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Seasonal patterns of photosynthates export were followed on 1- and 2-year-old trees of four Populus clones, belonging to the species Populus trichocarpa Torr. & Gray, Populus deltoides Bartr., and their interspecific hybrids grown in the field under short rotation. Differenttypes of branches (including sylleptic and proleptic) and cohorts ofleaves on the main stem were exposed to 14CO2 at monthly and bimonthly intervals in the first and second growing season. Patterns of photosynthates export were influenced by phenology and differed markedly among clones. Differences of translocation patterns were also found among various portions of the crown. Sylleptic branches exported carbon mainly to the lower stem and the roots, whereas main stem leaves, before bud set, contributed much to the height growth of the tree. Sylleptic branches also had greater translocation efficiency than proleptic, contributing more, on a per unit mass basis, to the growth of the tree. Within branches of the same order and among branches of different order, export of assimilates followed patterns similar to those found in the main stem. There was little exportof assimilates between adjacent branches of the same order or between branches and main stem leaves. Clones 11-11 and 1-12, which had thehighest number of sylleptic branches, were also the most productive clones, respectively, for the hybrids and the parental species. [ABSTRACT FROM AUTHOR]