Effect of Cations on Uptake, Translocation and Reduction of Nitrate in Wheat Seedlings

S ummary The effect of Ca 2+ , K + and Na + on uptake, transport and reduction of NO 3 − was studied in 10‐day‐old plants of Triticum aestivum L. cv. Sicco grown in water culture in relation to the ion balance in plant, medium and xylem exudate. When Ca 2+ (0.25 mM) was the only nutrient cation, NO 3 − uptake and accumulation of NO 3 − in the leaves were low as compared to treatments with K + or Na + (0.5 mM). With Na + , the uptake of NO 3 − was similar to that with K + , but virtually no Na + and 75 % less NO 3 − accumulated in the leaves. Medium pH dropped initially with 0.5 mM KNO 3 but rose immediately when Ca 2+ was the only cation. The carboxylate content rose in roots and leaves of K + plants and, to a lesser extent, in Na + plants. Roots of Ca(NO 3 ) 2 plants showed negative ash alkalinity indicating the presence of acidity. The pH of the xylem exudate increased with the cation/NO 3 − ratio in the exudate and was about one unit higher with Ca(NO 3 ) 2 than with NaNO 3 or KNO 3 . We conclude that roots of plants grown in the absence of monovalent cations have serious pH problems because of a high rate of alkalinity release to the medium in exchange for NO 3 − uptake, and the secretion of alkalinity into the xylem.