Natural chilling and limited inductive photoperiod affect flowering in two Asteraceae genera

To examine the effects of chilling and a limited inductive photoperiod (LIP) on flowering of Chrysanthemum X superbum Bergmans ex J. Ingram 'G. Marconi' and 'Snow Lady', Coreopsis grandiflora Hogg 'Sunray', and Coreopsis lanceolata L. 'Early Sunrise', seeds were sown and plants were maintained in the greenhouse in short days (SDs) for 7 weeks, followed by 4 months of natural outdoor chilling for all plants except 10 of each cultivar, which remained in the greenhouse under SDs for the duration of the experiment. Upon return to greenhouse conditions, 10 plants of each cultivar were placed in SDs, all other chilled plants were placed in long-days (LDs) and subsequently transferred to SDs after receiving 6, 8, 10, 12, 14, 16, 18, 20, or 22 LD cycles. Ten chilled plants of each cultivar remained in LDs for the duration of the experiment. Continuous SDs without chilling resulted in 70%, 40%, 20%, and 10% flowering in 'Snow Lady', 'Early Sunrise', 'Sunray,' and 'G. Marconi', respectively. Chilling, followed by a return to continuous SDs, improved powering in all cultivars compared to SDs without chilling. The number of LD cycles required for 100% flowering varied with cultivar. Ninety percent of the chilled 'Early Sunrise' plants flowered with no LD cycles, and 100% flowering was produced by as few as six LD cycles. In 'Snow Lady', 100% flowering occurred in chilled plants with no LD cycles and those receiving at least 10 LD cycles. With only six or eight LD cycles, 90% and 80% of these plants, respectively, still flowered. The percentage of 'G. Marconi' and 'Sunray' plants flowering ranged from 40% to 100% in the chilled treatments, with a general trend for more flowering with an increase in the number of LD inductive cycles. Continuous LDs following chilling resulted in as high or higher percent flowering than any other treatments (100% for 'G. Marconi' and 90% for 'Sunray'). The effects of LIP were evident in both Coreopsis cultivars but in neither Chrysanthemum X superbum cultivar. There was a linear relationship between the number of LDs received and stem length at first flower in 'Sunray' and 'Early Sunrise'. In both cultivars, a reduction in stem elongation of about 10 cm occurred when plants received only 6 LDs compared to 22 LDs. The number of days from the start of LDs to first flower increased linearly as the number of LD cycles before the transfer back to SDs increased.