Using remotely sensed spectral reflectance to indicate leaf photosynthetic efficiency derived from active fluorescence measurements

Chlorophyll fluorescence (ChlF) is an important signature of photosynthesis to evaluate plant response to the environment. We explored an approach to estimate an important leaf ChlF-derived parameter, the intrinsic efficiency of photosystem II photochemistry (Fv/Fm), using spectral indices calculated from leaf reflectance measured by a hyperspectral radiometer. It is observed that leaf chlorophyll content closely related to Fv/Fm in nonstressed leaves, thus the indices developed for chlorophyll estimation were successfully used to estimate Fv/Fm. For leaves under short-term stress, Fv/Fm dropped dramatically while leaf chlorophyll content remained almost the same. Compared to leaf chlorophyll content, reflectance was more sensitive to Fv/Fm variations. As Fv/Fm decreased, the slope of reflectance in the spectrum range of 700 to 900 nm obviously increased, and the first derivative reflectance in the red edge and infrared (NIR) regions was highly correlated with Fv/Fm. The indices using longwave red edge and NIR reflectance (NDRE740 and CI740) worked well for Fv/Fm retrieval in both stressed and nonstressed leaves with the coefficients of determination (R2) above 0.72 and normalized root-mean-square errors below 0.16. Note that the relationships NDRE740 and CI740 versus Fv/Fm were significantly different between nonstressed and stressed leaves, which may give a good implication to detect short-term stress occurrence.