Selection of Optimum Reflectance Ratios for Estimating Leaf Nitrogen and Chlorophyll Concentrations of Field‐Grown 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 (R 517 /R 413 ) ( r 2 = 0.65–0.78 ). Leaf Chl concentration was associated closely with reflectance ratios of either R 708 /R 915 or R 551 /R 915 ( r 2 = 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.