Significance of predawn water potential in heterogeneous soil water content distribution.

The development of practical field criteria for timing water application is required to improve crop productivity. The most simple physiological indicator of plant water status is the predawn leaf wafer potential. For several annual crops, this indicator is representative of relative evapotranspiration. The same correlation between relative evapotranspiration and predawn water potential is also found with young trees in containers. However, exceptions can occur for heterogeneous soil distributions of water content To examine this condition, a split-root experiment was performed with two compartments (20 and 80% of the total available soil volume respectively). Predawn and minimal stem wafer potential, sap flow, variation in trunk diameter were measured for control frees (two compartments irrigated) and stessed trees (one small compartment irrigated; 20% of total volume). The experiment showed that the predawn water potentials of the control (two compartments irrigated) and of the treatment (one compartment irrigated) do not differ significantly, while sap flow and variation in diameter clearly indicate stress. This experiment has been compared with drought treatment in homogeneous soil. We conclude that under this particuliar condition of heterogeneous soil water content distribution the use of this indicator is very questionable.