The expression of acrotony in deciduous fruit trees: A study of the apple rootstock M.9
One-year-old shoots of apple (Malus xdomestica Borkh.) rootstock Mailing 9 (M.9 clone Nic 29), ≃60 cm long without sylleptic side shoots, were selected randomly from a commercial stoolbed in Belgium, prepared, and forced at 25 °C and 16 hours daylength to follow bud developmental rate in different positions along a 1-year-old shoot. The number of buds that reached green tip was recorded daily until 50% of shoots exhibited budburst. The evolution of dormancy of the terminal, an upper lateral, and a lower lateral bud was followed throughout the dormant period. The influence of a distal disbudded shoot piece and Promalin [3000 mg·L-1 N-(phenylmethyl)-1H-purine 6-amine and 3000 mg·L-1 gibberellins A4+A7 without a wetting agent] application on the developmental rate of these buds was evaluated. Bud developmental rate decreased during winter and increased to a maximum before budburst in spring. The distal shoot-forming ability or acrotonic branching habit in apple appears to be mediated via a greatly increased developmental rate of the terminal bud relative to the upper and lower lateral buds, respectively. The proximal budbursting tendency or basitony exhibited in early winter was weak compared to the acrotony that developed in the last month before budburst in spring. Lateral buds showed a lower developmental rate than the terminals when inhibited by a distal disbudded shoot piece, so that resultant overall growth habit within the shoot remained acrotonic throughout the dormant period. Promalin application in winter did not restore the bud developmental rate associated with spring budburst. These results are discussed with reference to observations of trees growing under conditions of insufficient winter chilling.