The relative Pfr-level in a long-day duckweed,Lemna gibbaG3, was estimated by the null response method. The null%R value (%R in a R/FR-mixture that provides a null flowering response. This value was assumed to indicate the endogenous Pfr-level of the duckweed.) remained high during the initial hours of the 15 hr nyctoperiod then decreased rapidly, if a 12 or 33 hr photoperiod preceded the nyctoperiod. The null%R value dropped immediately after the start of the 15 hr nyctoperiod subsequent to a 1 or 24 hr photoperiod. Thus, the duration of the maintenance of a high Pfr-level changed rhythmically depending on the length of the preceding photoperiod. Nyctoperiods of up to 9 hr following a 12 hr photoperiod hardly affected flowering, but nyctoperiods given after a 24 hr photoperiod suppressed flowering in proportion to the length of the period. The Pfr-level in the nyctoperiod, therefore, seems to be important for flowering, and phytochrome change, as a function of the length of photoperiod, may serve as a photoperiodic timer. Although floral response to interruption with R or FR changed with the application period, the difference in response between R-treated and FR-treated plants was relatively constant during a 15 hr nyctoperiod combined with a photoperiod of any length other than 1 hr. Apparently, the floral response to the R or FR pulse was regulated by a shift in the Pfr-level caused by the light pulse.
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