[Foliar water use efficiency of Platycladus orientalis sapling under different soil water contents]

The determination of plant foliar water use efficiency will be of great value to improve our understanding about mechanism of plant water consumption and provide important basis of regional forest ecosystem management and maintenance, thus, laboratory controlled experiments were carried out to obtain Platycladus orientalis sapling foliar water use efficiency under five different soil water contents, including instantaneous water use efficiency (WUE_gs) derived from gas exchange and short-term water use efficiency (WUE_cp) caculated using carbon isotope model. The results showed that, controlled by stomatal conductance (g_s), foliar net photosynthesis rate (P_n) and transpiration rate (T_r) increased as soil water content increased, which both reached maximum va-lues at soil water content of 70%-80% field capacity (FC), while WUE_gs reached a maximum of 7.26 mmol·m^-2·s^-1 at the lowest soil water content (35%-45% FC). Both δ¹³C of water-soluble leaf and twig phloem material achieved maximum values at the lowest soil water content (35%-45% FC). Besides, δ¹³C values of leaf water-soluble compounds were significantly greater than that of phloem exudates, indicating that there was depletion in ¹³C in twig phloem compared with leaf water-soluble compounds and no obvious fractionation in the process of water-soluble material transportation from leaf to twig. Foliar WUE_cp also reached a maximum of 7.26 mmol·m^-2·s^-1 at the lowest soil water content (35%-45% FC). There was some difference between foliar WUE_gs and WUE_cp under the same condition, and the average difference was 0.52 mmol·m^-2·s^-1. The WUE_gs had great space-time variability, by contrast, WUE_cp was more representative. It was concluded that P. orientalis sapling adapted to drought condition by increasing water use efficiency and decreasing physiological activity.