Breeding Productive Tree Genotypes: The Role of Hydraulic Resistance Along the Root-Stem-Leaf Continuum in Constraining Growth.

Breeding productive tree genotypes is crucial for sustainable forestry, yet the hydraulic architecture along root-stem-leaf continuum that constrains biomass yield remains unclear. Here, six poplar hybrid genotypes with contrasting yield were used to quantify whole-plant hydraulic resistance, its partitioning patterns, and anatomical traits along the continuum. We observed substantial genetic variations in hydraulic resistance parameters. Roots contributed the largest proportion of whole-plant hydraulic resistance (> 54%). Components along the continuum were well-coordinated, and hydraulic resistance of all components was strongly correlated with yield (R² > 0.75), suggesting that hydraulic resistance is a strong predictor of yield. However, resistance partitioning patterns generally showed weak correlations with yield, with more productive genotypes partitioning a smaller proportion of resistance to leaves. Vessel diameter was a key determinant of hydraulic resistance at the root and leaf levels (R² ≥ 0.75), and vessel length significantly influenced stem hydraulic resistance (R² = 0.80). Additionally, genotypes with higher minor vein density and a lower ratio of palisade to spongy mesophyll thickness exhibited lower leaf resistance. Our results suggest that low hydraulic resistance throughout root-stem-leaf continuum is the functional basis for high yield, and the identification of key hydraulic and structural constraints will help overcome bottlenecks in breeding productive tree genotypes.