A laboratory investigation of a physical mechanism for the extended infrared absorption ('red shift') in wheat

Laboratory spectral measurements, on the components of both greenhouse and field grown winter wheat, were performed to identify the component and its appropriate response which gave rise to the extended infrared absorption or 'red shift' reported by Collins. Results of this study indicated that inherent intraplant adaxial (upper) leaf reflectances were of sufficient variability to suggest that an admixture of mechanisms may have utility on identifying the booting and head emergence stages in the life cycle of wheat. The physical mechanism for the shift was found to be relatively independent of the inherent variability in leaf spectra, and to be dependent upon the difference in the mode of deposition of cuticle upon the abaxial (lower) surface relative to that of the adaxial (upper) surface, the position of the flag leaf, and thus the surface exposed to the incident light during heading and after emergence of the head.