Relationships Between Sugarcane Leaf Hyperspectral Reflectance, Leaf Nitrogen Content, and Yield Components.

Leaf spectral reflectance has been used to estimate crop leaf chemical composition and other physiological characters. Leaf reflectance of sugarcane (Saccharum spp.) may be of use in evaluating genotypes.  The objectives of this study were: (1) to identify sugarcane genotypic variation in leaf hyperspectral reflectance (400-2500 nm), leaf chlorophyll and N contents, cane yield, sucrose content, and sucrose yield and (2) to determine relationships between leaf reflectance and these leaf chemical and yield variables. In Stage 2 of the 2008 and 2009 Canal Point Sugarcane Cultivar Development Program, three replicates of leaves at the top visible dewlap from each of 208 and 124 genotypes were used to measure reflectance and chlorophyll and N contents at four different times during the growing season (May, July, September, and October) in 2009 and 2010, respectively. The yield components were determined in early November on mature plants. Partial least-square regression was used to develop reflectance models for prediction of leaf chemical and yield components. Genotypic variation of leaf reflectance mainly occurred at wavelengths of 540-1200 nm. Leaf chlorophyll and N contents declined slightly as measurements progressed from May to October. Leaf chlorophyll and N contents among genotypes ranged from 28.7 to 53.6 and from 16.4 to 25.5 g kg^-1 with CVs of 10.8 and 7.4%, respectively and could be well predicted using leaf reflectance (P < 0.0001). The CVs of cane yield, sucrose content, and sucrose yield among genotypes were 26.0, 10.4, and 36.6%, respectively. The correlations of the predicted yield components with these measured yield traits were poor. Therefore, it might be difficult to use leaf reflectance for prediction of sugarcane yield components across genotypes.