Within woody angiosperms, vascular transport occurs predominantly within secondary xylem and phloem tissues, which are both produced by the vascular cambium during secondary growth. The vast majority of studies on conduit structure and function have been conducted on relatively small diameter organs (usually around 4-8 mm in diameter) and relatively little is known about how structure changes within the rest of the body of a plant. We were interested in examining how both sieve tube elements and vessels varied in structure from the main root tip to the main shoot tip of Populus trichocarpa trees.
We hypothesized that position within the tree would affect conduit structure. In xylem, we measured vessel diameter, pit membrane density, and the size of individual pit membranes. In phloem, we measured sieve tube diameters, sieve plate areas, and counted the number of sieve areas within each compound sieve plate. We excavated entire juvenile P. trichocarpa trees and measured vessel and sieve tube structural traits in 1 m increments spanning from the root tip to the shoot tip. Trees were >4 m tall and had roots that extended 4-5 m at their longest length. Only current year’s growth was measured at each sampled location.
We found that both sieve tube and vessel diameters greatly varied throughout the plant body, with roots consistently having larger diameter transport conduits than shoots (P < 0.001). Within xylem, pit membrane area and pit density were both strongly correlated with vessel diameter (r2 = 0.81 and 0.68, respectively; P < 0.001 for both). Within phloem, sieve plate area and number of sieve areas per sieve plate were correlated with sieve tube diameter (r2 = 0.39 and 0.73, respectively; P < 0.001 for both). Our results suggest that different organs within a plant are functionally different and that plant structure varies with position within the tree. Understanding plant vascular function will likely require increased knowledge of whole plant structure, since plant performance may be limited by any point along the transport pathway.