Spectral Dependence of Night-break Effect on Photoperiodic Floral Induction in <italic>Lemna paucicostata</italic> 441

Abstract

A single dark period of longer than 8 hr induced flowering in Lemna paucicostata 441 cultured in E medium. Monochromatic lights of 450, 550, 650 and 750 nm with a half-power bandwidth of 9 nm given for 10 min at the 8th hour of a 14-hr dark period inhibited flowering. The fluence rates required for 50% inhibition were 10, 0.5, 0.1 and 3 μmol m−2. sec−1, respectively. When applied between the 4th and the 10th hour of the dark period, lights of 450, 550 and 650 nm were inhibitory showing a maximum effect at the 8th hour. But 750-nm light completely inhibited flowering when applied at any time during the first 8 hr of the dark period. The inhibitory effect of 750-nm light given at the beginning of the dark period was totally reversed by a subsequent exposure to 650-nm light, and the fluence-response curves for the effect of 750-nm light given at the 0, 4th and 8th hour were essentially the same. This suggests that the presence of PFR is crucial for the floral initiation throughout the first 8 hr of the inductive dark period. The role of phytochrome in the photoperiodic flower induction of L. paucicostata is discussed.