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The uptake of K+ ion was studied in the roots of wheat (Triticum aestivum L. cv. GK Szeged) and cucumber (Cucumis sativus L. cv. Budai csemege) seedlings grown in nutrient solution under nitrogen and sulfate stress conditions. Seedlings pretreated with 1 or 10 mM NaNO3, absorbed more K+ than those treated with 0.1 mM NaNO3. However, the posteffect of NaNO3 was considerably influenced by the Na2SO4, treatment. The results suggest that, at least partly, a feed‐back regulation of K+ uptake may occur. However, due to the high Na+ contents of the roots, a Na+ effect in this process cannot be excluded. The growth and dry matter yields of the roots and shoots were strongly influenced by the SO2−/4 and NO−/3 supply of the plants. Appreciable differences were experienced between wheat and cucumber seedlings. The optimum SO2−/4 concentration of the growth solution for maximal growth varied considerably between the species, and was also different for the roots and the shoots in a given species. [ABSTRACT FROM AUTHOR]