After-effect of photoperiodism in relation to development and yield of rice plant : (II) On the premature heading, with special reference to "the premature heading inductivity"

"Aichi-Asahi", a late variety of the rice plant, was exposed to various photoperiodic treatments at the different stages of development in seed bed. The short-day treatment induced premature heading, ie, an abnormal heading response takes place during the earlier period in addition to the natural heading in the later period. The treatment increases the number of the panicles of the premature heading, and at the same time decreases the number of the days required for its heading. The effects of the treatments on inducing the premature heading were clearly expressed by an index : Premature Heading Inductivity = (Number of days from the Beginning of Premature Heading to That of Natural Heading) ÷ (Number of Days from the Beginning of photoperiodic treatment to That of Natural Heading) × 100. As the seedling grew, the shorter duration of shortday treatment was "suffieicut for inducing premature heading and the greater response was observed. These results suggest that the photoperiodic response becomes greater as the plant develops. Almost perfect correlation was found between "Inductivity" and the number of the panicles of premature heading. (Correlation coefficient r=0.96 ± 0.02 in the 1st experiment, r=0.95 ± 0.02 in the 2nd expt.) The long-day treatment followed to the short-day cancelled the latters' effect. More remarkable effect of inducting premature heading was observed with the seedlings transplanted to fields than that to pots. It may suggest that temperature, nutrition, etc. may somewhat modify the response of rice plant to photoperiods.