Analysis of the Impact of Climatic Conditions on Floral Transformation in Hydrangea macrophylla ‘Leuchtfeuer’

Hydrangea macrophylla is a horticultural plant of considerable commercial interest that has been widely studied with the aim to more effectively control the different stages of its development during production. However, although floral transformation is a key factor underlying the commercial quality of the product, it remains difficult to control despite these efforts. The floral transformation sequence consists of three successive phases: floral induction (B1), floral evocation (B2), and floral organogenesis (B3). The first is a phase of vegetative organogenesis without elongation leading to the formation of a bud composed of eight phytomer primordia under inductive climatic conditions. This work shows that climatic conditions favorable to floral transformation must be continuously applied without interruption throughout phase B1 to ensure the formation of the floral bud in Hydrangea macrophylla ‘Leuchtfeuer’. In the opposite case, floral transformation is stopped and vegetative growth begins once again.

Effect of Photosynthetic Photon Flux and Temperature on Floral Evocation and Development in the Vernalization-sensitive Ornamental Perennial Salvia ×superba `Blaukönigin'

Varying photothermal ratios (PTR) were supplied to Salvia ×superba Stapf `Blaukönigin' during pre-inductive vegetative development with the exception of a short germination period under uniform conditions. In addition, both unvernalized plants and plants receiving a saturating vernalization treatment of 6 weeks at 5 °C were given two photosynthetic photon flux (PPF) levels (50 or 200 μmol·m-2·s-1) during subsequent inductive 16-hour long days. There were no effects of PTR treatments during vegetative development on subsequent flowering. However, the higher PPF level during inductive long days significantly accelerated floral evocation in unvernalized plants, lowering the leaf number at flowering. The effect was practically negligent after the vernalization requirement was saturated. In a second experiment, varying periods (4, 7, 10, and 14 days or until anthesis) at a PPF of 200 μmol·m-2·s-1 during 20-hour days were given at the beginning of a long-day treatment, either with or without preceding vernalization treatment. Flowering percentage increased considerably as the period at 200 μmol·m-2·s-1 was extended compared with plants grown at a lower PPF of 50 μmol·m-2·s-1. However, the leaf number on flowering plants was not affected, except in unvernalized plants receiving the highest PPF continuously until anthesis, where leaf number was reduced by almost 50%. We propose that the PPF-dependent flowering is facilitated either by the rate of ongoing assimilation or rapid mobilization of stored carbohydrates at the time of evocation. Abortion of floral primordia under the lower PPF (50 μmol·m-2·s-1) irrespective of vernalization treatment indicates that the assimilate requirement for flower bud development is independent of the mechanism for floral evocation.