4种高大树木的叶片性状及WUE随树高的变化

Changes of leaf traits and WUE with crown height of four tall tree species

为了解西双版纳北热带雨林高大树木树顶叶片性状对通道阻力增长引起的水力限制增强及高光和季节性干旱等气候条件的响应,对该区乔木浆果乌桕(Sapium baccatum Roxb)、思茅木姜子(Litsea pierrei var.szemois liou)、小叶藤黄(Garcinia cowa Roxb)及共生木质藤本黑风藤(Fissisfigma polyanthum(Hook.f.et Thoms.)Merr.)的叶片形态解剖结构、光合色素、水分利用效率(WUE)等随冠层高度的变化及种间差异进行了研究。结果表明:小叶藤黄和思茅木姜子的叶片(310.14、319.73μm)和角质层(6.06、5.13μm)都较厚、细胞较大(21.48、27.09μm),光合色素含量则较低;黑风藤栅栏组织所占的比例最大、光合色素含量也最高,但叶片薄、WUE最低;浆果乌桕的WUE最高。随着冠层高度的增加,4种树木的叶厚、栅栏组织及角质层厚度、LMA、P/S和TPM/LT均增加、细胞变小,其中黑风藤的变幅最大。4树种的叶绿素和类胡萝卜素含量均随冠层的增高而减少,δ13C和WUE则随树冠增高而增大(黑风藤的变幅小于3种乔木);Δ则相反。上述结果表明4种树木冠层上部叶片偏向旱生结构和水分利用效率增加,暗示树顶叶片可能受到了水分胁迫,从而在结构上偏向于减少水分散失、功能上提高对水分的利用效率以适应水分亏缺;同时,随冠层增加光合色素含量减少,暗示其光合碳同化能力也降低。上述结果支持了水力限制假说中由于通道阻力增大引起树顶水力限制增强,大树可能会通过减少光和碳的获得而减慢树高生长的假设。

Leaves as the main photosynthetic organs are sensitive to exterior environments. Generally the structure and physiological characteristics of leaves are influenced Xishuangbanna in China, strong radiation and seasonal by water deficiency significantly. In the northern rainforest of 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, 319.73μm）, thicker cuticle （6.06, 5.13 μm） and bigger ceils （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 contentwere 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 δ^13C value and WUE of four tree species increased with the tree height, where the A 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.