干热河谷9种造林树种在旱季的水分关系和气孔导度

WATER RELATIONS AND STOMATAL CONDUCTANCE IN NINE TREE SPECIES DURING A DRY PERIOD GROWN IN A HOT AND DRY VALLEY

对元谋干热河谷人工混交林中 9个树种叶片的水势、饱和渗透势、气孔导度、渗透调节和季节变化进行了测定 ,同时对树高和胸径也进行了比较。水分生理特征表明 :1 )随着旱季的深入 ,除刺槐 (Robiniapseudoacacia)、黄荆 (Vitexnegundo)、滇榄仁 (Terminaliafranchetii)落叶外 ,其余常绿树种叶片的气孔导度、水势和饱和渗透势都呈下降的趋势 ,在最旱的三、四月份 ,它们适应干旱的方式有 4种 :①低水势、气孔导度近似关闭的厚荚相思 (Acacialepto carpa)、大叶相思 (A .auriculiformis)和肯氏相思 (A .cunninghamii) ;②低水势、低气孔导度的赤桉 (Eucalyptusdulebsis)、娟毛相思 (A .holosericea)和车桑子 (Dodonaeawiscosa) ;③较高水势、低气孔导度的柠檬桉 (Eucalyptuscitriodora)和新银合欢 (Leucaenaleucocephala) ;④较高水势、气孔导度近似关闭的马占相思 (Acaciamangium)。 2 )在干旱胁迫过程中 ,主要渗透调节物质出现的先后顺序是K+ 、游离脯氨酸、游离氨基酸和可溶性糖 ,K+ 和可溶性糖贡献最大 ,分别在干旱的前后期起渗透调节作用 ,游离脯氨酸和游离氨基酸在干旱的中期起渗透调节作用。

The valleys along the rivers in southwestern China are hot and dry because of the rain shadow effect. The primary vegetation on the slopes in these valleys is savanna. However, due to heavy human disturbance, the vegetation on these slopes has been devastated or severely degraded, resulting in severe water and soil erosion. Rehabilitation or revegetation of these slopes is urgently needed. Some eucalypt and legume trees have been introduced for the purpose of revegetation, and these have shown fast growth during the first seven years. However, it is not known how these trees cope with the hot and dry stresses during the dry period. In the present study, we monitored the water status, stomatal conductance and osmotic regulation, and their seasonal changes of 9 tree species in a hot and dry valley. The height and diameter of these trees attained over 5 years of growth also were compared. The mean height of the 9 species ranged from 0.7 to 7.6 m, the diameter from 2.6 to 6.7 cm. Four species ( Eucalyptus dulebsis, Acacia leptocarpa, A. auriculiformis and A. cunninghamii ) showed fast growth, two species ( E. citriodora, Leucaena leucocephala ) had intermediate growth and the remaining species ( A. mangium and Robinia pseudoacacia ) had slow growth. The physiological measurements showed that as drought conditions progressed, all species decreased their leaf water potentials and stomatal conductance. However, the decrease was more rapid in A. leptocarpa, A. auriculiformis, A. cunninghamii and Eucalyptus dulebis. During the peak drought period (March and April), these species also reached the lowest water potentials (predawn water potential lower than -1.0 MPa) and nearly closed their stomata ( A. leptocarpa, A. auriculiformis, A. Cunninghamii ) or remained at low stomatal conductance ( Eucalyptus dulebsis, A. holosericea and Dodonaea wiscosa ) while the remaining species maintained relatively high water potentials and a low stomatal conductance ( Eucalyptus citriodora and Leucaena leucocephala ) or nearly closed stomata ( A. mangium). Nevertheless, during the drought period, no species showed leaf wilting, except R. pseudoacacia and the native species Vitex negundo and Terminalia franchetii, which were deciduous. All species increased osmoregulation capacity, as indicated by the decrease of osmotic potentials at full saturation, and the increase of osmoregulation substances. Osmoregulatory substances increased over time in the following sequence: K +, free Pro, other free amino acids and soluble sugars. It appeared that K + was important in osmoregulation at the beginning of the drought, while soluble sugars were important in the late drought period. E. dulebsis, A. auriculiformis and A. cunninghamii exhibited the highest osmoregulation capacities. Our results show that most of the introduced species are suitable to grow in these hot and dry habitats because of, among other things, they have strong capacities of water_absorption and water conservation.