Assessment of Growth and Leaf Pigment Concentrations of Sweet Sorghum Using Canopy Reflectance.

A two year field experiment was initiated to investigate the relationship between canopy reflectance and vegetative growth parameters and leaf pigment concentration across five N fertilization rates of 0, 50, 100, 150 and 200 kg N ha^-1 in two cultivars of sweet sorghum (Topper and Dale). Plant height, stem and leaf biomass, leaf pigment concentration and canopy reflectance were measured 8-12 days interval from 25 days after emergence to panicle initiation. The normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), modified chlorophyll absorption reflectance index (MCAI), photochemical reflectance index (PRI), mean reflectance of Landsat Thematic Mapper bands (TM1-4), first derivatives of the reflectance in red edge (dR/dλ) as well as several other spectral indices and ratios were calculated. Across the measurements, highly significant N treatment effects were observed for growth parameters, chlorophyll (Chl) a, b, total Chl and carotenoids.  Leaf pigments initially increased linearly with N supply but leveled off at the 150 kg N ha^-1 treatment. In general, canopy reflectance was reduced drastically in the TM1, 2 and 3 regions accompanied with shifts of red-edge reflectance (λ = 685-745) towards longer wavelengths of the spectrum as N supply increased. Among the studied spectral indices, Chl a, b, TotalChl and carotenoids showed higher linear correlations with dR/dλ [(740-730)/10] (r² = 0.73-0.93); RI [740-480)/(400-700)] (r² = 0.66-0.99); MCARI (r² = 0.58- 0.97); PRI (r² = 0.64 – 0.95); ratios of R₁₀₇₅/R₇₃₅ (r²= 0.66- 0.97) and R₇₁₀/R₇₆₀ (r² = 0.63-0.99). These identified spectral indices and ratios may be useful for non-destructive estimation of plant growth and leaf pigment concentrations which may provide site-specific management of N in sweet sorghum.