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Home HortScience Uninterrupted Climatic Conditions Key to Successful Floral Transformation
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ANGERS, FRANCE--Hydrangea macrophylla is a popular ornamental plant sold throughout the world. Commercial production of the beautiful flowering plants is an intensive undertaking requiring several years and fraught with challenges for growers.

The flowering axes of hydrangea plants develop over 2 years of growth. During the first year, continuous plant growth ensures the formation of the stem. This is followed by floral transformation, a phase indicated by the formation of a mixed bud composed of scale leaves, phytomer primordia, and floral primordial. Floral transformation occurs at the end of the summer in temperate climates, and is followed by the onset of bud dormancy a period requiring cooler temperatures to restore growth capability. In the second year, terminal flowering is ensured by the elongation of all of the vegetative and floral organs that are formed in the mixed bud.

The commercial quality of potted hydrangea depends on the number key factors related to floral transformation, a sequence of three successive phases: floral induction (B1), floral evocation (B2), and floral organogenesis (B3). New research from HortScience focused on the impact of the transition from climatic conditions favorable to floral induction to those favorable to vegetative growth in Hydrangea macrophylla 'Leuchtfeuer'. Temperatures below 18 °C and a short photoperiod of less than 12 hours are considered "favorable" for floral transformation.

The study experiments showed that a change in climatic conditions during Phase B1, when the number of phytomer primordia in the bud is less than or equal to eight, leads to the end of the floral transformation process and a resumption of vegetative development. "We observed that climatic conditions favorable to vegetative growth did not interrupt the floral transformation process in the case of plants that had reached floral Stage 2 (beginning of Phase B3)," noted author Gilles Galopin. "Our study clearly showed that climatic conditions favorable to floral transformation must be continuously applied without interruption during the entire floral induction phase (B1) to ensure the formation of the floral bud in Hydrangea macrophylla."

The scientists concluded that the number of phytomer primordia present in the bud is a good indicator for monitoring development in Phase B1. "This indicator can be used during production to predict plant development in relation to climatic conditions," they wrote.


The complete study and abstract are available on the ASHS HortScience electronic journal web site:

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Original Article:

Analysis of the Impact of Climatic Conditions on Floral Transformation in Hydrangea macrophylla 'Leuchtfeuer'
Gilles Galopin, Laurent Crespel, Jean C. Mauget, and Philippe Morel
HortScience 46:1373–1376. [Abstract][Full Text][PDF]

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