Juvenility and flowering responses in Chrysanthemum x superbum and Coreopsis grandiflora and lanceolata

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Virginia Tech


Juvenility and flowering requirements were examined in Coreopsis grandiflora 'Sunray', C. lanceolata 'Early Sunrise', and Chrysanthemum x superbum 'G. Marconi' and 'Snow Lady'. C. grandiflora 'Sunray' and C. lanceolata 'Early Sunrise' grown from seed under continuous short days (SD) did not flower. 'Sunray' remained vegetative in long days (LD); however, LD induced flowering in 70 to 100% of the 'Early Sunrise' plants moved from SD to LD at true leaf stages beginning with 0 (cotyledons only) and progressing at 2 or 3 leaf increments to 24 leaves. The loss of juvenility in 'Early Sunrise' was gradual, with fastest flowering from onset of LD, 46 days, occurring when plants were transferred to LD at the 16 leaf stage. Plants moved to LD at six leaves flowered most rapidly from time of seeding, 84 days. Total leaf number at first flower increased as leaf number at transfer to LD increased. Chrysanthemum x superbum 'G. Marconi' was relatively unresponsive to LD, whereas all 'Snow Lady' plants flowered in LD treatments except the 24 leaf stage, which had 70% flowering. Although no 'G. Marconi' plants flowered under SD, 90% of the 'Snow Lady' plants flowered in continuous SD. In 'Snow Lady', transfer to LD at the cotyledon stage promoted fastest flowering from time of seeding, 75 days, and produced plants with the fewest number of leaves at first flower. Histological examination of apices of C. x superbum 'Snow Lady' revealed floral initiation in all 5 plants sampled following 3 weeks of LD. Initiation in SD started after the fifth week and was evident in all 5 plants sampled after the ninth week of SD.

The effects of chilling and a limited number of inductive photoperiods was examined in all 4 cultivars. Four months of natural outdoor chilling followed by at least 6 LD, promoted 40 to 100% flowering in Coreopsis grandiflora 'Sunray' and Chrysanthemum x superbum 'G. Marconi'. Chilling followed by SD increased flowering in each cultivar as compared to continuous SD with no chilling. The effects of limited inductive photoperiod (LIP) were evident in both Coreopsis cultivars, but not seen in either cultivar of C. x superbum. LIP inhibited stem elongation by approximately 10 cm in the chilled Coreopsis cultivars and also in C. lanceolata 'Early Sunrise' grown from seed with no chilling. LIP did not affect the scape length of either chilled or unchilled plants.