Selection of Optimum Reflectance Ratios for Estimating Leaf Nitrogen and Chlorophyll Concentrations of FieldGrown Cotton

Leaf N and chlorophyll (Chl) concentrations of cotton (Gossypium hirsutum L.) are important indicators of plant N status. Laboratory determinations of plant tissue N are time consuming and costly. Measurements of leaf reflectance may provide a rapid and accurate means of estimating leaf N and Chl. Studies were conducted to determine the relationships between leaf hyperspectral reflectance (400-2500 nm) and Chl or N concentration in field-grown cotton. One study consisted of four N rates of 0, 56, 112, and 168 kg N ha-1, and another study consisted of four mepiquat chloride (MC) rates of 0, 0.59, 1.17, and 2.34 L MC ha-1. Chlorophyll and N concentrations and reflectance of uppermost, fully expanded mainstem leaves were measured throughout the growing seasons. Reflectance at 556 and 710 nm increased significantly as N fertilizer rate decreased. Averaged across years and sampling dates, the percentage increase in reflectance at these two wavelengths was 8, 10, and 19% greater in the 112, 56, and 0 kg N ha-1 treatments, respectively, compared with the 168 kg N ha-1 treatment. The effect of MC on leaf reflectance was more complex than the N effect. In both the N and MC studies, a linear relationship was found between leaf N and a simple ratio of leaf reflectance at 517 and 413 nm (R517/R413) (r2 = 0.65-0.78***). Leaf Chl concentration was associated closely with reflectance ratios of either R708/R915 or R551/R915 (r2 = 0.67-0.76***). Our results suggest leaf reflectance can be used for real-time monitoring of cotton plant N status and N fertilizer management in the field.