Photoperiodism and rhythmic response to light

Abstract. Seedlings of Pharhitis nil show a circadian rhythm in the capacity to flower in response to the timing of a second red light pulse given at various times after a first saturating exposure to red when this is given together with a benzyladeninc spray. There are also changes in the photon irradiance required for half maximum response to the second red pulse. The photochemical properties of phytochrome in the photoperiodically sensitive cotyledons were also shown to change rhythmically. Oscillations in both p_r→ P_fr and P_fr→ P_r photoconversion characteristics persisted over at least two circadian cycles with a periodicity of about 12 h. There were, however, no significant oscillations in either P_fr peak absorbance or in Δ(ΔA). The changes in sensitivity for the photoconversion of P_r→ P_fr did not parallel the much larger changes in sensitivity of the flowering response to red light. The amplitude of the P_r→ P_fr rhythm was at least as great as that for P_r→ P_fr, but the flowering response to far-red light was not rhythmic, nor was there any large change in sensitivity. The changes in photoconversion properties may reflect a basic biochemical oscillation which affects both photoreceptor properties and sensitivity to photoreceptor input. There was also a marked rhythm in the P_fr/P ratio that would be established by a saturating pulse of red light and this too may have affected the flowering response to such a pulse. Far-red light inhibited flowering when given at any time during the inductive night. After 14 h in darkness, P_fr could still be measured in the cotyledons and it was concluded that far-red light inhibited flowering by removing P_fr As red light also inhibited flowering at this time, there may be two pools of phytochrome with different kinetic properties.