Remotely assessing photosynthetic nitrogen use efficiency with in situ hyperspectral remote sensing in winter wheat

Non-destructive and rapid methods for estimating nitrogen efficiency are helpful in quantifying and communicating the N use efficiency (NUE) in agricultural production. To accurately evaluate NUE, we analysed the quantitative relationship between leaf photosynthetic nitrogen use efficiency (PNUE) and wheat NUE, and investigated the quantitative relationship between PNUE and hyperspectral indices in winter wheat. Our field experiments were carried out during 2013-2016 across different growth stages, cultivars, planting densities, nitrogen (N) rates, and water treatments, and a nitrogen efficiency index (NEI) was developed to evaluate PNUE. Three of eight previously developed vegetation indices (VIs) performed well when regressed against PNUE; however, they exhibited bias between vegetative and reproductive stages. The NEI was obtained by integrating the reflectance at the red band (680 nm) into simple ratio index (SR (760,850)), to become a modified simple ratio index (mSR (760,850,680)), then we added a floating coefficient [1.8 + (R-680/R-850)] to mSR (760,850,680). Our novel index was better related with PNUE than previous VIs under different experimental conditions. There was no significant difference in the slope and intercept under the five experimental factors, suggesting that the new index is a superior tracker for PNUE changes under heterogeneous field conditions. These models accurately estimated PNUE in winter wheat, providing a theoretical basis and technical support for N fertilizer assessment and screening of N efficient wheat varieties.