Effects of drought stress on xylem permeability and transpiration control

许多树种在干旱条件下都可观察到枯死现象。以干旱条件下生长的柏树（Chamaecyparis obtusa）和杜松（Juniperus chinensis）幼苗为研究对象，比较了不同枯死发生率树种的木质部结构和水力导度。叶片比水导率从叶片基部到叶片顶部是恒定的，而柽柳叶片比水导率在叶片顶部高于叶片底部。可能是这些物种特有的水力导率造成了枯死发生率的差异。大多数柏科物种在一个个体中有针叶和鳞叶。以生长在中国半干旱区的柏科树种Sabina vulgaris为研究对象，研究了异叶性与生长条件的关系，以阐明异叶性与抗逆性的关系。在湿润和遮荫条件下生长的乳母植物幼苗中，针叶比例远高于鳞片叶片，而在干旱和空旷地生长的大型个体中，鳞片叶片比例远高于针叶叶片。叶片形态可能随光照和土壤水分条件的变化而变化，而异叶性使其在半干旱区维持种群数量。以白桦树（Quercus glauca）和芡实（Eurya japonica）幼苗为研究对象，对不同光照条件下2种树种的水分平衡进行了比较。结果表明，在强光条件下，不同树种间水分平衡存在显著差异，蒸腾速率增加。青花可能是通过气孔导度低和导管直径小来减少气孔空化，从而减少水分流失。粳稻可能通过气孔导度低和总叶面积小来减少水分流失

Dieback is observed in drought conditions for many tree species. Xylem structure and hydraulic conductivities were compared between tree species with different dieback incidences, using seedlings of Chamaecyparis obtusa and Juniperus chinensis growing under drought conditions. Leaf specific hydraulic conductivities were constant from the base to the top of the shoots in C. obtusa, whereas those of J. chinensis were higher in the top of the shoots than in the base. It is probable that these species specific hydraulic conductivities cause the differences of dieback incidencesMost of Cupressaceae species have needle leaves and scale leaves in an individual. We investigated the relations between these heterophylly and growing conditions for Cupressaceae species, Sabina vulgaris, growing in semi-arid areas of China, to demonstrate the relations of heterophylly to the stress tolerance. Ratios of needle leaves were much higher than those of scale leaves in the seedlings, which can grow in the moist and shade conditions under the canopies of nurse plants, whereas ratios of scale leaves were greatly higher than those of needle leaves in the large-sized individuals growing under the drought and open sites. It is likely that leaf form varies with the light and soil moisture conditions and the heterophylly permit S. vulgaris to maintain the population in semi-arid areas.We performed a comparison of water balance o ftwo tree species under the different light conditions, with seedlings of Quercus glauca and Eurya japonica. It was observed that water balance significantly varies with the species in the high light intensity, which induces an increase of transpiration rates. It is likely that Q. glauca reduces water loss by low stomatal conductance and small vessel diameters for reduction of cavitation. E. japonica probably reduces the water loss by low stomatal conductance and small amounts of total leaf area