Bimodal Floral Response of Lemna gibba G3 to Night Interruption: Photoperiodic Time Measurement

Lemna gibba G3 (M-1% sucrose medium, 26°C) showed a bimodalfloral response to a 2-hr light pulse scanning 21-, 18-, 15-and 12-hr nyctoperiods. With the simplified min-LD method, thelight pulse given early or late in these nyctoperiods was foundto signal false dusk or false dawn after two transient cycles.The magnitude of floral response to the light pulse dependedon the length of the asymmetric skeleton photoperiod comprisingeither the preceding main photoperiod and the false dusk orthe false dawn and the subsequent main photoperiod. No flowerwas induced by asymmetric skeleton photoperiods shorter thanthe critical daylength, 12 hr. In duckweed previously entrainedto an interrupted 15-hr nyctoperiod, false dawn or false duskwas physiologically equivalent to the light-requiring L1- orL2-phase of the critical photoperiod. Another light-requiringphase occurred 12 hr after or before the false dawn or falsedusk. These and relevant findings suggest that the timing ofthe L1- and L2-phases is under the control of the endogenouscircadian oscillator and that the skeleton as well as completephotoperiods are inductive only when both the L1- and L2-phases,whether they are shifted or not due to night interruption, areilluminated. (Received October 3, 1980; Accepted December 3, 1980)