Three-dimensional xylem networks and phyllode properties of co-occurring Acacia.

Reduced leaf size is often correlated to increased aridity, where smaller leaves demand less water via xylem conduits. However, it is unknown if differences in three-dimensional (3D) xylem connectivity reflect leaf-level adaptations. We used X-ray microtomography (micro-CT) to quantify 3D xylem connectivity in ∼5 mm diameter branch sections of co-occurring semi-arid Acacia species of varied phyllode size. We compared 3D connectivity to minimum branch water potential and two-dimensional (2D) vessel attributes derived from sections produced by micro-CT. 2D attributes included vessel area, density, vessel size to number ratio (S) and vessel lumen fraction (F). Trees with terete phyllodes had less negative water potentials than broad phyllode variants. 3D xylem connectivity was conserved across all trees regardless of phyllode type or minimum water potential. We also found that xylem connectivity was sensitive to vessel lumen fraction (F) and not the size to number ratio (S) even though F was consistent among species and phyllode variants. Our results demonstrate that differences in phyllode anatomy, and not xylem connectivity, likely explain diversity of drought tolerance among closely related Acacia species. Further analysis using our approach across a broader range of species will improve understanding of adaptations in the xylem networks of arid zone species.