Spatiotemporal Heterogeneity of Chlorophyll Content and Fluorescence Response within Rice (Oryza sativa L.) Canopies under Different Cadmium Stress

The spatiotemporal heterogeneity of rice canopy leaves at different leaf positions is very important for non-destructive monitoring under Cadmium (Cd) stress, and is one of the key problems that need to be solved urgently in modern agriculture. This study aims to explore the temporal and spatial heterogeneity of chlorophyll content and fluorescence in rice canopy leaves under different Cd stress concentrations. The responses of the relative chlorophyll content (SPAD) and Chlorophyll fluorescence (ChlF) parameters to Cd stress in the tillering, heading, and filling stages were analyzed through pot experiments with three Cd concentrations: 0.20 mg·kg−1 (CK), 0.60 mg·kg−1 (Cd1), and 1.60 mg·kg−1 (Cd2). Results showed that the canopy leaf SPAD value increased with increasing Cd concentration. Compared with CK, the leaf SPAD value in the Cd1 and Cd2 treatments were 1.91–3.45% and 5.52–12.01% lower, respectively. The SPAD value of the lower leaves was higher in the tillering stage, while the SPAD value of the upper leaves was higher in the heading and filling stages. The non-photochemical quenching coefficient (NPQ) of the third leaf (D3), the initial fluorescence yield (Fo) of the second leaf (D2), and the maximum photochemical efficiency (Fv/Fm) of photosystem II (PSII) in the tillering stage; the NPQ, Fo, and Fv/Fm of the D3 in the heading stage; and the NPQ, Fo, Fv/Fm, and Fv/Fm of the D2 in the filling stage were more sensitive to Cd stress than the other leaves and ChlF parameters.