CHANGES IN SPECTRAL CHARACTERISTICS OF CEREAL CROPS WITH PHYSIOLOGICAL DEVELOPMENT

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Abstract
Analyses of reflected solar radiation (350–800 nm), obtained with a high-resolution (< 1 nm) portable spectrophotometer from field plots of wheat, oats and barley, showed that changes in spectral features of cereal crops were closely related to stages of physiological development. The spectra were normalized at discrete 1-nm bandwidths to reduce variation among days due to changing sun and viewing angles. Results of the analyses showed five relatively narrow plateaux, between 350 and 800 nm, which were closely related to physiological development. As the crops neared maturity, three of the plateaux became indistinguishable (415–425, 450–480, 530–560 nm), whereas the two plateaux in the IR range, 730–750 and 770–790 nm, combined and decreased markedly in amplitude. The proportion of total reflected radiation in several bands was also related to stage of development. When analyzed in 100-nm bandwidths, the violet and blue bands remained fairly constant (5% and 20%), whereas the proportion of the IR decreased and in the red increased as plant development occurred. A marked crossover in the proportions occurred when the crop was more than 50% mature. The spectra of the lower portion of a crop canopy exhibited characteristics of more advanced stages of plant development than the spectra of the upper portion. Relating the radiation in a specific narrow band to total radiation (350–800 nm) conveniently reduced much of the day-to-day atmospheric variation and enabled comparisons to be made among crops and growth stages from data recorded on various dates. Discrete selection of narrow spectral bands closely related to physiological aspects of plants may reduce much of the "noise" presently in radiation data of "wide-band" detection systems. The results illustrate difficulty frequently encountered in distinguishing wheat, oats and barley from spectral data if the crops are near similar stages of physiological development.

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