CANOPY AIR-TEMPERATURE DIFFERENCES AND SOIL-WATER AS PREDICTORS OF WATER-STRESS OF APPLE-TREES GROWN IN A HUMID, TEMPERATE CLIMATE

Temperature differences between tree canopies and air (T(c) - T(a)) and between leaves and air (T1 - T(a)) of apples (Malus domestica Borkh. 'Royal Gala') grown in New Zealand were measured with infrared (IR) thermometry. Treatments included three orchard-floor management systems and irrigation withheld (WI) for part of the growing season. Measurements of soil moisture indicated that, under full irrigation (FI), an alfalfa orchard-floor system apparently had higher soil water content than herbicide-strip (H) or plastic-mulch systems, whereas under the drought stress of WI, the H system retained the most water. The T(c) - T(a) and T1 - T(a) of the WI treatment were significantly greater than those of the FI treatment after a soil-moisture differential was established. Linear regression between T(c) - T(a), or T1 - T(a), and vapor pressure deficit (VPD) exhibited variable responses among dates. A crop water stress index (CWSI) was calculated from environmental measurements. The calculated CWSIs were not related to soil-moisture measurements. Even 35 days after full irrigation had been reinstated on the WI plots, the T(c) - T(a), T1 - T(a), and CWSI of the WI plots were still significantly greater than those of the FI plots. These discrepancies in IR thermometry-based water-stress indices may be due to increased errors in the calculation of minimum CWSI at low VPDs and to fluctuating solar radiation and evapotranspiration, which are prevalent in humid, temperate climates.