HOLBROOK, N.M.*; ZWIENIECKI, M.A.; MELCHER, P.J.: The dynamics of “dead” wood: Hydrogel-mediated changes in xylem hydraulic properties

Xylem vessels are well known for their passive role in water transport and the possibility that these tubes comprised of dead cells might possess the ability for rapid flow control has never been considered. The prevailing view among plant biologists is that xylem conduits have two hydraulic states; they either transport water with a constant resistance or are blocked due to embolism. The suggestion that plants can refill embolized vessels, even when there is tension in adjacent conduits, may contribute to the dynamic stability of water transport capacity in plants. We discuss our empirical and modeling efforts to understand this repair phenomenon, focusing on the apparent contradictionbetween bubble dissolution and xylem tension. We then discuss our recent finding that xylem hydraulic resistance is a function of xylem sap ion concentration. We provide data demonstrating that the effect of ions on flow resistance results from the action of a hydrogel located in the pit membranes. We hypothesize that the effect of ions on xylem hydraulic resistance represents an important component in the regulation of water flow through plants. By providing a mechanism by which plants can alter the relative flow resistance through portions of their xylem, ion-dependent hydraulic resistance may allow plants to control the relative flows of water in a manner appropriate for a highly branched system