Determining mid-day stem water potential from sap flow measurements

Abstract

Trees tend to stabilize mid-day sap velocity through apparently coordinated active canopy conductance (Gc) responses to atmospheric evaporative demand. We recently reported a model describing this stabilization based on the linear relationship between canopy resistance (Rc) and vapor pressure deficit (VPD). The model was able to accurately reproduce the daily and seasonal courses of sap flow in nectarine trees for different leaf water potentials. Here, we studied inversion of the model to determine mid-day stem water potential, Ψmd, from sap flow measurements. The study was in a nectarine orchard in the hot Hula valley of northern Israel. Measurements included sap flux, weather variables, mid-day stem water potential and other parameters. Twelve trees were measured, where four were well-watered and irrigation was withheld from eight trees for eight weeks in the hot and dry summer. Ψmd declined from -1.05 to -2 MPa, and pre-dawn Ψ from -0.42 to -0.89 MPa. Similar to previous results, we found that the slope of the linear relationship between canopy resistance and VPD increased as Ψmd decreased in a monotonic manner, which was described by a linear relationship. Slopes of the relationship of canopy resistance to VPD varied linearly from 1.2 to 2.2 s mm-1 kPa-1 over the range of Ψmd values. Re-calculation of Ψmd using the relationship was very successful, giving an r2 of 0.83 between calculated and measured values.