| High biomass yields are essential for poplar cultivation. However, growth performance of trees is a complex function of various structural and process related components. The main objective of this Ph.D. thesis was to gain a fundamental insight in the effects of the genetic background and environment on poplar growth and its components, as well as their mutual interactions. The acquired knowledge was linked to the poplar genome by means of QTL (quantitative trait loci) analysis.Two interspecific poplar families sharing the same female parent ( Populus deltoides 'S9-2' x P. nigra 'Ghoy' (DxN, 180 F1 offspring) and P. deltoides 'S9-2' x P. trichocarpa 'V24' (DxT, 182 F1 offspring)) were investigated during four growing seasons (2003-2006) at two contrasting sites in Europe, i.e. northern Italy and central France. Both hybrid familles displayed higher biomass yields and growth capacity than either of their parents, but hybrid vigour or heterosis was more pronounced for the DxT family than for the DxN family. The interamerican (DxT) family was the most productive one irrespective of site, and showed more sylleptic branches, larger individual leaf area, and a longer growing season than the DxN family. The fertile Po valley of northern Italy (sufficient water availability, rich loam soil and sunny climate) was highly suitable for the cultivation of the studied poplar families. Significant genotype x site interactions were found for growth and biomass traits, although the ranking of the genotypes was quite stable between the two sites.Within both families, especially within the DxN family, there was a large genetic variation in growth vigour and growth components which can be exploited in selection and breeding programmes. Among the studied growth components, the number of sylleptic branches was the most robust growth predictor. Individual leaf area and leaf number increment were also strongly associated with growth performance, depending on the growth strategy of the family. Various QTL for growth and biomass traits were identified on the parental genetic maps. Via this approach many ecophysiological traits can be put on the genetic map of poplar in the near future. The functional validation of genes can potentially result in the genetic modification of poplars and other species. |
