THE EXPRESSION OF ACROTONY IN DECIDUOUS FRUIT-TREES - A STUDY OF THE APPLE ROOTSTOCK M9

One-year-old shoots of apple (Malus xdomestica Borkh,) rootstock Malli ng 9 (M.9 clone Nic 29), approximate to 60 cm long without sylleptic s ide shoots, were selected randomly from a commercial stoolbed in Belgi um, prepared, and forced at 25 degrees C and 16 hours daylength to fol low bud developmental rate in different positions along a 1-year-old s hoot, The number of buds that reached green tip was recorded daily unt il 50% of shoots exhibited budburst., The evolution of dormancy of the terminal, an upper lateral, and a lon er lateral bud was followed thr oughout 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 A(4)+A(7) without a wetting agent] applicati on on the developmental rate of these buds was evaluated, Bud developm ental rate decreased during winter and increased to a maximum before b udburst in spring. The distal shoot-forming ability or acrotonic branc hing habit in apple appears to be mediated via a greatly increased dev elopmental rate of the terminal bud relative to the upper and lower la teral buds, respectively. The proximal budbursting tendency or basiton y exhibited in early winter was weak compared to the acrotony that dev eloped in the last month before budburst in spring, Lateral buds showe d a lower developmental rate than the terminals when inhibited by a di stal disbudded shoot piece, so that resultant overall growth habit wit hin the shoot remained acrotonic throughout tile dormant period, Proma lin application in winter did not restore the bud developmental rate a ssociated with spring budburst, These results are discussed with refer ence to observations of trees growing under conditions of insufficient winter chilling.