Abstract: Field measurements were made of leaf net photosynthetic rate (Pn), apparent quantum yield (AQY), dark respiration (Rd), transpiration (Tr), water use efficiency (WUE), stomatal conductance (Gs), intercellular CO2 concentration (Ci), leaf temperature (TI), stomatal density, air temperature (Ta) and relative soil water content (SWC) for Adenophora lobophylla Hong in August, 1996. The species grows in the region from an altitude of 2 300 m to 3 400 m on the eastern boundary of the Qing-Zang Plateau in Sichuan Province, China. Leaf gas exchange, water use efficiency and plasticity of populations in field were compared among different altitudes to evaluate the possible interactions between adaptation of A. lobophylla and environmental factors in these habitats. Pn and AQY at low altitudes were lower than those at high altitudes. They strongly responded to SWC and Ta. On the other hand, Rd at low altitudes was higher than that at high altitudes because of the higher air temperature there. The growth rates at low altitudes were associated with the increases in Rd and a relevant less Pn. Stomata showed strong responses to leaf-to-air vapor pressure deficit at the leaf surface (Vpdl) and Tl in these habitats. Increasing stomatal limitations to photosynthesis appeared to be responsible for the reduction in Pn at high Tl, Vpdl and low available soil water for A. lobophylla at low ahitudes. Nonstomatal limitation to photosynthesis also happened at extreme soil water deficits and high Tl and Vpdl at an altitude of 2 300 m. Tr had a close relationship with stomatal conductance and was also affected by leaf temperature and leaf-to-air vapor pressure deficit at the leaf surface among habitats grown in different ahitudes. WUE increased with altitude. Increasing stomatal densities showed different plasticity of A. lobophylla as altitude increased. SWC and Ta appeared to be important factors to limit carbon assimilation in A. lobophylla at low altitudes, primarily through the process of stomatal closure. The overall results are in consistence with the hypothesis that strong pressure from tmfavorable environmental factors to gas exchange and wateruse of A. lobophylla may prevent their population expansion at low altitudes, which indicate that the above-mentioned restrictions might lead to the endangerment of A. lobophylla.

Key words: Adenophora lobophylla, Endangered plant, Gas exchange, Water use efficiency