Hydrangea macrophylla is an ornamental horticultural plant grown in pots for indoor and outdoor use. The commercial quality of potted plants is defined by the number of flowering axes, their morphometric homogeneity, and the synchronization of flower bloom. The flowering axis develops over 2 years of growth (Galopin, 1995). Continuous plant growth over the first year ensures the formation of the stem. It is followed by floral transformation that is morphologically identifiable by the formation of a mixed bud composed of scale leaves, phytomer primordia, and floral primordia (Galopin et al., 2008; Uemachi and Nishio, 2000; Zhou and Hara, 1988). This floral transformation occurs at the end of the summer in temperate climates. It is followed by the onset of endodormancy of the bud that requires cooler temperatures to restore growth capability (Fuchigami and Wisniewski, 1997; Wallerstein, 1981). The second year, terminal flowering is ensured by the elongation of all of the vegetative and floral organs preformed in the mixed bud.
When plants are in production, we frequently observe either heterogeneous flowering axes at the top of the plant or axes without inflorescences that totally degrade the quality of the product. Improved knowledge of the morphogenetic processes involved in floral transformation and, in particular, inductive environmental factors would make it possible to avoid these losses in quality.
Many researchers have attempted to identify the climatic factors favorable to floral induction. Thus, temperatures below 18 °C (Bailey and Weiler, 1984; Post, 1942) and a short photoperiod of less than 12 h (Guo et al., 1995; Morita et al., 1980; Shanks et al., 1986) are favorable to floral transformation. Despite some interesting results, this research has not yet made it possible to control floral transformation during production. More recent studies have led to a detailed and chronological description of floral transformation in H. macrophylla (Fig. 1). The floral transformation sequence consists of three successive phases: 1) floral induction with the formation of the bud composed of eight vegetative preformed primordia (B1); 2) floral evocation with an increase in the size of the meristem and an activation of the entire apical zone (B2); and 3) floral organogenesis with a differentiation of floral primordia (B3) (Galopin et al., 2008, 2010).
The floral induction phase (B1) is defined as the transition from the capacity of the meristem to produce leaves to its capacity to produce flowers after the perception of a climatic (O'Hare, 2004) or agronomic (Chaikiattiyos et al., 1994) signal. The research presented here focuses on observations of the impact of the transition from climatic conditions favorable to floral induction to those favorable to vegetative growth in H. macrophylla during this phase (B1). This change in climatic conditions can be found during production, which is why it is important to assess its consequences on plant development.
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