Selecting optimal hyperspectral bands to discriminate nitrogen status in durum wheat: a comparison of statistical approaches

Hyperspectral data can provide prediction of physical and chemical vegetation properties, but data handling, analysis and interpretation still limit their use. In this study, different methods for selecting variables were compared for the analysis of on-the-ground hyperspectral signatures of wheat grown under a wide range of nitrogen supplies. Spectral signatures were recorded at end of stem elongation, booting and heading stages in 100 georeferenced locations, using a 512-channel portable spectroradiometer operating in the 325-1075 nm range. The following procedures were compared: i) an heuristic combined approach including lambda-lambda R2 (LL R2) model, principal component analysis (PCA) and stepwise discriminant analysis (SDA); ii) variable importance for projection (VIP) statistics derived from partial least square (PLS) regression (PLS-VIP); iii) multiple linear regression (MLR) analysis through Maximum R-square improvement (MAXR) and stepwise algorithms. Discriminating capability of selected wavelengths was evaluated by canonical discriminant analysis. Leaf-nitrogen concentration was quantified on samples collected at the same locations and dates and used as response variable in regressive methods. The different methods resulted in differences in number and position of the selected wavebands. Bands extracted through regressive methods were mostly related to response variable, as shown by the importance of the visible region for PLS and stepwise. Band selection techniques can be extremely useful not only to improve the power of predictive models but also for data interpretation or sensor design.