No abstract

Regulated Deficit Irrigation (RDI) strategies, often applied in tree crops, require precise monitoring methods of water stress. The Crop Water Stress Index (CWSI), based on canopy temperature measurements, has shown to be an good indicator of water deficits in field crops but has seldom been used in trees. The CWSI was measured on a continuous basis in a Central California mature pistachio orchard, under full and deficit irrigation. Two treatments -control, returning the full evapotranspiration (ET c ) and RDI, irrigated with 40% ET c during the Stage two of fruit grow (shell hardening). During Stage two, the canopy temperature -measured continuously with infrared thermometers -of the RDI treatment was consistently higher than the control during the hours of active transpiration; the difference decreasing after irrigation. The Non-Water-Stressed Baseline (NWSB), obtained from clear-sky days canopy-air temperature differential and vapour pressure deficit (VPD) in the control treatment, showed a marked diurnal variation in the intercept, mainly explained by the variation in solar radiation. By contrast, the NWSB slope remained practically constant along the day. Diurnal evolution of calculated CWSI was stable and near zero in the control, but showed a clear rising diurnal trend in the RDI treatment, increasing as water stress increased around midday. The seasonal evolution of the CWSI detected large treatment differences throughout the RDI stress period.While the CWSI in the well-irrigated treatment rarely exceeded 0.2 troughout the season, in the RDI it reached values of 0.8-0.9 near the end of the stress period. The CWSI responded to irrigation events along the whole season, and detected clearly mild water stress, suggesting extreme sensitivity to variations in tree water status. It correlated well with mid-day leaf water potential (LWP), but was more sensitive than LWP at mild stress levels. We conclude that the CWSI, obtained from continuous nadir-view measurements with infrared thermometers, is a good and very sensitive indicator of water stress in pistachio. We recommend the use of canopy temperature measurements taken from 1200 to 1500h, together with the following equation for the NWSB: (T c -T a ) = -1.33·VPD + 2.44. Measurements of canopy temperature with VPD < 2 kPa are likely to generate significant errors in the CWSI calculation and should be avoided.

No abstract

No abstract