Effects of growth light intensities on photosynthesis in seedlings of two tropical rain forest species
WANG Bo-Yi~1, FENG Yu-Long~(1,2*) (1. Kunming Division, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Kunming 650223, China; 2. College of Life Sciences, Hebei University, Baoding 071002, China).
Acclimation to growth light intensities and the strategies of photoprotection were explored in seedlings of two tropical rainforest tree species, Pometia tomentosa and Trema orientalis. The former is a late-successional species in forest, and its seedlings distribute in understory or small canopy gap. Whereas the latter is a pioneer tree species, and its seedlings occur in canopy gap. The plant materials were raised under four light regimes [a. 3.1%/12.5% (Before/after fog disappear, percent of sunshine); b. 12.5%/12.5%; c. 9%/36%; d. 25%/36%] for about 3 months during the foggy and cool season in 2002 at Xishuangbanna, Yunnan Province, China. Then maximum net photosynthetic rate (P_(max)), chlorophyll fluorescence parameters, chloroplast pigments content, and lamina mass per unit area (LMA) were measured. We manipulated morning light intensities to simulate the effect of fog which could shield sunlight for about 60%～90% in the morning. Fog presents almost every day in this season. The main purpose of this study was to determine whether decreasing morning light intensity (the effect of fog) could ameliorate photoinhibition of photosynthesis when the light intensity was the same in the afternoon. If so, we could conclude that fog could protect tropical rainforest tree species from photoinhibition through screening sunlight in Xishuangbanna. For the two species, with the increase of growth light intensity LMA, non-photochemical quenching coefficient (NPQ), the carotenoid content expressed as unit leaf dry weight and expressed as unit chlorophyll, light saturation point and light compensation point increased, while chlorophyll concentration decreased. Seedlings of the two species could acclimate to different growth light regimes in this study through changes of physiological and morphological traits. Diurnal photoinhibition of photosynthesis, as judged by maximum efficiency of PSⅡ photochemistry, was significantly severer in P. tomentosa than in T. orientalis in all growth light regimes. The acclimation capacity to high light regimes was stronger in T. orientalis than in (P. tomentosa.) With the increase of growth light intensity, P_(max) increased significantly, but NPQ not in T. orientalis. While the reverse trends occurred in P. tomentosa. At the same light regimes thermal dissipation was much lower, but P_(max) was much higher in T. orientalis than in P. tomentosa. This indicates that photodamage was avoided mainly through increasing light energy utilization by photosynthesis in T. orientalis, a pioneer tree species, but through increasing thermal dissipation in (P. tomentosa,) a late-successional species. In addition, photosynthetic pigments content and LMA were lower in T. orientalis than in (P. tomentosa,) indicating that the percent of light energy absorbed by the former is lesser. This can ameliorate excitation pressure on photosystem Ⅱ. The effects of decreasing light intensity in the morning was significant in P. tomentosa, but not in (T. orientalis.) Decreasing light intensity in the morning could ameliorate diurnal photoinhibition of photosynthesis significantly in P. tomentosa, but not in T. orientalis. Fog can decrease light intensity by 60%～90% in the morning in foggy and cool season in Xishuangbanna. Shading by fog in the morning might be important to P. tomentosa and other late-successional species, especially to chilling sensitive species. So we hypothesized that fog be important to the existence of tropical rainforest in Xishuangbanna, and one of its role be to ameliorate photoinhibition of photosynthesis through decreasing light intensity in the morning.
【CateGory Index】： Q945
【CateGory Index】： Q945