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. The photochemical reflectance index (PRI) has emerged tobe a pre-visual indicator of water stress. However, whether the varyingshaded-leaf fractions, which may be caused by multiple view angles or thechanging crop density in the field, affect the performance of PRI indetecting water stress of crops is still uncertain. This study evaluated theimpact of the varying shaded-leaf fractions on estimating relative watercontent (RWC) across growth stages of winter wheat using seven formulationsof PRI. Results demonstrated that for the control treatment the mean PRI ofsunlit leaves was slightly higher than those of shaded leaves, but thedifference between PRI of sunlit and shaded leaves increased as waterresources became more limiting. Despite the difference between PRI of sunlitand shaded leaves, the significance of the linear relationship between RWCand most studied formulations of PRI did not show obvious variations withshadow fractions, except for the 100 % shaded-leaf condition. Among thestudied formulations of PRI, PRI3 based on reflectance at 512 nm as thereference band provided the most accurate estimates of RWC with varyingshaded-leaf fractions, except for the 100 % shaded-leaf condition. Theslope and the intercept of linear regression models with PRI3 also showedminimized variations with shaded-leaf fractions. We then applied a uniformRWC prediction model to the data of varying shaded-leaf fractions and foundthat the accuracy of RWC predictions was not significantly affected in themixture of sunlit and shaded leaves. However, RWC estimated with PRI of the100 % shaded-leaf condition had the highest root mean square error (RMSE), implying that PRI of thepure shaded leaves may yield inaccurate estimates of plant water status.

Assessing shaded-leaf effects on photochemical reflectance index (PRI) for water stress detection in winter wheat