Nonphotochemical Transformations of Phytochrome in Vivo

Some of the photochemical properties of this pigment, which has been dubbed phytochrome, have been described previously (5, 7, 8). The present paper is concerned with the nonphotochemical transformations of phytochrome in vivo. Dark transformations of phytochrome were indicated from studies of the germination of lettuce seeds (2) and the flowering of photoperiodic plants (3). The far-red absorbing form of phytochrome, P730, was found to revert to the red-absorbing form, P660, in darkness. This dark reaction was postulated to be the basis of the time-sensing mechanism of photoperiodism (4). The photometric measurement of phytochrome (7) permits the dark conversion of P730 to P660 to be measured in vivo. In dark-grown seedlings, this dark conversion is accompanied by another dark reaction which results in a loss of reversible phytochrome. It is not known whether this loss is due to a destruction of phytochrome or merely to a loss of photoreversibility. The present paper reports on these 2 nonphotochemical processes which appear to involve phytochrome as a reactant.