Sodium Transport and Compartmentation in Spergularia marina: Partial Characterization of a Functional Symplasm

In this paper, a combination of tracer uptake, efflux, and pulse-chase techniques is applied to the problem of compartmentation of Na(+) ((24)Na(+)) in the roots of intact, midvegetative Spergularia marina (L.) Griseb. plants. An approach is presented for conducting useful compartmental analysis when it is known that the assumptions required for straightforward interpretations of influx and efflux studies are invalid. Linear rates of (24)Na(+) accumulation in both roots and shoots were attained within at most a few minutes following the start of labeling. Shoot (24)Na(+) contents equaled root contents within about 20 minutes. Analysis of root accumulation rates, and compartmental and pulse-chase efflux studies indicated that the unidirectional flux rates involved were at least an order of magnitude greater than linear rates of root and shoot accumulation. These rapid fluxes involved only a small portion of the total root Na(+) (about 1%). The results suggest the existence of a small symplastic compartment, distinct from the ;bulk cytoplasm,' rapidly exchanging with the medium, and responsible for delivery of Na(+) to the xylem. The physical identity of this compartment and its physiological significance are discussed with respect to precedents in the literature.