Nonphotochemical Transformations of Phytochrome in Vivo

Abstract

Phytochrome was measured spectrophotometrically in intact, dark-grown, maize seedlings and was found to be present entirely as P660. If the seedlings were given a single, brief irradiation with red light, the P730, formed by the light, reverted to P660 during the ensuing dark period and approximately 2/3 of the photoreversible phytochrome was lost. If the seedlings were placed in continuous red light, almost all of the reversible phytochrome was lost. The rate of loss of phytochrome was measured with seedlings in continuous light of different spectral qualities. The rate of loss was maximal if the light maintained 10% or more of the phytochrome as P730. A measurable rate of loss occurred even under far-red sources which maintained less than 1% of the phytochrome as P730. These measurements show that long irradiations with far-red light may have the same effect as red-light treatments in phytochrome systems and suggest that the action of P730 saturates at relatively low levels of P730.