Genetic Variation of Stomatal Traits and Carbon Isotope Discrimination in Two Hybrid Poplar Families (Populus deltoides 'S9-2' x P. nigra 'Ghoy' and P. deltoides 'S9-2' x P. trichocarpa 'V24')

BACKGROUND AND AIMS: Stomata play an important role in both the CO₂ assimilation and water relations of trees. Therefore, stomatal traits have been suggested as criteria for selection of clones or genotypes which are more productive and have larger water-use efficiency (WUE) than others. However, the relationships between plant growth, WUE and stomatal traits are still unclear depending on plant material (genus, species, families, genotypes) and, more precisely, on the strength of the relationships between the plants. In this study, the correlations between these three traits categories, i.e. plant growth, WUE and stomatal traits, were compared in two related poplar families. METHODS: Stomatal traits (stomatal density, length and ratio adaxial : abaxial stomatal densities) of a selection of F₁ genotypes and the parents of two hybrid poplar families Populus deltoides 'S9-2' x P. nigra 'Ghoy' (D x N family, 50 F₁) and P. deltoides 'S9-2' x P. trichocarpa 'V24' (D x T family, 50 F₁) were measured, together with stem height and circumference. Carbon isotope discrimination (Δ) was determined and used as an indicator of leaf-level intrinsic WUE. KEY RESULTS: Leaves of hybrids and parents were amphistomatous, except for the P. trichocarpa parent. Both families displayed high values of heritability for stomatal traits and Δ. In the progeny, the relationship between stem circumference and Δ was weak for the D x N family, while abaxial and total stomatal density were positively associated with stem dimensions for the D x T family only. CONCLUSIONS: Genetic variation in stomatal traits and Δ was large within as well as between the different poplar species and their hybrids, but there were no direct relationships between stomatal traits and plant growth or Δ. As already noticed in various poplar hybrids, the absence of, or the weak, relationship between Δ and plant growth allows the possibility of selecting poplar genotypes combining high productivity and high WUE. In this study, stomatal traits are of limited value as criteria for selection of genotypes with good growth and large WUE.