Effect of seedling age, photoperiod, and temperature on bolting and inflorescence quality in Delphinium

To examine the effect of seedling age on bolting and inflorescence quality in Delphinium × cultorum (dwarf pacific type), seedlings with 2 to 3, 4 to 5, or 6 to 7 expanded leaves were grown in a greenhouse under 8- and 24-hr photoperiods. Concurrently, the interaction between photoperiod and temperature on the development and quality of inflorescences was investigated. 1. Seedling age did not significantly affect bolting response, although a 24-hr photoperiod accelerated bolting compared with a 12-hr photoperiod. Under the long days, seedlings with 2 to 3 expanded leaves produced only 22 florets, whereas under the 12-hr photoperiod, the seedlings with the same number of leaves produced good spikes with almost 40 florets. 2. Seedlings grown in growth chambers under 12- to 24- hr photoperiods with temperature regimes of 17/12 and 24/19°C (day/night) for 18 weeks, the days to flowering and the number of leaves at bolting were increased under shorter daylength under both temperature treatments. These photoperiodic responses were more significant at 17/12°C. All plants tested at 24/19°C bolted under a 16-hr or longer photoperiods. At 17/12°C, only 50% of plants under a 16-hr photoperiod bolted, whereas all plants bolted under the 20- and 24- hr photoperiods. Under the 16- to 20- hr photoperiods, more florets were initiated at 17/12°C than at 24/19°C.