catherine Bastien

A Populus deltoides × Populus trichocarpa F1 pedigree was analyzed for quantitative trait loci (QTLs) affecting ectomycorrhizal development and for microarray characterization of gene networks involved in this symbiosis. A 300 genotype progeny set was evaluated for its ability to form ectomycorrhiza with the basidiomycete Laccaria bicolor. The percentage of mycorrhizal root tips was determined on the root systems of all 300

The objective of this study was to evaluate the environmental, temporal and genetic stability of the relationships between growth and a selection of tree architectural, leaf and phenological traits (selection based on the conclusions of previous studies carried out on the same experimental trial). Therefore, the growth of two hybrid families, Populus deltoides 'S9-2' x Populus nigra 'Ghoy' (D x N family, 180 F(1)) and P. deltoides 'S9-2' x Populus trichocarpa 'V24' (D x T family, 182 F(1)), was investigated during a 3-year period at two sites, i.e., in northern Italy and central France. At the end of the second growing season, all trees were coppiced and the resprouts were thinned to a single stem. At the end of each growing season, stem circumference and height were measured for all F(1) hybrids. The number of sylleptic branches, individual leaf area (LA) and petiole length of the largest leaf along the main stem, production of new leaves, bud flush and bud set were estimated for a selection of genotypes (31 F(1)) per family at each site during the course of the 3-year experiment. The D x T family was clearly the most productive family and displayed the highest heterosis values. However, there appeared to be a compromise between good growth at a given site and stability between the two different sites, both at family and at genotype levels. Particularly, the less performing trees were stable between Italy and France. Among the studied growth components, the number of sylleptic branches and individual LA of the largest leaf along the main stem were the best growth predictors, irrespective of site and family. Growth strategies in terms of leaf development differed between the two families. Hence, leaf production rate was strongly associated with growth of the D x N family only. These results have important consequences for the use of the studied traits as selection criteria in breeding programmes.