ABSTRACIThe Na+ and K+ transport characteristics of Spergularia marina (L.) Griseb. were considered in order to compare the systems by which these two physiologically different cations are managed during initial acquisition and subsequent partitioning in midvegetative plants. Uptake of 22Na and 42K and redistribution of labels in pulse-chase studies were compared under steady state growth conditions or with the concentration of one of the ions elevated. At high external concentrations, the initial 42K+ accumulation and transport to the shoot was associated with a small, rapidly exchanging, cellular compartment similar to that previously indicated for Na+ (D Lazof, JM Cheeseman 1986 Plant Physiol 81: 742-747). At 1 mol m3, K+ was conducted to the shoot through a root compartment, the specific activity of which rose much more slowly than the rapidly exchanging compartment. After a lag of approximately 5 minutes, 42K+ translocation approached a constant rate with a half-time of 14 minutes compared to 5 minutes for 22Na+ or for 42K+ at higher external levels. At all external levels, prolonged translocation of 42K+ was measured when a 10 minute pulse was followed by an unlabeled chase, again suggesting a conducting compartment distinct from that for Na+. It is suggested that the K+ conducting compartment, possibly the 'bulk cytoplasm,' is associated with the active K+ transport system generally found in higher plants.The partitioning of mineral resources between the above and below ground parts of a plant is controlled by the root. It is largely mediated through transmembrane events, with a negligible contribution of apoplastic movements (11,17). The systems responsible for providing a controlled nutrient supply to a rapidly growing shoot and maintaining the ion levels in the root itself, however, are still not well understood. Despite the desirability of 'simple' systems for transport studies, nutrient management is better considered within the complexity of an intact growing plant.Among the difficulties facing organismal transport studies is interpretation of isotope flux studies when unidirectional fluxes cannot be measured. In a previous report (16), we addressed these problems, considering the transport and accumulation of labeled Na+ in Spergularia marina growing at moderate salinity. Using the approach of integrated pulse-chase protocols, transport events related to an internal root compartment other than 'bulk cytoplasm' and 'vacuole' were distinguished.Even in a halophyte, Na+ may well be a special case, particu- larly when the external concentrations to which the plants are exposed are high, both absolutely and relative to other cations. In this report, therefore, we extend our studies to lower salinity conditions and to K+. Our objective is to consider Na+ and K+ transport under comparable conditions, with particular concern for the mechanisms of initial acquisition, for compartmentation within the roots, and for delivery of the ions to the xylem.
MATERIALS AND METHODSSpergularia marina (L.) Griseb. see...