[Sensitivity study of a revised leaf photochemical reflectance index (PRI)]

Photochemical reflectance index (PRI) defined as a normalized difference index using two narrow reflectance bands at 531 and 570 nm that are closely related to xanthophyll cycle pigment content has been successfully used to estimate leaf photosynthetic light use efficiency (LUE) across species which vary in water content and nitrogen concentration. Previous research demonstrated that a consistent relationship could be established between PRI and LUE calculated from gas exchange measurements at the leaf, small canopy, and full forest or crop canopy scales. However, a number of problems, such as the saturation of PRI when LUE exceeds 0.03 mol CO2 mol(-1) PPED (photosynthetic photon flux density) and disjunctive relationships of PRI and LUE in seasonal changes, still existed and need to be handled in order to evaluate LUE more accurately. A sensitivity study of a revised PRI with four leaf parameters was performed based on PROSPECT model in the present article to study the effects of different biochemical concentrations on leaf SR-PRI (simple ratio PRI). Sensitivity study proved that leaf SR-PRI is more sensitive to leaf mesophyll structure parameter (N) and chlorophyll a + b content (c(ab)) than parameters of dry matter content (c(m)) and equivalent water thickness (c(w)), indicating that leaf mesophyll structure parameter (N) and chlorophyll a + b content (c(ab)) should be especially considered when acquiring leaf SR-PRI. And changes in the two parameters would cause large variation in SR-PRI which would reduce the precision for estimating light use efficiency. Validation study of SR-PRI was carried out in the analysis and the results proved that SR-PRI can also be a feasible index of estimating LUE for four species of plants with correlation coefficients better than that of PRI and LUE. The advantage of SR-PRI compared to PRI is its much clearer physical meaning and its sensitivity to the changes in reflectance at 531 nm which serves as a core parameter to evaluate light use efficiency.