A brief summary at this stage may be helpful. The night-break inhibition of flowering in SDP is clearly dependent on Pfr; thus it seems that the Pfr present at the end of the photoperiod declines to a level which is not effective in the night-break reaction before the light-sensitive phase of the photoperiodic rhythm is reached. The Pfr-requiring reaction, which is essential for flowering, is also clearly dependent on Pfr. In this case, however, the Pfr is highly stable in darkness. The dusk signal which initiates dark time measurement is usually attributed to a thermochemical reduction in Pfr, although this has not unequivocally been proved. If Pfr is involved in the dusk signal, it must be highly unstable in darkness since, at physiological temperatures, dark time measurement appears to begin within less than an hour after transfer to darkness. Thus the phytochrome that is associated directly with the photoperiodic mechanism (dusk signal perception and night-break inhibition) and that controlling the Pfr-requiring reaction must be different at least in the stability of Pfr.
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