Canopy-Level Photochemical Reflectance Index from Hyperspectral Remote Sensing and Leaf-Level Non-Photochemical Quenching as Early Indicators of Water Stress in Maize

In this study, we evaluated the effectiveness of photochemical reflectance index (PRI) and non-photochemical quenching (NPQ) for assessing water stress in maize for the purpose of developing remote sensing techniques for monitoring water deficits in crops. Leaf-level chlorophyll fluorescence and canopy-level PRI were measured concurrently over a maize field with five different irrigation treatments, ranging from 20% to 90% of the field capacity (FC). Significant correlations were found between leaf-level NPQ (NPQ(leaf)) and the ratio of chlorophyll to carotenoid content (Chl/Car) (R-2 = 0.71, p < 0.01) and between NPQ(leaf) and the actual photochemical efficiency of photosystem II (Delta F/F-m') (R-2 = 0.81, p < 0.005). At the early growing stage, both canopy-level PRI and NPQ(leaf) are good indicators of water stress (R-2 = 0.65 and p < 0.05; R-2 = 0.63 and p < 0.05, respectively). For assessment of extreme water stress on plant growth, a relationship is also established between the quantum yield of photochemistry in PSII (Phi P) and the quantum yield of fluorescence (Phi F) as determined from photochemical quenching (PQ) and non-photochemical quenching (NPQ(leaf)) of excitation energy at different water stress levels. These results would be helpful in monitoring soil water stress on crops at large scales using remote sensing techniques.