Water deficit in arid and semiarid regions affects whole-plant sap flow and leaf-level water relations. The objectives of this study were to clarify how sap flow of Calligonum arborescens responds to different drought stress conditions and to understand its acclimation mechanism to drought environments. A field experiment was conducted for C. arborescens during the growing season to evaluate the effects of deficit irrigation on the daily and seasonal variations of trunk sap flow in the shelterbelt along the Tarim Desert Highway, Xinjiang, China. Three dif- ferent water regimes （2,380, 1,960 and 1,225 m3/hm2） were applied at different stages of plant growth. From 1 May to 30 October 2007, a heat-balance stem flow gauge was used to monitor the sap flow dynamics of C. arborescens under different water regimes. Atmospheric evaporation demand and soil moisture conditions for differentially irri- gated C. arborescens were also monitored. The result showed that sap flow exhibited a clear diurnal pattern re- gardless of treatments; the diurnal patterns of sap flow and vapour pressure deficit were very similar under different water regimes and growing seasons, while the slope of the linear regression of this correlation confirmed an in- creasing water regime. The sap flow decreased under reduced water regimes and there was nocturnal sap flow regardless of water regimes, which was mainly contributed to nocturnal transpiration and water recharge. The sap flow peaked before midnight and dropped afterwards with obviously higher values in summer than in other seasons. It is speculated that the water consumption of C. arborescens during the day can be supplemented through the sap flow at night, which increased with increasing irrigation amount. Net radiation was the most significant correlated factor that influenced sap flow velocity and transpiration under different water regimes （R2〉0.719）. Compared with the commonly practiced water regime, the growth of C. arborescens was significantly slower in the stress deficit
