Effect of Notching, 6-Benzyladenine, and 6-Benzyladenine plus Gibberellin A4 + A7 on Budbreak and Shoot Development from Paradormant Buds on the Leader of Young Apple Trees

Abstract

Notching is a technique to stimulate shoot growth from paradormant buds with the long-term objective of increasing the cropping potential of fruit trees by developing new fruiting scaffolds. Notching is not commonly practiced by apple (Malus ×domestica) growers in the southeastern United States because results can be inconsistent; notched buds frequently develop into weak spurs producing a few leaves rather than strong extension shoot growth. Therefore, a study was initiated to investigate the potential of 6-benzyladenine (6-BA) to enhance the shoot growth response from notched buds on 2- or 3-year-old wood. Notching paradormant buds on the leader of young ‘Granny Smith’/‘M.26 EMLA’ (‘M.26’) trees at the pink bud stage resulted in new growth developing on 59% of notched buds; however, fewer than 4% of these extended beyond 1.0 cm in length. Spray application of a solution of 1.5 g·L−1 6-BA into the notch increased budbreak to 95% and the proportion of new growth that developed into shoots to 89%, with a mean new shoot length of 7.9 cm. In a timing study on ‘Granny Smith’/‘M.26’, the optimum budbreak and shoot growth responses to notching plus 6-BA treatment were observed between budbreak and 2 weeks after budbreak. Notching paradormant buds on the leader of ‘Fuji’/‘M.9 T337’ (‘M.9’) trees increased budbreak, with 83% of the new growth developing into extension shoots. Spray application of 0.75 g·L−1 6-BA into the notch did not increase budbreak or extension growth of ‘Fuji’/‘M.9’ compared with notching alone. Spray application of a solution of 6-BA directly into a notch made above a paradormant bud may greatly enhance budbreak and shoot extension responses in cultivars or situations where a poor response to notching alone is expected.