Using portable photosynthesis system (LI-6400),we measured leaf conductance of Populus euphratica and Tamarix ramosissima grown under conditions with high groundwater and without groundwater availability to explore their stomatal sensitivity in response to changes in leaf-to-air vapor pressure deficit (LVPD), which were obtained by regulating the relative humidity in the chamber in our experiment. Stomatal sensitivity to LVPD was calculated by fitting the linear relationship between stomatal conductance(Gs)and ln-transformed LVPD. The results indicate that the stomatal sensitivity to LVPD for P. euphratica was significant higher than T. ramosissima under high groundwater availability while similar under no groundwater condition. Under no groundwater condition, P. euphratica showed significantly reduced stomatal sensitivity to LVPD, while similar for T. ramosissima, in comparison to high groundwater condition. Thus the higher stomatal sensitivity to LVPD under high groundwater condition indicates that P. euphratica is relatively conservative in water use when environmental water supply is favorable. In contrast, T. ramosissima exhibited an inert stomatal response to LVPD, a small reduction in Gs and a parallel Pn when no groundwater was available, which suggest that P. euphratica is more susceptible to environmental water deficit, and from a perspective of plant-water relation, T. ramosissima exhibits advantages in adapting to the desert riparian zones characterized by highly variable water environment and great transpirational demand.
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