Abstract:Leaves as the main photosynthetic organs are sensitive to exterior environments. Generally the structure and physiological characteristics of leaves are influenced by water deficiency significantly. In the northern rainforest of Xishuangbanna in China, strong radiation and seasonal drought occur frequently. With the increasing of tree height, the xylem pathways increase, resulting in the increasing of water gravity and pathway resistance. In the rainforest, tree is higher and is liable to be influenced by hydraulic limitation. The response of treetop leaves to hydraulic limitation and seasonal drought in higher trees is particularly important to the study of seasonal rainforest. In order to understand the adaptation strategies of rainforest trees, the changes of leaf anatomical structure, pigment content, and water use efficiency (WUE) under varied tree heights of a liana plant Fissisfigma polyanthum (Hook. f. et Thoms.)Merr. and three co-existing arbors of Sapium baccatum Roxb, Litsea pierrei var. szemois liou and Garcinia cowa Roxb were studied in the rainforest of Xishuangbanna, China. Differences in leaf structure, pigment content and WUE were compared among the four tree species. The results showed that the G. cowa and L. pierrei have thicker leaf (310.14, 319.73 μm), thicker cuticle (6.06, 5.13 μm) and bigger cells (21.48,27.09 μm). In addition, their photosynthetic pigments content were lower compared to the other two species. The palisade tissue occupied larger proportion of the leaf transverse section, and the pigments content were higher in the F. polyanthum than three arbor species. However, the F. polyanthum had thinner leaf (147.67 μm) and lowest WUE, and the S. baccatum had the highest WUE. The leaf thickness, palisade tissue and cuticle thickness, LMA, P/S, and TPM/LT of measured four tree species increased with the increasing of crown height, and the F. polyanthum had the largest increasing. Meanwhile, the leaf chlorophyll and carotenoid content of four tree species reduced with the increasing of crown height. The leaf δ13C value and WUE of four tree species increased with the tree height, where the △ value has an opposite trend. The WUE of F. polyanthum had a small change along tree height than the arbor species. The above results showed that the upper crown leaves of four tree species exhibited xeromorphic structure and high WUE. These xeromorphic structure and physiological characters suggests that the trees may suffer from water stress. In order to reduce the influences of relative water shortage, the water loss of treetop leaves was minimized through reducing stomatal conductance, and the WUE was increased resultantly. Simultaneously, the reduced stomatal conductance led to the decreasing of photosynthesis. The fact of low photosynthetic pigment contents in the upper crown leaves indicates the reduced ability of photosynthetic carbon assimilation in treetop leaves. Xeromorphic structure may limit cell division and expansion, then the gas exchange and carbon assimilation capability are restricted resultantly. However, respiration consumption increases with the spread of tree crown. The nutrient shortage limits carbon investment on new leaf growth, and the tree growth is limited ultimately. These findings support the hypothesis of hydraulic limitation that the height growth of big trees slowed down through reduced absorption of irradiance and carbon, which is caused by the hydraulic limitation at treetop arising from increased pathway resistance.
