Effects of drought stress on xylem permeability and transpiration control

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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