Monitoring water status in apple trees using a sensitive morning crop water stress index*

Abstract

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, CWSI mo , and over solar noon hours, CWSI md , 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 md ( R 2 = 0.32–0.60, p mo and SWP ( R 2 = 0.38–0.69, p mo and SWD, while SWP was less correlated with CWSI md ( R 2 = 0.33–0.55, p mo and CWSI md was observed using a nadir view orientation thermal measurement. The CWSI mo showed higher sensitivity to SWP than SWD.

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