Use of Ground-Based Canopy Reflectance to Determine Radiation Capture, Nitrogen and Water Status, and Final Yield in Wheat

Spectral reflectance of plant canopies provides an accurate indication of ground cover fraction, which is highly correlated with radiation capture. If nutrients and water do not limit growth, radiation capture is highly correlated with daily growth rate and ultimate yield. However, if nutrients and water limit growth, none of the common spectral indices are able to separate a more developed, stressed canopy from a less-developed, rapidly growing canopy. We have long known that nitrogen stress is highly correlated with reduced chlorophyll concentration and increased reflectance of green radiation. Conversely, water stress inhibits leaf expansion, which increases chlorophyll concentration and decreases green reflectance. In a two-year study, we measured ground-based canopy reflectance of wheat plots in nitrogen and water stressed environments and found that all of the common spectral indices were highly correlated with ground cover fraction, but none of them could distinguish the intensity of leaf color from ground cover fraction. Here we report a reflective index that can differentiate nitrogen and water stress over a wide range of ground cover. The index is based on the ratio of the green and red variants of the normalized difference vegetation index (NDVIgreen/NDVIred). The new index was able to 32 distinguish N and water stress from satellite data using wavelengths less than 1000 nm. This index should be broadly applicable over a wide range of plant types and environments.