Changes in ABA levels in vegetative and flower buds during dormancy in camellia

Camellia japonica L. is an evergreen acidophilic perennial, member of Theaceae Mirb. (Theales), that includes more than 3,000 named cultivars of great ornamental value. Bud dormancy is a key adaptive trait during the annual cycle of overwintering temperate woody plants and is also present in Camellia. There is sufficient evidence that abscisic acid (ABA) is responsible for the initiation and, in a number of species, also for the maintenance of endodormancy. Increase of endogenous ABA is generally observed at the onset of endodormancy, while loss of bud dormancy is linked to declining ABA levels. To deepen our understanding of the role of ABA in C. japonica during dormancy, both vegetative and flower buds were studied. In a first experiment, we assessed the seasonal dynamics of ABA in vegetative buds from the onset of dormancy to its release. In this case, the results described a complex type of response, with several peaks of ABA in the middle of dormancy, and a major peak followed by a drastic reduction right before dormancy release of the vegetative bud. The second experiment concerned the quantification of ABA in flower buds during a cold treatment (7°C) performed after dormancy onset. Here, the results indicated a clear downward trend of flower buds ABA levels in response to increasing periods at constant low temperatures. Overall morphological and physiological data confirmed the critical role of cold in the release of endodormancy in camellia. Future studies concerning the release of bud dormancy and correlated ABA levels in response to cold should focus on a wide range of C. japonica cultivars. This could allow selection of genotypes that bloom later in spring on the one hand (garden camellias) or genotypes that can be easily forced in flowering (flowering pot plants) on the other hand. This knowledge could also be exploited in breeding programs.