A pharmacological analysis of high-affinity sodium transport in barley (Hordeum vulgare L.): a 24Na+/42K+ study

Abstract

Soil sodium, while toxic to most plants at high concentrations, can be beneficial at low concentrations, particularly when potassium is limiting. However, little is known about Na(+) uptake in this 'high-affinity' range. New information is provided here with an insight into the transport characteristics, mechanism, and ecological significance of this phenomenon. High-affinity Na(+) and K(+) fluxes were investigated using the short-lived radiotracers (24)Na and (42)K, under an extensive range of measuring conditions (variations in external sodium, and in nutritional and pharmacological agents). This work was supported by electrophysiological, compartmental, and growth analyses. Na(+) uptake was extremely sensitive to all treatments, displaying properties of high-affinity K(+) transporters, K(+) channels, animal Na(+) channels, and non-selective cation channels. K(+), NH(4)(+), and Ca(2+) suppressed Na(+) transport biphasically, yielding IC(50) values of 30, 10, and <5 μM, respectively. Reciprocal experiments showed that K(+) influx is neither inhibited nor stimulated by Na(+). Sodium efflux constituted 65% of influx, indicating a futile cycle. The thermodynamic feasibility of passive channel mediation is supported by compartmentation and electrophysiological data. Our study complements recent advances in the molecular biology of high-affinity Na(+) transport by uncovering new physiological foundations for this transport phenomenon, while questioning its ecological relevance.

Fig. 1.

Concentration dependence of unidirectional, high-affinity Na⁺ influx, measured in 7-d-old barley seedlings grown with 200 μM Ca²⁺ (I, circles), 100 μM Na⁺ (II, squares), or full nutrient medium (III, triangles). Values represent means ±SEM (n=4–6). (This figure is available in colour at JXB online.)

Fig. 2.

Influences of (A) K⁺, (B) NH₄⁺, and (C) Ca²⁺ on high-affinity Na⁺ influx (and of Na⁺, NH₄⁺, and Ca²⁺ on high-affinity K⁺ influx, respectively; insets). Fluxes were measured at 100 μM Na⁺ or K⁺. Dashed lines represent IC₅₀ and IC₇₅ values for the inhibition of control fluxes. Values represent means ±SEM (n=3–14). (This figure is available in colour at JXB online.)

Fig. 3.

Traces of ²⁴Na⁺ efflux from roots of intact barley seedlings grown and measured at 100 μM Na⁺. Diamonds, steady-state conditions with no inhibitors; squares, sudden application of 0.1 mM amiloride as shown by the arrow (at 19.5 min); triangles, 5 mM K⁺ present in the labelling medium and during subsequent elution. Each point on the efflux traces represents the mean of nine replicates for control and amiloride treatments, and the mean of four replicates for the K⁺ treatment. Tabular inset: flux parameters derived from compartmental analysis by tracer efflux, under steady-state conditions (mean ±SEM; n=9). (This figure is available in colour at JXB online.)

Fig. 4.

Relationship between membrane electrical potential and unidirectional Na⁺ influx, and in response to NH₄⁺ and K⁺. NH₄⁺ and K⁺ concentrations represent IC₅₀ and IC₇₅ values for the suppression of Na⁺ influx by these ions (see Fig. 2). Na⁺ fluxes were measured at 100 μM [Na⁺]_ext. The data represent means of 4–23 replicates ±SEM. (This figure is available in colour at JXB online.)

Fig. 5.

Tissue Na⁺ content (squares) and fresh weight (circles) of barley seedlings was determined in plants grown at 0.01, 0.1, 1, 5, 10, 25, and 50 mM Na⁺ steady state. Values represent means ±SEM (n=6–10). (This figure is available in colour at JXB online.)