The role of phytochrome in photoperiodic time measurement and its relation to rhythmic timekeeping in the control of flowering in Chenopodium rubrum.

To follow changes in the status of phytochrome in green tissue and to relate these changes to the photoperiodic control of flowering, we have used a null response technique involving 1.5-min irradiations with mixtures of different ratios of R and FR radiation.Following a main photoperiod of light from fluorescent lamps that was terminated with 5 min of R light, the proportion of Pfr in Chenopodium rubrum cotyledons was high and did not change until the 3rd hour in darkness; at this time, Pfr disappeared rapidly. When the dark period began with a 5-min irradiation with BCJ or FR light to set the proportion of Pfr low Pfr gradually reappeared during the first 3 h of darkness and then disappeared again.The timing of disappearance of Pfr is consistent with the involvement of phytochrome in photoperiodic time measurement. Reappearance of Pfr after an initial FR irradiation explains why FR irradiations sometimes fail to influence photoperiodic time measurement or only slightly hasten time measurement. A R light interruption to convert Pr to Pfr delayed, the timer by 3 h but only for interruptions after and not before the time of Pfr disappearance. Such 5-min R-light interruptions did not influence the operation of the rhythmic timekeeping mechanism. Continuous or intermittent-5 min every 1.5 h-irradiations of up to 6 h in duration were required to rephase the rhythm controlling flowering. A skeleton photoperiod of 6 h that was began and terminated by 5 or 15 min of light failed to rephase the rhythm.The shape of the curves for the rhythmic response of C. rubrum to the length of the dark period are sometimes suggestive of "clocks" operating on the principle of a tension-relaxation mechanism. Such a model allows for separate timing action of a rhythm and of Pfr disappearance over the early hours of darkness. Separate timing action does not, however, preclude an interaction between the rhythm and phytochrome in controlling flowering.