Role of the Photoperiod Preceding a Flower-Inductive Dark Period in Dark-Grown Seedlings of Pharbitis nil Choisy

Flowering responses to a single photoperiod, of various durations and irradiances, followed by an inductive dark period were investigated with dark-grown seedlings of Pharbitis nil Choisy. The number of flower buds induced in each plant (NFB) increased with the increase of both duration and irradiance of the photoperiod. Reciprocity did not hold for this photoresponse within the range of 0-16 h and 2.5-10 W-m-2, NFB depending on the duration rather than the irradiance. With lengthening of the dark period following a photoperiod of 8 h or less, two different phases alternately appeared so that NFB sharply increased at 20-24 h and 40-43 h after the onset of the photoperiod, then gradually decreased. When the photoperiod was longer than 8 h, NFB sharply increased at 12–16 h after the end of the photoperiod and remained around the saturated value with longer dark periods. Far-red light given immediately after the photoperiod inhibited flowering, the inhibitory effect being stronger the shorter the photoperiod. This far-red effect is mediated by phytochrome and PFR seems to be required during the inductive dark period following a short photoperiod for floral induction.