Monitoring the leaf equivalent water thickness of kiwifruit in high temperature using leaf spectral reflectance

Abstract Leaf water status information is highly needed for assessing high temperature stress. Leaf equivalent water thickness (fresh weight − dry weight)/area) is commonly used functional plant trait. Retrieval of leaf equivalent water thickness based on hyperspectral remote sensing has been shown to be rapid and accurate. However, a universal index that is applicable to various plants remains a considerable challenge. Few attempts had been made to monitor kiwifruit leaf equivalent water thickness using hyperspectral vegetation indexes under high temperature stress. In this study, a high temperature stress experiment was conducted in ambient conditions on 29 July–1 August 2021 during the kiwifruit expansion period to obtain a leaf spectrum dataset in the laboratory after the high temperature (above 35 °C) duration time for 1, 2, 3, and 4 days. This study systematically analyzed the performance of hyperspectral vegetation indexes of kiwifruit in estimating leaf equivalent water thickness with the range of 0.02–0.04 g/cm2. Newly developed and published spectral indexes were calculated to estimate kiwifruit leaf equivalent water thickness. Results showed that the newly developed three-band vegetation index (R2039 − R2438)/R752 generated the optimal testing performance for estimating kiwifruit leaf equivalent water thickness, with the coefficient of determination of 0.771, root mean square error of 0.0024 g/cm2, and residual prediction deviation of 2.09, respectively. This study suggests that the kiwifruit leaf equivalent water thickness under high temperature stress could be accurately estimated based on leaf level spectral indexes.