Intra-organismal variation in the structure of plant vascular transport tissues in poplar trees

Abstract

Phloem and xylem conduit structure vary greatly throughout the body of Populus trichocarpa trees, particularly between roots and shoots. This has implications for understanding organ and whole plant vascular function. Woody plant vascular transport occurs predominantly within secondary xylem and phloem, which are both produced by the vascular cambium during secondary growth. We examined how vessel and sieve tube structure varied throughout the plant body of P. trichocarpa trees and whether xylem and phloem conduit structure was correlated across different positions within the plant. We excavated entire juvenile P. trichocarpa trees and measured vessel and sieve tube structural traits of current-year growth in 1 m increments along the main root:shoot axis. Trees were > 4 m tall and had roots that extended 4–5 m at their longest length. We found that both sieve tube and vessel diameters greatly varied throughout the plant body and with organ diameter. Roots had wider diameter conduits than shoots. Sieve tube diameter was strongly correlated with vessel diameter, which may be related to their common developmental origin. Other structural traits, such as pit membrane area and pit density for xylem, and sieve plate area and number of sieve areas per plate for phloem, also varied and were correlated with changes in conduit diameter. The median air-seeding pressure of vessels (Pm) and vessel length did not differ between roots and shoots. Understanding plant vascular function will likely require increased knowledge of whole plant structure and function, since plant performance may be limited by any point along the transport pathway. Considering intra-organismal variation may be a way to evaluate structure–function hypotheses while controlling for confounding sources of variation that may impact inter-specific comparisons.