Effects of different irrigation regimes on characteristics of transpiring water- consumption of three desert species

The sap flow velocity and leaf water potential of three desert shrubs (Tamarix ramosissima, Haloxylon ammodendron, and Calligonum arborescens), grown under three different volumes of irrigation water in the Tarim Desert Highway shelterbelt, which is located at Taklimakan Desert of China, were measured using a heat-balance stem flow gauge and press chamber. The sap flow velocity differed among the species and among the irrigation volume treatments. Under the same irrigation volume, the sap flow velocity of the three species followed the rank order T. ramosissima > C. arborescens > H. ammodendron. The sap flow velocity of the same species decreased significantly with reduction in irrigation volume. The night-time transpiration of the three species as a proportion of total diurnal transpiration followed the order H. ammodendron (18.68%) > C. arborescens (17.48%) > T. ramosissima (12.82%). These results indicate that the water consumption of the three species during the day can be supplemented through the sap flow at night, which results in increased drought resistance. The water potential of leaf-canopy-root of H. ammodendron was higher, and the compensatory sap flow at night was higher, than that of the other species and thus conferred stronger drought resistance. The diurnal variations in sap flow velocity of T. ramosissima and C. arborescens were similar. In both species, the diurnal variation in sap flow velocity was represented by a single-peaked curve, the sap flow velocity values were higher and the extent of variation was wider under irrigation volumes of 35 and 28 kg times plant super(-1) times time super(-1). With an irrigation volume of 17.5 kg times plant super(-1) times time super(-1) the diurnal variation in sap flow velocity was represented by a two-peaked curve and the sap flow velocity values were lower than those of the other treatments. The diurnal variation in stem sap flow velocity of H. ammodendron showed a single-peaked curve under each of the irrigation volume treatments. These results indicate that water deficit occurs in T. ramosissima and C. arborescens when irrigation volume decreased to 17.5 kg times plant super(-1) times time super(-1), and the species adapt to drought stress by adjustment of stomatal conductance. However, water deficit is not found in H. ammodendron at an irrigation volume of 17.5 kg times plant super(-1) times time super(-1) and this species exhibits greater drought resistance. With decreasing irrigation volume