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 (Pfr) was twice as rapid as after 10 min red light. A small fraction of Pfr was stable. After de‐etiolation by a period of white light (6 h or longer) the rapid decrease of Pfr during the first 30 min was accompanied by a rapid increase of the red absorbing form of phytochrome (Pfr). This rapid increase of Pfr is probably due to dark reversion. Long term synthesis of phytochrome was inhibited by the presence of Pfr. Phytochrome synthesised in darkness showed the etiolated‐plant type characteristics and underwent rapid destruction upon photoconversion to Pfr. The stable Pfr identified here is possibly that pool of phytochrome associated with the long term promotive process in flower induction, and the rapidly reverting Pfr is that pool associated with the night break inhibition of flowering.