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Jacques J. Crabbe

The flushing behavior of shoot growth and its consequences on shoot differentiation are important features in fruit tree development, with regard to flowering ability. In this respect, two different approaches were applied to young `M26' apple trees. First, poorly branched 2-year-old trees were headed back, either in the second-year or in the first-year wood, at different times from right before to 6 weeks after budbreak. Early pruning resulted in rapid and prolonged regrowth, with a final very similar shaping of the tree to that of the intact controls. Late pruning, in contrast, leads to a two-step reaction (late spring and summer flushes), sometimes stronger on 2-year-old than on 1-year-old wood. In a second experiment, buds and young shoots were sampled on pruned trees in locations where they could be supposed to remain short shoots or grow long, with one or two flushes. They were weighed, their leaves and internodes measured, and the plastochron evaluated. During budbreak and the first month afterwards, shoot differentiation appears achieved. The primary difference between long and short shoot types does not consist in faster internode elongation but, rather, in faster production (reduced plastochron) of larger leaves.

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Viviane Planchon and Jacques J. Crabbe

During 5 years from grafting, all annual shoots or growth units appearing on unpruned apple trees were recorded, and their main characteristics (length, node number, location of flower buds) introduced in a database through a code permitting quick and unequivocal identification. From this comprehensive description, the most and highly probable structure of the tree was deduced from year to year. Both cultivars display very similar gross structures, despite minor differences in shape, flowering mode, and productivity. The development occurs in two phases. In the first one, a frame composed of trunk and several storied sets of branches is built. Flowering is rather strictly biennial along each branch system. The second phase is one of reinforcement of the branches by axillary growth but even more by sympodial branching due to increasing and repeated terminal flowering (successive “bourse” shoot formation). `Cox's Orange Pippin' mainly differs from Jonagold by earlier transition to the second phase and regular production of more “bourse” shoots (two to three instead of one).