STEM-WATER POTENTIAL AS A SENSITIVE INDICATOR OF WATER-STRESS IN PRUNE TREES (PRUNUS-DOMESTICA L CV FRENCH)

The relative sensitivity of plant- and soil-based measures of water availability were compared for prune trees subjected to a range of irrigation regimes under field conditions. Over the growing season, leaf- and stem-water potentials (PSI) measured at midday exhibited clear differences between frequently irrigated trees and unirrigated trees that were growing on stored soil moisture. Stem-PSI was less variable than leaf-PSI, and the daily variability in stem-PSI was closely related to daily variability in evaporative demands, as measured by vapor pressure deficit (VPD). As a result of lower variability, stem-PSI reflected the small stress effect of a moderate, 50% soil moisture depletion irrigation interval, whereas leaf-PSI did not. The relation between soil water content and estimated orchard evapotranspiration (ET) was influenced by local differences in soil texture within the experimental plot. The relation between stem-PSI and ET, however, was not influenced by soil texture and, in addition, was very similar to the relation between stem-PSI and leaf stomatal conductance. Both relationships indicated that a 50% reduction in leaf and canopy level water loss characteristics was associated with relatively small reductions (0.5 to 0.6 MPa) in stem-PSI. Stem-PSI appears to be a sensitive and reliable plant-based measure of water stress in prune and may be a useful tool for experimental work and irrigation scheduling.