Thermal and Light Requirements for Flower Differentiation of Snapdragon

In order to define critical thermal and light requirements for flower differentiation, 4 cultivars of Snapdragon: Mistral, Monaco, Vincenot and Potomac, were tested under 4 thermal-photoperiodic regimes obtained by combining 2 thermal treatments [(night/day) 22/28 and 16/22°C] and 2 photoperiods [Short Day = 8 hours and Long Day = 14 hours]. The research was carried out on potted plants in a growth chamber. The effects of treatments were studied in terms of plant growth, cut stem production and flowering time. The time of transition from vegetative to floral meristem was assessed by transmitted light microscopy on longitudinal sections of apical meristems. Floral transition at the meristematic stage was related to morphological parameters at the whole plant level (height, number of leaves, leaf area). The cut stem yield did not change among the cultivars and was unaffected by thermal and photoperiodic treatments. The number of days from transplanting to flowering increased from 59 (Mistral) to 87 (Potomac). Long-day conditions anticipated flowering of 40 days compared to the short-day treatment and reduced both stem length and leaf number at the anthesis. Thermal treatments did not affect the flowering time and the main cut stems characteristics. Microscopy analyses revealed that while it is possible to control the floral induction by choosing specific genotypes and modulating environmental parameters, the time between meristem induction and flower opening appears to be unaffected by thermal and light regimes. This time corresponded, in our experimental conditions, to approximately 20 days. A critical size of the plant, in terms of leaf area, seems to be required for the flower induction.