The effects of short-day (SD) treatment on differentiation, development and anthesis of flower buds, and on cut flower quality were investigated in a leading cut sunflower (Helianthus annuus L.) cultivar, Sunrich Orange, a quantitative SD plant. Plants 15 were cultivated under long-day (LD) conditions with night interruption lighting from 2200 to 200 HR. SD treatment (11.5 h photoperiod) for 1, 2, or 3 weeks was commenced at the cotyledon, two true leaves, or four true leaves stage. The control plants were 2 grown under LD conditions throughout the experimental period. SD treatment for 1 or 2 weeks promoted flower bud initiation even when delivered at the cotyledon stage (9 days after sowing), indicating that the juvenile phase is very short in this cultivar.Flower differentiation and development were also accelerated by the SD treatment, and resulted in 19-39 days earlier anthesis. The quality of cut flowers was improved by SD 5 treatment to desirable characteristics such as decreased weight, shortened stem and reduced stem diameter. The present study suggests that under LD conditions such as found in summer, SD treatment for 2 weeks from the cotyledon or two true leaves stage can promote LD-delayed flowering of quantitative SD sunflower plants without reducing cut flower quality. 10