FIELD MEASUREMENTS OF CROP WATER STRESS IN GRAPEVINES USING THERMAL DIFFUSIVITY AND VAPOUR PRESSURE DEFICIT SENSORS
Single-point thermal diffusivity (TD) sensors were implanted in the xylem tissue at the base of canes in the canopy of Cabernet-Sauvignon grapevines in a commercial vineyard during the 2011-2012 growing season. Vapour pressure deficit (VPD) measurements derived from precise air temperature and relative humidity measurements within 10 cm of the thermal diffusivity sensor were used as a surrogate for the atmospheric demand that drives sap flow and changes in cellular water content. Soil moisture tension profiles were measured as an independent marker of plant stress, based on traditional irrigation scheduling techniques. These sensors were located below the vines in sandy-loam soil at depths of 20 cm, 40 cm and 80 cm. Rainfall and ETo were measured above the canopy 10 m away. The 1.5 mm diameter spherical thermal diffusivity sensor dissipates a precise 25 mW ± 25 μW into its environment while simultaneously monitoring temperature rise at the centre of its heat field. A differential temperature measurement technique eliminates background temperature variations from the thermal diffusivity readings. The sensors were calibrated in agar-gelled water and glycerol to obtain sensor-specific thermal diffusivity calibration values having a resolution of 0.0001 mm2 s-1. Field results over a three month period showed a strong correlation between VPD and thermal diffusivity. During that part of the diurnal cycle where maximum transpiration occurs (between dawn and mid-afternoon), well-watered vines showed an increase in TD due to an increase in the effective thermal conductivity of the vine tissue due to sap flow. This convective enhancement of thermal diffusivity subsided as plants came under water stress and sap flow declined. Nocturnal TD also changed as plant tissue re-hydrated after periods of high daily sap flow, changing the heat capacity of the plant tissue in the absence of sap flow. Ninety-six 15 minute readings of TD and VPD for each 24 h cycle from dawn to dawn were studied using a vector-averaging technique which amplified the phase-shift between VPD and TD caused by sap flow, irrespective of the magnitude of daily atmospheric demand. During one ten-day period when irrigation was withheld (due to pump failure), the linear correlation between this daily crop water stress indicator and the more traditional average soil moisture tension was r2 = 0.95, shifting abruptly back from high stress levels to average levels when irrigation was turned back on. This potentially inexpensive sensor has been shown to accurately reflect changes in the vines driven by transpiration demand and may be a viable alternative to more traditional soil-based measurements in commercial horticulture.
Skinner, A.J. (2014). FIELD MEASUREMENTS OF CROP WATER STRESS IN GRAPEVINES USING THERMAL DIFFUSIVITY AND VAPOUR PRESSURE DEFICIT SENSORS. Acta Hortic. 1038, 145-154
sap flow, plant water status, effective thermal conductivity, cellular water content, irrigation scheduling