Maintenance of high Pfr-level in the dark period in relation to flowering in Lemna gibba G3

The relative Pfr-level in a long-day duckweed, Lemna gibba G3,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-levelof the duckweed.) remained high during the initial hours ofthe 15 hr nyctoperiod then decreased rapidly, if a 12 or 33hr photoperiod preceded the nyctoperiod. The null % R valuedropped immediately after the start of the 15 hr nyctoperiodsubsequent to a 1 or 24 hr photoperiod. Thus, the duration ofthe maintenance of a high Pfr-level changed rhythmically dependingon the length of the preceding photoperiod. Nyctoperiods ofup to 9 hr following a 12 hr photoperiod hardly affected flowering,but nyctoperiods given after a 24 hr photoperiod suppressedflowering in proportion to the length of the period. The Pfr-levelin 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 responseto interruption with R or FR changed with the application period,the difference in response between R-treated and FR-treatedplants was relatively constant during a 15 hr nyctoperiod combinedwith a photoperiod of any length other than 1 hr. Apparently,the floral response to the R or FR pulse was regulated by ashift in the Pfr-level caused by the light pulse. (Received April 2, 1979; )