We present results and insights gained from the characterization of the hydraulic architecture of eight field-grown Populus deltoides × Populus nigra genotypes, already known to differ in terms of growth performance, water-use and xylem anatomy (Marron et al. 2005, Monclus et al. 2005, 2006; Fichot et al. 2009). Under well-watered conditions, we provide evidence that (1) there are wide genotypic variations for hydraulic efficiency inferred from the leaf specific hydraulic conductance (kSL) and for resistance to drought-induced cavitation inferred from the water potential inducing 50% loss in hydraulic conductance, (2) a strong trade-off between water transport efficiency and xylem safety occurs at the whole plant level, indicating that genotypes with lower kSL apparently compensate the greater risks of embolism by building a safer xylem, (3) higher relative growth rate is tightly associated with lower whole-plant hydraulic efficiency and higher resistance to cavitation, (4) estimates of leaf water-use efficiency (intrinsic water use efficiency and carbon isotope discrimination against 13C) are weakly associated with whole-plant hydraulics. Under limited water supply, the characterization of xylem resistance to cavitation provides additional evidence that (5) xylem safety acclimates to moderate drought in a genotype-dependent manner, (6) there is no clear relationship between xylem safety and either xylem water transport efficiency or xylem biomechanics, contrary to what is observed at inter-specific scales. Our results provide interesting insights for the possible role of whole plant hydraulic architecture and xylem hydraulics in mediating key aspects of whole-plant physiology in poplar, such as growth and water-use efficiency. Finally, our results also indicate that there may be great opportunity for intra-specific comparisons to unravel specific issues that cannot be fully addressed by interspecific comparisons.
