面积叶绿素含量和净光合速率明显下降 ,生长在中等光强下的幼苗比低光强下的幼苗下降的比率大 ,滇南红厚壳比玉蕊下降的比率大。在降温初期 ,即从 10月到 12月初 ,2种光强下 2种植物SOD和APX活性上升 ,CAT活性出现波动 ,黎明光化学效率和MDA含量基本没有变化 ,光合机构没有受到氧化破坏。 12月末 ,中等光强下滇南红厚壳黎明光化学效率降低 ,SOD和APX活性有所下降 ,MDA含量显著上升 ;而低光强下的滇南红厚壳和 2种光强下玉蕊SOD和APX活性仍在上升 ,MDA含量变化较小。这表明 ,2种光强下 2种热带树苗是以不同的方式响应自然降温 ,低温对中等光强下的植物比低光强下的影响大 ,上层树种滇南红厚壳受到的影响比中层树种玉蕊大。同时 ,根据降温过程中抗氧化酶活性和MDA含量的变化 ,大致可将西双版纳热带雨林幼苗对低温的响应分为 2个阶段 ,即第 1阶段为低温适应阶段 ,抗氧化酶活性上升 ,光合机构没有受到明显的氧化破坏 ;第 2阶段 ,中等光强下滇南红厚壳抗氧化酶

The Response of Photosynthetic Characteristics and Enzymatic Antioxidant System in Leaves of Two Tropical Seedlings Growing in Shade Conditions as Temperature Fall

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.