| Summary: | The widespread species Populus balsamifera L. exhibits large intra-specific variation in photosynthetic rates and phenology. Northern populations have a tendency towards higher photosynthetic carbon assimilation rates (An) than trees from the south. However, because bud set occurs earlier in northern trees they accomplish far less height growth than do southern trees. Assuming that there are no physiological constraints to combining high An and long growing season, the progeny of intra-specific crosses between northern and southern populations may accomplish more growth in one growing season than their parents (i.e., heterosis). High performing F₁s could be used for a variety of agroforestry projects. Full reciprocal crosses were conducted between individuals from two northern (N) and two southern (S) populations found at the extremes of the P. balsamifera range. Representative selections of progeny and parental material were planted in a greenhouse and in the field and characters including shoot elongation rate, photosynthetic rates, stomatal conductance (gs), water-use efficiency (WUE) leaf mass per area (LMA), internal conductance (gm) in a greenhouse and phenology in the field were evaluated. Although the F₁ families did not display any evidence of heterosis, photosynthetic rates and phenology were uncorrelated in the NxS families, suggesting an uncoupling of traits. Additionally, a number of individuals which possessed a combination of high growth potential and late growth cessation (bud set) were observed and could be useful for a variety of potential deployment areas. The high An in northern populations has been partially attributed to gm, and appears to be a consequence of leaves with greater LMA, which have an increased mesophyll surface area available for carbon uptake. WUE was also correlated with LMA, suggesting that in facilitating CO₂ diffusion for carbon assimilation an increase in gm over an increase in gs provides a clear advantage in not promoting further water loss. === Forestry, Faculty of === Graduate
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