The Photochemical Reflectance Index (PRI) as a Water Stress Indicator in Peach Orchards from Remote Sensing Imagery

摘要

Early detection of water stress is important to avoid the risk of yield losses in commercial orchards. Pre-visual assessment of water stress has been successfully achieved in the major herbaceous crops since the late seventies with remote sensing using thermal infrared radiation. Much more recently, the Photochemical Reflectance Index (PRI), function of xanthophyll pigment absorption, has been proposed for short-term stress detection. However, the assessment of vegetation stress with the PRI cannot be properly conducted without considering leaf and canopy structural effects on the spectral index. This is because reflectance from bands at 531 and 570 nm is affected by both leaf and canopy parameters. Consequently, modelling work at leaf and canopy scale is needed to enable the operational application of PRI to map water stress in non-homogeneous canopies, such as peach trees, where structural changes play a main role in the reflectance signature. PRI is presented here as a pre-visual water stress indicator for peach, together with a new modelling method that has been developed. The methodology simulates the PRI-based threshold for non-stress conditions, therefore enabling the comparison between non-stress (modelled) and stress (imagery) levels. Forwater stress detection, the relationships between crown PRI and crown temperature yielded a determination coefficient of r~2=0.8, demonstrating the sensitivity of PRI for tree water status. Deficit-irrigation treatments yielded higher PRI values than the simulated PRI (sPRI), which provides the benchmark for non-stress conditions derived from model inversion. Fully-irrigated trees, on the contrary, showed PRI values around/below the simulated non-stress sPRI baseline. These results suggest that PRI canbe used as an indicator of water stress in peach orchards, provided that modelling approaches to characterize non-stress conditions as function of canopy structure are used.