Assessment of growth, leaf N concentration and chlorophyll content of sweet sorghum using canopy reflectance

Remote estimation of leaf nitrogen (N) or pigments through hyperspectral reflectance offers an opportunity to non-destructively diagnose plant N status. Two sweet sorghum (Sorghum bicolor [L.] Moench) cultivars (Top 76-6 and Dale) were grown with 0, 56, 112, 168, and 224 kg N ha−1 in 2009 and 2010. Reflectance measurements were coupled with plant height, main-stem node number, leaf N concentration, and total chlorophyll content to establish the relationship of these traits with canopy reflectance. Canopy reflectance was most sensitive to N status in the visible region, specifically near green (595 nm) and red (701 nm) wavebands. Simple-ratio spectral models comprised of visible wavebands or wavebands from the visible and near infrared region outperformed models developed using only the most sensitive single-waveband. Based on the cross-validation of spectral models between data from two years and two cultivars, the simple-ratio models comprising the reflectance (R) ratios of 595 nm vs. 1676 nm and 595 nm vs. 508 nm predicted leaf N concentration and chlorophyll content with the greatest accuracy (highest r2 and lowest relative error, RE). These simple-ratio models were used to develop general-purpose spectral models to derive coefficients to estimate leaf N concentration (-66.63 × R595/R1676 + 34.14; r2 0.52; RE 16.8%) and chlorophyll content (-49.12 × R595/R508 + 107.47; R2 0.64; RE 17%). The identified spectral models can be used to assess growth, diagnose sweet sorghum N status and may be useful to make N management decisions for site-specific fertilizer applications.