Monitoring Water Status In Apple Trees Using A Sensitive Morning Crop Water Stress Index*

The crop water stress index (CWSI) has shown to be a good indicator of water status in fruit trees. Conventional CWSI measured over solar noon is widely used to monitor plant water status. This study compared the theoretical CWSI averaged over morning hours, CWSImo, and over solar noon hours, CWSImd, of apple trees. This study also assessed their sensitivity to the changes in soil water status (soil water deficit [SWD, %] and soil water potential [SWP, kPa]) at different root zone depths. Four different types of commercial and experimental apple orchards with different characteristics growing under semi-arid conditions in Washington State, United States, were chosen to assess CWSI(mo)against CWSI(md)sensitivity to soil water status. In some of these locations, soil water status ranged from fully watered to severely stressed (low water deficit to high water deficit). Ground-based thermal and microclimate measurements were recorded continuously at 10-s intervals and acquired for the 15-min average on a daily basis throughout the study period. The linear relationship between SWD and CWSI(mo)at the effective root zone depth (average of 0-150 mm and 0-460 mm) resulted in a slightly higher correlation (R-2= 0.42-0.64,p< .001) compared to the traditional CWSImd(R-2= 0.32-0.60,p< 0.001). The correlation between CWSI(mo)and SWP (R-2= 0.38-0.69,p< 0.001) was found to be larger than the correlation between CWSI(mo)and SWD, while SWP was less correlated with CWSImd(R-2= 0.33-0.55,p< 0.001). Interestingly, a better relationship between soil water status and both CWSI(mo)and CWSI(md)was observed using a nadir view orientation thermal measurement. The CWSI(mo)showed higher sensitivity to SWP than SWD. (c) 2020 John Wiley & Sons, Ltd.