Phytochrome in Norflurazon-treated seedlings of Pharbitis nil

The properties of phytochrome have been measured by dual-wavelength spectropho-tometry in the cotyledons of the short-day plant Pharbitis nil Choisy cv. Violet, where it is known to play a role in flower induction. In plants de-etiolated by a single white light period (4 h or longer), destruction of the far-red absorbing form of phytochrome (P_fr) was twice as rapid as after 10 min red light. A small fraction of P_fr was stable. After de-etiolation by a period of white light (6 h or longer) the rapid decrease of P_fr during the first 30 min was accompanied by a rapid increase of the red absorbing form of phytochrome (P_fr). This rapid increase of P_fr is probably due to dark reversion. Long term synthesis of phytochrome was inhibited by the presence of P_fr. Phytochrome synthesised in darkness showed the etiolated-plant type characteristics and underwent rapid destruction upon photoconversion to P_fr. The stable P_fr identified here is possibly that pool of phytochrome associated with the long term promotive process in flower induction, and the rapidly reverting P_fr is that pool associated with the night break inhibition of flowering.