Woody root and shoot xylem anatomy reveal patterns of whole-plant axial variation and suggest structure-function...

Plants must balance multiple functions across their large woody bodies, including water transport, biomechanical support, and storage. While vessel diameters within woody shoots have been intensively studied, woody roots compose a substantial portion of the plant body and remain comparatively understudied. Moreover, little is known about axial variation in xylem fibers and parenchyma. We excavated root systems of six Populus balsamifera L. spp. trichocarpa trees and examined xylem traits at 1-m increments from root tip to shoot tip. At each sampled location we measured several anatomical and functional traits. Anatomical traits included the percentage area of vessels, fibers, secondary wall area, and parenchyma; fiber diameter and wall thickness; vessel element diameter, wall thickness, and length; and inter-vessel pit area and density. Hydraulic functional traits included hydraulic conductivity and embolism resistance. Vessel diameter increased from organ apices toward the root-shoot junction in both roots and shoots, reaching maximum values in older root tissues. Model comparison strongly favored piecewise regressions over a single power-law relationship reflecting the inability of a single global scaling relationship to capture the pronounced structural transitions between root and shoot tissues. Axial trends for other traits differed between roots and shoots, resulting in unique combinations of vessel and non-vessel traits at each position along the tree axis. Across the plant axis, roots exhibited higher vessel and parenchyma fractions and lower fiber fractions than shoots, whereas shoots showed greater secondary wall investment. Structural and functional traits were highly correlated with one another, with roots generally showing higher hydraulic conductivity and greater embolism vulnerability relative to shoots. Together, these results show that whole-plant xylem structure is highly coordinated yet strongly divergent across different regions of the large woody plant body. Whole-plant xylem structure and function, especially that of woody roots, is essential to our understanding of whole-plant structure and function.