Phytochrome in Pharbitis nil during and after de‐etiolation

Phytochrome (P) was measured by in vivo spectrophotometry in the cotyledons of Pharbitis nil Choisy cv. Violet. In etiolated plants exposure to red light (R) results in a rapid fall in P content by destruction of the far‐red absorbing form of phytochrome (Pfr). In continuous R or white light the P content falls as a result of destruction to a stable 3% of that originally present. In Norflurazon‐treated white light de‐etiolated plants, Pfr undergoes reversion in darkness to the red absorbing form of phytochrome (Pr) with a half life of 4 h at 19°C, while total P remains constant. Synthesis of Pr after de‐etiolation occurs at a slow rate and is enhanced by terminating the de‐etiolation light treatment with far‐red light. The results are discussed in relation to the P control of flowering.