Response of China aster to daylength and gibberellic acid

Abstract

1. Plants of Callistephus chinensis cv. ‘Giant Comet’ were subjected to photoperiods of 8 or 16 hours. In short day (SD) a much greater number of leaves was formed than in long day (LD) before the terminal inflorescence was initiated. By transferring plants from LD to SD at weekly intervals it was established that in LD floral induction is completed about 6 weeks after sowing. 2. Inflorescence development proceeds faster in SD than in LD. Plants in continuous SD or LD flower almost simultaneously. Plants transferred from LD to SD six weeks after sowing, flower about a month earlier than those plants grown entirely in LD. 3. Gibberellic acid (GA3) did not affect inflorescence initiation in LD. In SD, however, GA3 applications reduced the number of leaves formed before the inflorescence. The maximal effect, obtained by spraying with a solution of 25 mg GA3/1 during 30 days, halved the difference in leaf number of untreated plants in LD and SD. The same effect was obtained when plants were sprayed 10, 20 or 30 times during this 30 day period. 4. Manipulation of the photoperiod allowed the development of a method to grow two generations within one year. This method, which is being used to speed up a breeding project, is briefly described.