Is the ranking of poplar genotypes for leaf carbon isotope discrimination stable across sites and years in two different full-sib families?

- Introduction Because of its vigorous growth, poplar can play an important role for sustainable production of woody biomass to cover renewable energy needs. Hence, the selection of suitable genotypes has to be based on relevant traits, among which intrinsic water use efficiency (Wi, estimated through leaf carbon isotope discrimination, Δ) may be a key trait. Besides a large genetic variation in Δ among the frequently planted poplar hybrids, the use of Δ in deployment or breeding programmes requires insights in the robustness of the genotype ranking for Δ across environments and years. - Methods Two F1 full-sib families of poplar (Populus deltoides × Populus nigra and Populus deltoides × Populus trichocarpa) were grown at two sites in Europe, i.e. northern Italy and central France. For each family, leaf samples from 31 F1 genotypes collected during different field studies were used (1) to assess the effect of genotype, site and year on Δ in leaves, as well as their mutual interactions, and (2) to elucidate the relationships between Δ, leaf morphology and tree dimensions. - Results Under the well-watered conditions of our study, a low to moderate genetic variability was observed in the two poplar families. Within-family broad-sense heritability values ranged from 0 to 0.49. The ranking of genotypes for Δ was more stable between years than between sites. - Conclusions The study confirmed the occurrence of some degree of genetic variability ofΔ in the studied poplar families and the possibility to identify genotypes with low, stable Δ values across years. However, the significant genotype-by-site interactions in our study suggest that selection for larger water use efficiency or lower Δ in these families has to consider specific responses in different environments