During the growing seasons 1994-1995 and 1995-1996, weekly measures of length and number of internodes and morphological observations were made on annual shoots of 11- to 21-year-old Nothofagus dombeyi (Mirb.) Blume trees growing in northern Patagonia, Argentina. Growth period and shoot size at the end of the extension period were highly variable. Four main types of annual shoots could be identified according to the death or persistence of the apical meristem and the development or not of axillary branches during the growth season of main stem extension. For all shoots the maximum growth rate took place in late November and was followed by a slower growth rate or cessation of growth in early December to early January. For those shoots with a long growth period, a second growth peak took place in late January-February, and only those shoots with a growth rate higher than 35-40 mm/week developed axillary branches as they elongated. Results from bud dissections suggest that the early growth rate peak corresponded to the development of those structures preformed at the time of bud break. The death of the apical meristem of a shoot affected both the position and the relative size of axillary branching on that shoot.Key words: Nothofagus, annual shoot, growth dynamics, branching pattern.
Buds in different positions along trunk, main branch, secondary branch, and short branch parent shoots of young Nothofagus dombeyi (Mirb.) Blume trees were dissected, and the number of organs of their rudimentary shoots was counted. Bud contents were compared with the number of organs of sibling shoots developed in positions equivalent to those of the dissected buds. Cataphyll number was relatively constant for all buds. The number of green leaf primordia differentiated in each bud depended both on the position of the bud on the parent shoot and on the size of the parent shoot. Sibling shoots derived distally from large parent shoots had more nodes than the rudimentary shoots of buds in a similar position. Proximal sibling shoots derived from large parent shoots and all sibling shoots derived from small parent shoots are entirely preformed. In N. dombeyi, the size gradient of the sibling shoots derived from a particular parent shoot relates mostly to variation in organ preformation, organ neoformation, and internode extension. The expression of each of these sources of variation is related to the position of the sibling shoot on the parent shoot and on the position of the parent shoot on the tree. Consideration is given to the role of environmental conditions on tree development, in view of the species' morphogenetic gradientsKey words: branching, bud content, sylleptic branching, leaf primordia, Nothofagus, preformation.
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