Evaluating Capability of Reflectance Spectrometry for Estimating the Wheat Chlorophyll and Nitrogen

Leaf chlorophyll and nitrogen, due to their important role in photosynthesis are among the major biological parameters of plant physiological status. The ability to quantify chlorophyll and nitrogen can provide important information for precision agricultural activities, plant and agricultural resource management planning, and modeling ecosystem services and production capabilities. This study aimed to assess the capability of indices for estimating the amount of chlorophyll and nitrogen in wheat using spectral data at the canopy level and also determine the most suitable spectral regions and absorption features for this purpose. This research was carried out in a greenhouse environment and the spectroscopic measurements were performed using ASD Fieldspec-3 full-range spectral spectroradiometer. Four plant band indices were classified into two groups of ratio- (NDVI, RVI, and DVI) and soil-based indices (SAVI2) for the raw spectrum and the first derivative of the spectrum for the total samples, and the results were compared. The parameters of position, depth, area, asymmetry and width were calculated for seven absorption features extracted from continuum-removed spectra, and the correlation of these indices with chlorophyll and nitrogen content of wheat was examined. The results showed that SAVI2 had a stronger correlation (RMSE = 0.12, R2 = 0.85) with the chlorophyll content NDVI (RMSE=0.30, R2=0.69) had a higher correlation with the nitrogen content, while using the first derivative with NDVI provided better results. Moreover, area and depth parameters of 430-760 nm absorption spectrum were the best indicators for estimating the amount of chlorophyll and nitrogen in wheat, respectively.