Measurement of the Critical Nyctoperiod by Lemna paucicostata 6746 Grown in Continuous Light

The short-day duckweed Lemna paucicostata 6746 could be inducedto flower in two days at 26C when continuous illumination forentrainment was followed by continuous darkness. This 48-h darkperiod or the minimum darkness requirement for floral inductionwas called the induction period. The length of the inductionperiod (IP) was routinely computed as the number of 24-h cyclesusing the equation of regression of flower number in logarithmon culture time. A light pulse given about 7 h after the startof the induction period increased the apparent IP value fromtwo to three, suggesting that the interrupted first day hadfunctioned as a noninductive day. A pulse given at any otherpart of the induction period did not modify the IP value. Thelight-sensitive part is probably the inducible phase, and thefirst 7-h period of darkness terminated by it seems to be thecritical nyctoperiod. These and relevant facts suggest thatthe light-off oscillator measures the critical night length,7 h. Either red or far-red irradiation at the inducible phase extendedthe IP value by one. No red/far-red photoreversibility was detected.As expected, however, red or far-red irradiation of any otherpart of the critical nyctoperiod could not modify the IP value. (Received February 8, 1985; Accepted May 14, 1985)