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 vascular plexus linking branch and main root vessels has been examined using cryoSEM, and the limiting porosity of the network determined with tracers (dye, and particles of known size: latex, polystyrene and gold sols). Dye and water move freely throughout the xylem network, while particles are constrained to follow tortuous vessel-like conduits of irregularly-shaped elements linked by large-diameter perforations. These conduits end at special pit membranes (boundary pit membranes) at the periphery of main root vessels. Particles accumulate on the outer side of these filters, often filling the terminal elements of these conduits adjacent to the main root vessels. Some vessel elements within the plexus are isolated from the convoluted conduits by normal pit membranes. and often also from each other, by pit membranes and still-intact end walls in otherwise mature elements. These extra-conduit elements may be an auxiliary filtering system. The boundary pit membranes filtered out particles with mean diameters as small as 4.9 +/- 0.7 nm, indicating a pore size one or two orders of magnitude smaller than most previous measurements for pit membranes, but close to pore sizes determined For hydrated primary cell walls. It is concluded that boundary pit membranes at branch-root junctions are efficient filters for microbes and particulates entering damaged branch roots. They would also restrict entry of air/water interfaces when main root xylem tension was less than approx. 100 MPa. (C) 2000 Annals of Botany Company.