The Interaction of Photoperiod and Temperature on the Flowering Response of Rice

Eight varieties of the species Oryza sativa L. and two varieties of O. glaberrima Steud. were grown under controlled conditions in combinations of three photo-periods (8½ 10½ and 11½ hours light per day), each giving the same total daily radiant energy, and four temperature régimes (‘night’ and ‘day’ temperatures of 25-35° 30-35° 30-40° and 35-35° respectively). The flowering responses were measured as the number of days from sowing to the first appearance of the panicle. Under the range of conditions investigated, one variety (Kogbati 3) was completely insensitive to both photoperiod and temperature. The remainder were affected by both factors and the optimum photoperiod (i.e. the photoperiod resulting in earliest flowering) varied between 8½ and 10½ hours. In general, the higher the temperature régime, the longer the duration of the vegetative phase. In some varieties, but not all, there was some indication that the value of the optimum photoperiod increased with increasing temperature. No specific effects of night temperatures as opposed to day temperatures on the flowering response could be detected, but there was some evidence that high night temperatures were more deleterious to vegetative growth than high day temperatures. The 8½-hour photoperiod also resulted in particularly poor vegetative growth by comparison with other photoperiodic treatments.