Capability of Photochemical Reflectance Index to Track Maize Canopy Radiation Use Efficiency and Its Drivers Under Soil Drying

Photochemical reflectance index (PRI) has been a promising indicator for estimating vegetation photosynthetic efficiency. However, its capability to track drought stress-induced changes in canopy radiation use efficiency (RUE) and the underlying mechanisms remains insufficiently explored, largely due to the confounding effects of soil background and canopy characteristics. This study aimed to explain how the canopy PRI responds to drought stress and quantify the relative contributions of soil moisture and canopy characteristics to its variability. Using maize field experimental data across varying drought treatments, we found that drought significantly altered the PRI-RUE relationship, with canopy PRI exhibiting a stronger correlation with RUE under increasing soil drying. This enhancement in the PRI-RUE relationship was primarily attributed to changes in canopy structure and physiological characteristics. Specifically, the fraction of absorbed photosynthetic available radiation (fAPAR), canopy water content (CWC) and canopy chlorophyll content (CCC) were more related to PRI than leaf area index (LAI). While available soil water content (ASWC) was not directly linked to PRI, a positive linear relationship emerged after accounting for the effects of canopy characteristics, particularly fAPAR. Furthermore, fAPAR and LAI were identified as the most important direct and indirect factors influencing canopy PRI, respectively. These findings underscore the importance of considering fAPAR's contribution to accurately estimate photosynthetic efficiency and monitor crop stress under soil drying scenarios. By demonstrating how drought strengthens the PRI-RUE relationship and elucidating its underlying mechanisms, this study provides insights for improving crop stress monitoring and photosynthetic capacity assessment.