Phytohormones and shoot growth in a three-generation hybrid poplar family

Hybrid vigor for secondary growth in poplar has been linked with increased gibberellin (GA) concentration in cambial tissue, but the relationship between concentrations of GAs and hybrid vigor of primary growth has not been investigated. We explored associations between concentrations of GAs, abscisic acid (ABA) and indoleacetic acid (IAA) and shoot extension in a hybrid family of Populus deltoides Bartr. ex Marsh. (Clone ILL-129) and P. trichocarpa Torr. & Gray (Clone 93-968) and two F₁ and 67 F₂ genotypes. Rapidly elongating subapical internodes from shoots of 4-year-old clonal saplings were selected for hormone analysis. The F₁ hybrids displayed heterosis (hybrid vigor) for primary growth as a result of the complementation of dominance for increased internode length from the P. trichocarpa parent and dominance for increased diameter from the P. deltoides parent. Internodes from the faster-elongating shoots of the P. trichocarpa parent had a fourfold higher concentration of bioactive GA₁ and higher concentrations of GA₂₀, GA₄₄, GA₂₉ and GA₈ than the P. deltoides parent. However, the two fast-growing F₁ hybrids had low concentrations of all five GAs, with concentrations similar to those of the slower-elongating P. deltoides parent. Concentrations of ABA and IAA were correlated with GA concentrations and there was thus no evidence of a consistent promoting or inhibiting effect of ABA or IAA within the F₁ family. These results indicate that heterosis for internode growth was not primarily regulated by hormone concentration. The segregating population of F₂ hybrids was analyzed to assess the possible utility of hormone screening as a selection tool. The internodes of most of the F₂ hybrids were smaller than those of their F ₁ parents and the larger P. trichocarpa parent. Among the F ₂ hybrids, mature internode length varied fourfold, and was correlated with lengths of young, subapical internodes from which hormones were analyzed. In these internodes, GA₁ concentrations were negatively correlated with length (r = 0.41), diameter (0.33) and mass (0.50). Indoleacetic acid concentration was negatively correlated only with diameter (r = 0.37) and positively correlated with GA₁ (r = 0.46), whereas ABA concentration was not correlated with any parameter. Thus, in the F₂ population, variation in hormone concentration was not primarily responsible for the variation in shoot growth, indicating that selection for individuals with high GA concentration may not be an effective strategy for identifying vigorous hybrid genotypes.