Blacquiere et al. (2002) reported that the critical photoperiod for sunflower was around 12 h; therefore, the day-length under SD conditions was 11.5 h (530–1700 h) and created by automatically opening and closing photo-protective plastic film. The duration of SD treatment was 1, 2, or 3 weeks. After the SD treatment, the plants were returned to the LD conditions and were cultivated until flowering. Control plants were grown under the LD conditions described above throughout the experimental period.