SIGNIFICANCE OF PREDAWN WATER POTENTIAL I N HETEROGENEOUS SOIL-WATER CONTENT DISTRIBUTION

The development of practical field criteria for timing water applicati on is required to improve crop productivity. The most simple physiolog ical indicator of plant water status is the predawn leaf wafer potenti al. For several annual crops, this indicator is representative of rela tive evapotranspiration. The same correlation between relative evapotr anspiration and predawn water potential is also found with young trees in containers. However, exceptions can occur for heterogeneous soil d istributions of water content To examine this condition, a split-root experiment was performed with two compartments (20 and 80% of the tota l available soil volume respectively). Predawn and minimal stem wafer potential, sap flow, variation in trunk diameter were measured for con trol frees (two compartments irrigated) and stessed trees (one small c ompartment irrigated; 20% of total volume). The experiment showed that the predawn water potentials of the control (two compartments irrigat ed) and of the treatment (one compartment irrigated) do not differ sig nificantly, while sap flow and variation in diameter clearly indicate stress. This experiment has been compared with drought treatment in ho mogeneous soil. We conclude that under this particuliar condition of h eterogeneous soil water content distribution the use of this indicator is very questionable.