An experiment was conducted on Haloxylon ammodendron C.A. Mey, a small xeric tree. Soil water content, soil evaporation, leaf water potential, leaf transpiration rate and stomatal conductance were measured at the two sites that contrast in soil texture: sandy and heavy textured, 8 km apart on the southern periphery of Gurbantonggut Desert, Central Asia, during the 2005 and 2006 growing seasons. Leaf specific hydraulic conductance was calculated from the measurements, and root distributions of plants grown at the two sites were quantified by whole-root system excavation. In general, plants grown in sandy soil experienced better water status than in heavy textured soil. Low soil evaporation loss is not the main reason for this better plant water status at sandy site. Plants in sandy soil developed much deeper root systems, larger root surface areas and higher root: leaf surface area ratio than in heavy textured soil, which facilitated plants acquiring more water and surviving the prolonged drought period. Plants growing at light textured sites should have an advantage in acclimatising to the changed water conditions of the future. Plants at the more sandy sites have a larger buffering capacity to excessive variation in ambient conditions.

The effects of soil moisture distribution on water uptake of drip-irrigated corn were investigated by simultaneously monitoring the diurnal evolution of sap flow rate in stems, of leaf water potential, and of soil moisture, during intervals between successive irrigations. The results invalidate the steady-state resistive flow model for the continuum. High hydraulic capacitance of wet soil and low hydraulic conductivity of dry soil surrounding the roots damped significantly diurnal fluctuations of water flow from bulk soil to root surface. By contrast, sap flow responded directly to the large diurnal variation of leaf water potential. In wet soil, the relation between the diurnal courses of uptake rates and leaf water potential was linear. Water potential at the root surface remained nearly constant and uniformly distributed. The slope of the lines allowed calculating the resistance of the hydraulic path in the plant. Resistances increased in inverse relation with root length density. Soil desiccation induced a diurnal variation of water potential at the root surface, the minimum occurring in the late afternoon. The increase of root surface water potential with depth was directly linked to the soil desiccation profile. The development of a water potential gradient at the root surface implies the presence of a significant axial resistance in the root hydraulic path that explains why the desiccation of the soil upper layer induces an absolute increase of water uptake rates from the deeper wet layers.

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