Sodium Transport and Compartmentation in Spergularia marina

Abstract

ABSTRACrIn this paper, a combination of tracer uptake, efflux, and pulse-chase techniques is applied to the problem of compartmentation of Na ' (4Nal) 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 staightforward interpretations of influx and efflux studies are invalid. Linear rates of 2'Na' accumulation in both roots and shoots were attained within at most a few minutes following the start of labeling. Shoot 2'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.The ultimate significance of the study of root level ion transport and compartmentation in higher plants lies in the understanding of the dual role of roots as organs of ion absorption, with physiological needs of their own, and as organs responsible for nutrient resource partitioning within the whole plant. Analysis of the transport processes has been facilitated by the use of isotope techniques which allow increased resolution ofion movements with respect to both time and a large background of nontransported ions.The There has, to our knowledge, never been a consideration of the requirements for interpretable use of isotopes in transport studies using whole plants under conditions in which one could be assured that unidirectional influx was not being measured, and in which the arrangement of compartments was demonstrably not serial. In this paper we will present such a consideration, showing in particular that by combining influx and compartmental efflux techniques, each of which in itself is theoretically invalid under these circumstances, with pulse-chase studies, it is possible to characterize the compartmentation accounting for the observed ion fluxes and the partitioning between root and shoot. We will present results obtained using intact, vegetative Spergularia marina plants growing on 0.2 x sea water medium in solution culture. The particular virtues of this system have been discussed elsewhere in detail (5,6). For the present purposes, we will restrict our consideration to sodium uptake, accumulation, and translocation because of its relatively straightforward transport characteristics. MATERIALS AND METHODSSpergularia marina (L.) Griseb. seeds were collected from plants growing in our growth chambers and were germinated, transferred to solution cultur...

Characterization of the Na+delivery from roots to shoots in rice under saline stress: Excessive salt enhances apoplastic transport in rice plants

“…Lazof and Cheeseman (1986) suggested that a small and labile symplastic pool that was involved in the upward transport of Na+. Based on the fact that membrane-impermeable tracers, such as La3+ ), raffinose (Perry and Greenway 1973), and PTS (Hanson et al 1985;Moon et al 1986;Yeo et al 1987), could be detected in the shoots or xylem exudate when the tracers were applied to intact roots, the occurrence of long-distance transport which bypasses membranes had been demonstrated.…”

Localization of Nitrate Absorption and Translocation within Morphological Regions of the Corn Root

“…The second factor is the efficiency of the rinsing procedure in removing apoplastic "5N with minimal loss of symplastic "5N. Similar rinsing procedures have previously been shown to effectively remove cationic radiotracers from the free space without significant loss of label from membrane-bound pools (10). Loss of internal NO3-was tested in preliminary studies, exposing intact seedlings to 0.1 mM [5N]NO3-for 3 d, rinsing roots in 8 mm CaSO4 at 0°C for 2 min to remove free space NO3-, and then rinsing in a large volume of 8 mm CaSO4 at 0°C (5 I/plant) for 30 min.…”

“…Results from short-term time courses of uptake by intact roots indicate that symplastic ion transport across the root to the xylem can occur within a few minutes (e.g. 10,12,15,21), so that considerable interregional transport is likely to occur with exposure to labeled nutrients for 30 min or more.…”

“…Changing accumulation rates during short time courses (<15 min) have been used previously to distinguish unidirectional influx and rates of translocation through intact roots (10,11 …”

The integration of activity in saline environments: problems and perspectives

“…This exacerbates the energy problem; it is not simply a problem at the plasmamembrane. Clearly, there is still something critical we are missing about the status of Na + in cells (Lazof and Cheeseman 1986). …”