Abstract— Qualitative changes in the spectral energy distribution (SED) of natural daylight within the 400–800 nm wavelength band have been followed within a wheat canopy. Selective attenuation of radiation by the canopy caused large decreases in the blue and red wavebands and, to a lesser extent, the green; far-red was largely transmitted. This resulted in a decrease in the red:far-red ratio from the values observed in natural daylight.
The spectral energy distribution below the canopy was found to be partially dependent on solar elevation and sky condition; it was also dependent on the age, height, leaf area index and chlorophyll content of the crop. The possible ecological significance of the wide variety of spectral energy distributions which have been observed are discussed in relation to phytochrome function.