Effect of different Al supplies during vegetative phase on sepal blueing in hydrangea inflorescence

In Hydrangea macrophylla (hydrangea), the quality of the flowering plant depends both on its architecture and the color of the inflorescences. Indeed, the presence of delphinidin in the sepals allows a pink coloration and a blue when aluminum (Al) is present in the substrate. The relation between quantities of Al supplies and sepal blueness is unclear. Also, with a same amount of Al supply, some cultivars turn deep blue while other display only a faint color. The lack of knowledge on the interaction between Al levels, genotype, and sepal blueing limits design of adapted Al fertilization scheme for each cultivar and likely excessive supplies of Al are used in production. Within the framework of the MATCH project, which aims to reduce chemical inputs during production, one of our objectives is to reduce Al supplies, in light with Al toxicity for the environment. Traditionally, Al is added at the end of the vegetative phase, until a foliar concentration of 400 mg Al kg^‑1 DM (dry mass) (Leuween et al., 2003). This threshold allows development of a deep blue color of sepals. To optimize the amount of Al applied, experiments were conducted where different amounts of Al (as in production=100, 50% and 0) were applied during the outdoors vegetative phase to three hydrangeas cultivars with a gradient of blueing capacity. Al quantifications in different organs were performed during vegetative phase. The inflorescence color was objectively characterized with a spectrocolorimeter. In addition, plant phenotyping was carried out at flowering to compare the quality of the plants obtained with different Al supplies. First results are presented on both the visual quality of the plants and the evolution of Al contents in the organs in relation to plant growth and inflorescences color. The results showed that Al treatments can be decrease during vegetative phase for the easy-to-blue cultivar without changing both visual quality and sepal color during flowering phase.