Phytochrome Action in the Induction of Plowering in Short-day Plants: Effect of Photoperiod Quality

Seedlings of Phavbitis nil and Chenopodium rubrum flower in response to a single inductive dark period preceded and followed by continuous fluorescent light. However, when a far-red irradiation for 1 h or longer was substituted for fluorescent light, the flowering response to an ensuing dark period could be completely inhibited or enhanced up to threefold depending on when the far-red exposure commenced. Evidence of red/far-red photoreversibility established phytochrome as the photoreceptor controlling these responses. There was no indication of the involvement of photosynthetic pigments. For P. nil prolonged exposure (1.5-6 h) to far-red radiation during the photoperiod could shorten (2-3 h) or lengthen (2 h) the duration of darkness required for flowering. The degree of change depended on whether the far-red radiation was imposed just prior to darkness (shortening) or about 9 h prior to darkness (lengthening). In a similar manner the spectral composition of the photoperiod influenced the timing during darkness for earliest sensitivity to brief (5 min) red light interruptions. The shorter the critical dark period, the earlier in darkness the seedlings became sensitive to red light interruptions of darkness. It is clear that the form of phytochrome during the photoperiod influences the timing of phytochrome- linked processes in darkness. Two explanations discussed are a coupling via rhythmic changes in substrate on which phytochrome acts, and an effect on phytochrome reactions which alters the timing of Pf, disappearance during a subsequent dark period.