Hydraulic function of medullary bundles in a tropical root climber.

PREMISE: Medullary bundles, which are vascular bundles in the pith, have evolved multiple times. However, their functional significance remains poorly understood. Here, we investigated how medullary bundles are involved in the vascular architecture and hydraulic function in a tropical root climber, Adelobotrys scandens (Melastomataceae) to understand the functional significance of this vascular architecture in a wet-adapted root climber species as compared to what is known about medullary bundles in the lineages in hydraulically limited arid/saline environments.

METHODS: We used light microscopy to examine the organization of the medullary bundles in the stem. We measured the hydraulic conductivity of intact stems and compared it with those of stems that were experimentally reduced to pith tissue. These analyses allowed estimation of hydraulic conductivity through the pith tissue system.

RESULTS: Adelobotrys adscendens has a polycyclic eustele, with procambium-derived medullary bundles of two types: peripheric bundles forming leaf traces and cauline bundles forming sympodia at nodal regions of stems. Although the pith has low intrinsic hydraulic conductivity, pith tissues contributed up to 80% of total stem hydraulic conductivity near the apex, before secondary xylem becomes the dominant hydraulic pathway.

CONCLUSIONS: Our experimental study enhances the understanding of atypical vasculature in woody plants. We demonstrated that medullary bundles function in plant water balance, particularly in water transport processes in leaf traces and young shoots, where the pith dominates stem cross-sectional investment. We discuss how this type of vasculature is involved in the evolution of life forms that develop aerial adventitious roots.