High-chilling requirement apple cultivar has more accentuated acrotony than low-chilling one in mild winter region
Résumé
Key message: This study contributed to the understanding of acrotony in apple cultivars in a mild winter region, contrasting high-and low-chilling requirement apple cultivars, and the role of this behavior in plant adaptability.
Abstract: When there is insufficient chilling accumulation during the cold season, apple trees present erratic and poor budbreak. However, there is a lack of information regarding carbohydrate metabolism and their influence on budbreak and branch architecture in apples grown in subtropical humid climates. This work aimed to evaluate budbreak and the availability of carbohydrates in distal and proximal portions of 1-year-old apple tree shoots, and to verify their relationships with axillary shoot formation in spring. Two apple cultivars 'Eva' and 'Fuji', with low and high chilling requirements, respectively, were assessed in a mild winter region of Brazil. From April to October of 2012, 2013 and 2014, 25 one-year-old shoots of each cultivar were sampled biweekly. These shoots were divided into distal and proximal portions. Wood from five shoots was sampled to quantify the nonstructural carbohydrate and alpha-amylase content for each portion. The single-node cutting test was used to observe mean time to budburst for the remaining 20 shoots. In generally, our results suggest that under mild winter conditions, the apple tree does not enter into deep endodormancy. Furthermore, there were biological and biochemical differences between the distal and proximal portions of the bearing-shoots for all the studied variables, likely related to the higher sink force in the distal portion. Consequently, the buds from the distal portion sprout first, thereby establishing a primigenic dominance over the buds of the proximal portion. Our study also suggests that the shallow endodormancy of high chilling requirement cultivars cultivated in mild winter regions would make paradormancy predominant, emphasizing acrotonic branching in these cultivars. There was greater availability of nonstructural carbohydrates and alpha-amylase activity in the wood of the distal portion of 1-year-bearing shoots of apple trees under mild winter conditions, regardless of the cultivar.