Abstract
Abstract— Phytochrome photoequilibria have been measured in dark‐grown Phaseolus uulgaris L. and Cucurbita pepo I. hypocotyl hooks which had been exposed to various natural and artificial radiation sources. Mean phytochrome photoequilibria (φ) varied from 0.20 within a wheat canopy to 0.54 above, although lower values were occasionally observed in densely shaded areas. Greater variation in phyto chrome photoequilibria and lower levels of Pfr were recorded within a sugar beet canopy. The range of photoequilibria was φ= 0.04 in dense shade to φ= 0.54 above the canopy. Photoequilibrium was achieved within 5 s in mid‐day sunlight and approximately 30 s in dense canopy shade.A close correlation was found between φ and the ratio of the quantum flux in the red and far‐red wavelength bands (ζ) in broad spectrum (400–800 nm) radiation. This relationship allows direct prediction of φ from a knowledge of ζ. Phytochrome showed greatest sensitivity to spectral changes in the range ζ= zero to ζ= 1.0, which is the range found in the natural environment.The observations provide support for the hypothesis that phytochrome is involved in the detection of shading by plants.
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