Dark transformations of phytochrome in cotyledons of Pharbitis nil

Pharbitis seedlings grown in total darkness or continuous far-red irradiation were exposed to 30 min of red irradiation followed by a dark period, and in vivo phytochrome in their cotyledons was photometrically assayed at various times. Loss of photo-reversibility of Pfr after the exposure to red light occurred without dark reversion to Pr in cotyledons of both seedlings. Pfr decay in the former cotyledons was mostly prevented in the first 30 min under red light illumination, while that in the latter occurred without such a lag phase. Pfr was no longer photometrically detectable by the eighth hr after irradiation at both 18°C and 25°C. No evidence has yet been obtained to show a correlation between photometrically detectable phytochrome in vivo and the red far-red reversible responses of flowering.