We present a new “physiological reflectance index” (PRI) isolated from narrow waveband spectral measurements of sunflower canopies. This index correlates with the epoxidation state of the xanthophyll cycle pigments and with the efficiency of photosynthesis in control and nitrogen stress canopies, but not in water stress canopies undergoing midday wilting. It is analogous in formulation to the broadband normalized difference vegetation index (NDVI) and uses reflectance at 531 nm and at a reference wavelength to minimize complications associated with diurnal sun angle changes. In conjunction with other methods, this index may lead to improved remote and ground-based estimates of canopy photosynthetic function.