Threshold intensity of twilight illumination for photoperiodism ofrice based on solar altitude

摘要

We conducted a study to determine the threshold intensity of twilight illumination for photoperiod-ism in rice (Oryza sativa L.) based on solar altitude in an open-field, day-length-extension experiment and utilization of a crop model. Light supplementation was provided by incandescent lamps and controlled based on solar altitude using four japonica rice cultivars: Hitomebore, Koshihikari, Nip-ponbare, and Nikomaru. First, pot experiments were conducted individually with supplementary light treatmentin the morning and in the evening. By comparing the difference in heading time between rice with and without supplementary light, we determined the threshold intensity of twilight illumination. The threshold was between a solar altitude of 0° and -2°in the morning and evening civil twilight. Second, a field experiment was conducted with light supplementation in the morning and evening civil twilight. Based on the results, we concluded that the threshold intensity of illumination would be around a solar altitude between —1° and —2° in the morning and the evening civil twilight.