Vegetative and optical characteristics of four-row crop canopies

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Abstract
The estimation of canopy vegetative parameters from reflected radiation would provide valuable information for assessing crop stress. Vegetative and optical properties of cotton, soya beans, grain sorghum and sunflower were measured at three growth stages at different solar zenith angles. Scattering and absorption coefficients and asymptotic reflectance in thematic mapper bands TM 1, 2, 3 and 4 were determined from measured reflectance, transmittance and absorptancc. Plant height, ground cover, leaf area index, leaf overlap index, foliage density and leaf angle were measured at each sampling date. Increasing leaf area increased the scattering coefficient value in band TM4 while the band TM 3 coefficient value remained unchanged. Absorption coefficient value changes in bands TM 4 and TM 3 with leaf area were opposite to those of scattering coefficients. Increases in solar zenith angle caused increased scattering in TM 4 for all crops; however, in TM 3 increases occurred only in cotton and soya beans. Asymptotic reflectance in band TM 3 was reached in all crops at LAIs ≥ 30. Band TM 4 asymptotic reflectance occurred in cotton and sunflowers at LAIs ≥ 30, but for soya beans and grain sorghum LA Is ≥ 6·0 were required. Canopy optical properties were related to Sun angle and canopy vegetative parameters which were classified into three types representing Sun angle, leaf quantity and canopy architecture. Solar zenith angle, leaf area index and foliage density produced the best estimates of canopy optical properties for all crops.