Architecture of Branch-root Junctions in Maize: Structure of the Connecting Xylem and the Porosity of Pit Membranes

The architecture of the connecting xylem network in the vascularplexus linking branch and main root vessels has been examinedusing cryoSEM, and the limiting porosity of the network determinedwith tracers (dye, and particles of known size: latex, polystyreneand gold sols). Dye and water move freely throughout the xylemnetwork, while particles are constrained to follow tortuousvessel-like conduits of irregularly-shaped elements linked bylarge-diameter perforations. These conduits end at special pitmembranes (boundary pit membranes) at the periphery of mainroot vessels. Particles accumulate on the outer side of thesefilters, often filling the terminal elements of these conduitsadjacent to the main root vessels. Some vessel elements withinthe plexus are isolated from the convoluted conduits by normalpit membranes, and often also from each other, by pit membranesand still-intact end walls in otherwise mature elements. Theseextra-conduit elements may be an auxiliary filtering system.The boundary pit membranes filtered out particles with meandiameters as small as 4.9 ± 0.7 nm, indicating a poresize one or two orders of magnitude smaller than most previousmeasurements for pit membranes, but close to pore sizes determinedfor hydrated primary cell walls. It is concluded that boundarypit membranes at branch-root junctions are efficient filtersfor microbes and particulates entering damaged branch roots.They would also restrict entry of air/water interfaces whenmain root xylem tension was less than approx. 100 MPa. Copyright2000 Annals of Botany Company Zea mays, air-seeding, branch-root junction, cryoSEM, embolisms, maize, pit membrane, pore size, xylem, water transport