The Function of Phytochrome in the Natural Environment—iii. Measurement and Calculation of Phytochrome Photoequilibria

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 P fr 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.