Reflectance of Cotton Plants At Different Spatial Scales Related to Nitrogen (N) Treatments.

This study was conducted to examine the reflectance of cotton plants measured at three different spatial scales: individual leaf, canopy, and scene, in relation to N treatment effects, and consequently to select the best spatial scale(s) for estimating chlorophyll or N contents. At the leaf scale, N treatments effects were most apparent at 550 nm and 700 nm. N treatments did not significantly affect the internal leaf structure, and consequently the NIR reflectance. Wavelengths sensitive to N fertilization shifted to 600 nm and 700 nm when the measurements were made at the canopy level. NIR reflectance began to increase with increased N fertilization, as N treatments promoted increased biomass production and thus, multiple scattering by the leaves. At the leaf and canopy scales, reflectance measured in the visible region was a function of chlorophyll concentration. In contrast, measurements made at the canopy scale reflected the effects of increasing biomass production in the NIR region. At the scene level, effects of N treatments were most sensitive at 685 nm to 690 nm wavelengths. NIR reflectance also increased with the amount of N applied. Only measurements made at the scene scale had a consistent relationship between the amount of N fertilization and reflectance at both the visible and NIR regions. However, the primary contribution to this sensitivity was percent ground cover as influenced by N fertilization, instead of chlorophyll concentration. Since N fertilization primarily affect cotton chlorophyll content and ground cover, selecting only one scale may not be sufficient to represent the effects of N applications. Therefore, combining spatial scales that correspond to chlorophyll content (which are either at the leaf or canopy level) with the scale that best explains the variation in ground cover (which is at scene level) might be a practical approach for estimating chlorophyll or N content of crops.