Studies on the Photoreceptors for the Promotion and Inhibition of Flowering in Dark-Grown Seedlings of

Three-day-old etiolated seedlings of Pharbitis nil were exposed to red light for 10 min and sprayed with N6-benzyladenine before transfer to a 48-h inductive dark period, after which they were grown under continuous white light. A second red irradiation promoted flowering when given at the 5 and 24th hour of the inductive dark period but inhibited flowering at the 10 and 15th hour. Far-red light inhibited flowering when given at any time during the first 24 h of the dark period. Red/far-red reversibility was clearly observed at the 0, 5, 10 and 24th hour, but not at the 15th hour when both red and far-red lights completely inhibited flowering. The action spectrum for the inhibition of flowering at the 15th hour of the inductive dark period had a sharply defined peak at 660 nm and closely resembled the absorption spectrum of the PR form of phytochrome. The photoreceptors involved in these photoreactions are discussed. Key words: Circadian rhythm — Flowering — Morning glory (Pharbitis nil)