Continuous in situ measurements of crop water stress in ‘Shiraz’ grapevines using a thermal diffusivity sensor

Abstract

A new measure of crop water deficit stress (CWS) using a thermal diffusivity sensor was evaluated under field conditions in southern Australia in early 2014 in mature 'Shiraz' grapevines over a two-month period. The trial tracked the correlation between thermal diffusivity (TD) of xylem (sap and tissue) and vapour pressure deficit (VPD) as a measure of CWS. In-canopy VPD measurements were used as a surrogate for atmospheric demand. Matric potential sensors in the soil profile below the vine were used as an independent measure of potential water deficit stress. The TD vs. VPD correlation coefficient was derived from 96 quarter-hourly readings over each day, starting at 06:00 h daily when the vine was expected to have reached hydraulic equilibrium with the soil. Correlation coefficients were around 0.9 under well-irrigated conditions but dropped to around 0.6 before each irrigation was applied. As the soil profile dried over the course of the season and tissue rehydration increasingly failed to recover after high-stress summer days where sap flow was low, TD did not correlate as well with VPD. The sensor output closely tracked irrigation cycles.