Genotypic variation and water deficit effect on morphological traits related to productivity in six Populus deltoides × P. nigra hybrids growing in glasshouse in well-watered (white bar) or in water deficit (black bar) conditions. Studied traits encompassed: (A) stem biomass, (B) root biomass, (C) height of plants, (D) total leaf area, (E) leaf number and (F) specific leaf area. For each graph, G indicates the genotype effect, T the treatment effect and (GxT) genotype by treatment effect. Means are accompanied by their standard errors SE (n = 6). Significant differences between well-watered and water deficit conditions are indicated by asterisk: * P ≤ 0.05, ** P ≤ 0.01 and *** P ≤ 0.001.