Scaling relationships and vessel packing in petioles

While tradeoffs among mechanical and conductive functions have been well investigated in woody stems, these tradeoffs are relatively unexplored in petioles, the structural link between stems and laminas. We investigated size-independent scaling relationships between cross-sectional areas of structural and vascular tissues, relationships between tissue areas of xylem and phloem, vessel packing within xylem, and scaling of vascular and structural tissues with lamina traits.

We examined allometric relationships among petiole tissues and as a function of lamina and petiole size variation on eleven species of Pelargonium. From transverse sections of methacrylate-embedded tissue, we measured the cross-sectional areas of all tissues within the petiole and vessel lumen, and cell wall areas of each vessel. Allometric scaling relationships were analyzed using standardized major axis regressions.

Pelargonium petiole vessels were packed as predicted by Sperry's packing rule for woody stems. In contrast to woody stems, there was no evidence of a tradeoff between vessel area and fiber area. Within cross-sections, more xylem was produced than phloem. Among bundles, xylem and phloem scaling relationships varied with bundle position. Except for lamina dry mass and petiole fiber cross-sectional area, petiole and lamina traits were independent.

Petioles share vascular tissue traits with stems despite derivation from leaf primordia. We did not find evidence for a tradeoff between structural and vascular tissues, in part because fibers occur outside the xylem. We propose this separation of conduction and support underlies observed developmental and evolutionary plasticity in petioles.