Contrasting bloom dates in two apple cultivars linked to differential levels of phytohormones and heat requirements during ecodormancy

Abstract

Apple production in temperate regions is continuously imperiled by spring frosts. Economic losses caused by frost damage can be enormous, and the risk of such losses is projected to rise due to global climate change. Induction of bloom delay has been suggested as a potential frost avoidance strategy. However, applicable practices toward this goal are largely lacking, primarily due to the complexity of mechanisms governing dormancy and blooming in deciduous woody perennials like pome fruits. Cultivars differing in bloom time provide a suitable platform for investigating the regulatory mechanisms of these phenological processes. In this study, we investigated the hormonal regulation of bud dormancy and blooming in two apple cultivars, ‘Honeycrisp’ and ‘Cripps Pink’, which represent late- and early-blooming apple cultivars, respectively. Our results indicated that the two cultivars showed similar chilling requirements but differed significantly in the number of growing degree hours (GDH) required for bud burst and flowering. Abscisic acid (ABA) levels were remarkably elevated in the dormant buds of both cultivars during endodormancy, but they were significantly higher in ‘Honeycrisp’ buds, especially during the transition from endo- to eco-dormancy. Through investigating ABA biosynthetic and catabolic genes, we identified two biosynthetic genes, NCED1 and NCED2, and one catabolic gene CYP707A 2.3, whose differential transcript levels can explain the distinctive ABA patterns in the two apple cultivars. Our data also showed higher cytokinin (CK) levels in the buds of ‘Cripps Pink’ compared to ‘Honeycrisp’, shortly before the time of budburst. The results of this study indicate that differences in bloom times among apple cultivars can be explained, at least partially, by the differential heat requirements and ABA levels during eco-dormancy. Future investigations, especially at the omics levels would provide better understanding of molecular networks linking ABA hemostasis with bud dormancy and bloom regulation in apple.