作 者 ：张教林, 曹坤芳
期 刊 ：植物生态学报 2002年 26卷 6期 页码：639-646
Keywords:Antioxidants, Heat dissipation, Photoinhibition, Photosynthetic acclimation, Reactive oxygen, Tropic,
摘 要 ：研究了生长于不同光照条件下（100%、25%和8%光强）热带雨林冠层树种绒毛番龙眼（Pometia tomentosa）和中层树种滇南风吹楠（Horsfieldia tetratepala）幼苗的光合能力、热耗散、活性氧和保护性酶的活性。结果表明，绒毛番龙眼的最大光合速率随着生长光强的增加而提高，而滇南风吹楠在全光条件下的最大光合速率反比25%光照条件下的低。全光条件下两个树种光系统II的最大光化学效率（Fv/Fm）都显著降低，表明发生了长期光抑制。当把生长于遮荫条件下的幼苗移到全光下，从凌晨到中午随着光强的增加光抑制加剧，日落时生长于8%光照条件下的绒毛番龙眼及生长于8%和25%光照条件下的滇南风吹楠的光抑制不能完全恢复。非光化学猝灭对光强的响应曲线表明，随着生长光强的增加滇南风吹楠的热耗散能力增强，而生长在全光和25%光照条件下的绒毛番龙眼的热耗散能力都比滇南风吹楠的弱。两个树种叶片中O-[]·2、H2O2含量、SOD和CAT活性均随着生长光强的增加而提高；在同一光照条件下，绒毛番龙眼叶片中O-[]·2、H2O2含量、SOD和CAT活性显著高于滇南风吹楠。上述结果表明，在光抑制条件下，冠层树种绒毛番龙眼较大程度通过提高保护性酶的活性来保护光合机构免受损伤，而中层树种滇南风吹楠却较大程度通过增强非光化学猝灭来耗散过量光能；滇南风吹楠对强光的适应性差。
Abstract:Tropical rain forest trees often experience several episodes of different light environments during their growth from seedlings into canopy trees, such as in canopy gaps with high irradiance and in forest understorey with very low irradiance. Shade tolerant trees usually have low photosynthetic capacities and low plasticity in photosynthetic acclimation to different light environments. They may suffer from excess light when growing in high light environment, resulting in photoinhibition and photoxidation. Also, when forest understorey plants are suddenly exposed to high irradiance such as canopy gap creation, they may suffer severe photodamage. Plants have two major mechanisms to prevent photodamage, i.e., thermal dissipation through the development of an increased transthylakoid pH gradient and xanthophyll cycle, and antioxidant system to scavenge oxygen free radicals resulting from excess light energy. When growing in high light, plants may increase photosynthetic energy utilization or increase photoprotection capacities or both. This study investigated the effect of growth light level on the capacities of photosynthesis, heat dissipation and antioxidation in seedlings of two tropical rainforest tree species, Pometia tomentosa (Sapindaceae), an important canopy species, and Horsfieldia tetratepala (Myristicaceae), a subcanopy species. The seedlings were cultivated under shading nets with three different light levels, i.e. 100%, 25% and 8% daylight, respectively. The maximum photosynthetic rate (Pmax) of P. tomentosa increased with the growth light level. In contrast, Pmax of H. tetratepala grown in 100% daylight was smaller than that in 25% daylight. Under either 100% or 8% daylight, P. tomentosa had larger Pmax values than H. tetratepala. The values of both photosynthetic light saturation point and chlorophyll content per unit mass increased with growth light level for both species, and under the same light levels, P. tomentosa had greater values than H. tetratepala. Chlorophyll a/Chlorophyll b ratios were not significantly different for both species under the three different light levels; however, P. tomentosa had significantly larger values than H. tetratepala. Compared to the shaded situations, in the fully open site, the maximum photochemical efficiency of photosystem II (Fv/Fm) of both species were reduced, indicating that sustained photoinhibition occurred in the seedlings grown under full sun. When the shaded seedlings were transferred to full sun, on the first day, photoinhibition (as indicated by the decrease of ΔF/Fm′) accelerated from dawn to noon, and the recovery from photoinhibition occurred gradually in the afternoon but was not completed until sunset in P. tomentosa seedlings from the 8% daylight site and in H. tetratepala seedlings from both 8% and 25% daylight sites. The light response curves of non-photochemical quenching showed that heat dissipation capacity of H. tetratepala increased with growth light level. Whereas, P. tomentosa had lower heat dissipation capacity in the fully open site compared to in shaded sites. In the fully open site and in 25% daylight site, P. tomentosa had lower heat dissipation capacity than H. tetratepala. The contents of O-[]·2, H2O2 and activities of SOD and CAT in the seedlings in the open site were the greatest for both species, and these were higher in P. tomentosa than in H. tetratepala under the same light levels. With the pooled data from both species under all light levels, there were significantly correlations among O-[]·2, H2O2 content, and SOD and CAT activities. This indicated that oxygen free radicals in seedlings were scavenged through the enhancement of activities of antioxidative enzymes. The present study shows that the canopy species P. tomentosa has higher photosynthetic plasticity but lower thermal dissipation capacity compared to the subcanopy species H. tetratepala. Under photoinhibitory conditions, the photoprotection of P. tomentosa was more dependent on the enhancement of activities of antioxidative enzymes, while H. tetratepala was more dependent on the non_radiative heat dissipation. H. tetratepala showed poor acclimation to grow in fully open sites.