Estimation of High Biomass Sorghum Growth and Leaf Characteristics by Canopy Reflectance.

A two year field experiment was initiated to investigate the relationship between canopy reflectance, vegetative growth parameters and leaf pigment concentration across five N fertilization rates of 0, 56, 112, 168 and 224 kg N ha^-1 in two cultivars of high-biomass sorghum (ES-5200 and ES-5201).  Plant height (PH), node number (NN), leaf pigment concentrations and canopy reflectance were measured 64, 85, 99 and 118 days after planting (DAP).  Spectral reflectance regions associated with measured parameters were evaluated along with existing spectral indices and ratios including modified chlorophyll absorption reflectance index (MCARI), total canopy chlorophyll content index (TCCCI) and normalized difference vegetation index (NDVI).  Across the measurements, no significant genotype × treatment interactions were observed for PH, NN, SER, NAR and TChl across treatments.  However, highly significant N treatment effects were found for PH, NN, stem elongation rate (SER), node addition rate (NAR), chlorophyll (Chl) a, b, total chlorophyll (TChl) and carotenoid concentrations.  The highest coefficients of determination (r²) for TChl was observed with spectral ratios of R710/R1427 (r² = 0.84) whereas, MCAR and TCCCI exhibited r² of 0.72 and 0.77, respectively.  The spectral ratio of R695/R670 (r² = 0.62) and R695/R510 (r² = 0.74) exhibited the highest correlations with carotenoid concentration and ratio of TChl/Carotenoid, respectively.  The relationship between spectral reflectance and measured parameters will be explored further to develop and validated suitable spectral algorithms to accurately estimate high biomass sorghum growth, leaf pigments and plant nitrogen status under field condition.