The demonstrationin vivo of more than one form ofP fr.

Phototransformation studies of pumpkin phytochromein vivo indicate two populations, a slow and a fast one. The ratio between the two populations can be altered by different light-dark-pretreatments. The destruction kinetics ofP fr also show a deviation from a first order curve which indicates the existence of two populations ofP fr with respect to decay. No significantP fr →P r dark reversion could be detected. Two models are discussed, which could explain the shape of the destruction kinetics observed and the different ratios of the two populations obtained from phototransformation measurements.