The absorption, translocation and accumulation of sodium in young squash plants

Abstract

In most plants the concentration of sodium is much lower in the leaves than in the roots. In other species, however, there seems to be an equal distribution of this element between the shoots and the roots. The squash plant, Cucurbita pepo, used in the present study is an example of the type of plant that in respect to the roots maintains a relatively low sodium concentration in its leaves. When the plants were grown in a medium containing equivalent amounts of sodium and potassium the hypocotyls and leaves contained less sodium than potassium. When the roots were excised or killed by immersion in boiling water similar amounts of both elements moved to the upper portion of the plant. Thus the living roots exerted a large effect in discriminating against the uptake of sodium. The low amount of sodium in the leaves was accounted for, in part, by translocation of this element from the leaves to the roots. Suspension of phloem activity by steam or localized chilling to girdle the hypocotyl resulted in accumulation of sodium in the leaves and the hypocotyl above the girdle. Similar accumulation resulted from the treatment with 2,4-dinitrophenol, a phosphorylation uncoupling agent. Calcium was found to reduce significantly the amount of sodium accumulated by excised and intact roots. However, this effect on the roots was not consistently reproduced in the shoot. The predominant anion in the medium seemed to influence the effect of calcium. Thus under transpiring conditions calcium inhibited sodium translocation to the shoot if the predominating anion in the medium was nitrate. On the other hand calcium increased the amount of sodium in the shoot if the predominant anion was sulfate. Chloride and phosphate did not seem to have any effect on the calcium response. Under conditions where root pressure was active (decapitated roots or guttating plants), the amount of sodium in the xylem sap was reduced by the presence of calcium in the root medium.