Thalassodendron ciliatum (Forssk.) Den Hartog: root structure and histochemistry in relation to apoplastic transport

Abstract

Root structure, histochemistry and apoplastic permeability were investigated in the seagrass Thalassodendron ciliatum (Forssk.) Den Hartog. The free space tracers Cellufluor and lanthanum nitrate were used to study apoplastic transport. Tracer penetration occurred only in a few actively growing primary roots near the rhizome tip and was restricted mainly to the anterior region of the root hair zone. The extent of penetration of tracers into tissues of these roots was governed by the stage of differentiation of the cells and the chemical composition of cell walls. Close to the root tip where walls were composed mainly of cellulose and were relatively unmodified, an apoplastic pathway existed from the external medium to the free space of vascular tissues. The presence of tracers in both the walls and lumens of xylem elements suggested that they were transported by these elements. With increase in distance from the root apex, cell walls of the epidermis, outer cortex and endodermis become modified by deposition of suberin and lignin. These wall modifications prevented apoplastic transport. Older roots which comprised more than 95% of the root system, possessed strongly lignified and suberized cell walls and were impermeable to the tracers. On the basis of the results obtained, it would seem that the main function of the roots is that of anchorage and support, with limited absorption and transport occurring via the apoplast.