Apoplastic and Symplastic Pathways in Avicennia marina (Forsk.) Vierh. Roots Revealed by Fluorescent Tracer Dyes

The apoplastic permeability of Avicennia marina (Forsk.) Vierh. (grey mangrove) roots was investigated by examining the composition of xylem sap collected under pressure from root systems immersed in 25% seawater containing 0.01% trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate (PTS), a mobile fluorescent dye which is confined to ihe apoplast. Xylem sap from intact root systems contained between 3 and 18% of the NaCl, and less than 0.2% of the PTS concentrations present in the external bathing medium. The low levels of PTS in the xylem sap indicate that apoplastic transport was minimal. By contrast, xylem sap from disturbed roots that had been removed from the sand in which they were growing and transferred to the PTS treatment solution contained an average of 57% of the NaCl and 12% of the PTS concentrations present in the external solution, demonstrating the importance of using undisturbed root systems for such studies. Using Cellufluor, an apoplastic fluorescent dye which binds to cellulose, an apoplastic barrier was located at the periderm of mature roots, and at the exodermis and endodermis of progressively younger regions of fine roots. Uranin, a symplastic tracer dye, was taken up only by cells of the distal 17 mm of fine roots, the region of the root where the periderm had not yet developed. These results suggest that the uptake of ions and water in intact roots of A. marina occurs mainly via the symplast of younger regions of the third-order and fourth-order roots.