The Photochemical Reflectance Index (PRI) 20 years later: what have we learned?

The Photochemical Reflectance Index (PRI) was originally derived as a measure of diurnal xanthophyll cycle activity and photosynthetic light-use efficiency (LUE). Based on experimental studies of plant canopies in the early 1990's, PRI is derived from reflectance at two wavelengths: 531 nm (the xanthophyll cycle wavelength) and a reference wavelength (unaffected by the xanthophyll cycle). Like any index approach, it has key advantages and disadvantages. Advantages include data reduction and a demonstrated correlation with biochemical and physiological processes tied to photosynthetic light regulation. Disadvantages include sensitivity to a number of confounding factors that include pigment pool sizes, vegetation canopy structure, and sun and view angle effects. Despite these challenges, many studies have shown some promise in using this index for airborne or satellite assessment of photosynthetic activity. This presentation will review key challenges in using this index for global assessment of photosynthetic activity, and will propose avenues for further research that might strengthen our understanding of the dynamic 531 nm reflectance signal. Alternative and complementary approaches to this index will also be briefly discussed.