Water Potential-Water Content Relationships In Apple Leaves

Abstract

Three methods for determining the relationship between xylem pressure potential as measured in a pressure chamber (an estimate of leaf water potential) and leaf relative water content were compared for apple leaves. A range of leaf water contents was obtained either by sampling leaves in the field at different times of day and on days with differing evaporative demand, or by allowing evaporation from excised leaves in the laboratory, or by expressing sap by overpressurization in a pressure chamber. The first two methods gave very similar results, but the last tended to give rather lower water potentials at any given water content. A possible explanation for these results and their implications for the estimation of osmotic potentials using pressure-volume curves are discussed. Some osmotic adjustment was observed in trees droughted for 3 months, with estimated osmotic potentials, both at full turgor and zero turgor, being nearly 0.3 MPa lower than in irrigated controls.