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 X 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 tempera ture on the development and quality of inflorescences was investigated. 1. Seedling age did not significantly affect bolting response, although a 2 4-hr photoperiod accelerated bolting compared with a 12- hr photoperiod. Un der the long days, seedlings with 2 to 3 expanded leaves produced only 22 f lorets, whereas under the 12-hr photoperiod, the seedlings :vith the same n umber 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 degrees C (day/night) for 18 weeks, the days to flowering and the number of leaves at bolting were increased un der shorter daylength under both temperature treatments. These photoperiodi c responses were more significant at 17/12 degrees C. All plants tested at 24/19 degrees C bolted under a 16-hr or longer photoperiods. At 17/12 degre es C, only 50% of plants under a 16-hr photoperiod bolted, whereas all plan ts bolted under the 20- and 24-hr photoperiods. Under the 16- to 20-hr phot operiods, more florets were initiated at 17/12 degrees C than at 24/19 degr ees C.