Effect of the Supply of Potassium, Calcium, and Magnesium on the Absorption, Translocation, and Assimilation of Ammonium- and Nitrate-Nitrogen in Wheat Plants

Forty-two-day-old wheat (Triticum aestivum L. var. Asakazekomugi) plants were treated with complete, K-free (—K), Ca-limited (—Ca), and Mg-free (—Mg) nutrient solutions for 10 days using 2 mM NH4NO3 as the nitrogen source, which was replaced with 4 mM 15 NH4C1 or Na15NO3 for the subsequent 2 days to investigate the absorption, translocation, and assimilation of inorganic nitrogen in relation to the mineral supply. In another experiment plants were grown on NO3 −, NH4 +, NH4N03, and K-free and Ca-limited NH4N03 nutrient solutions for 10 days, and then in the latter three treatments the nitrogen source was replaced with NO3 − and half of the —K plants received K for 6 days to examine the changes in the nitrate reductase activity (NRA). Wheat plants absorbed NH4 −N and NO3-N at a similar rate. Influence of K on the absorption of N03-N was stronger than that on the absorption of NH4-N in wheat plants. The supply of K to the —K plants increased the absorption of NO3-N, while the absorption of NH4-N still remained at a lower rate in spite of the addition of K. A limited supply of Ca and lack of Mg in nutrient media slightly affected the absorption of NH4-N. The influence of K was stronger on the translocation of nitrogen from roots to shoots, while Ca and Mg had little effect. When K was supplied again to the —K plants the translocation of NO3,-N was more accelerated than that of NH4-N. Incorporation of NH4-N into protein was higher than that of NO3-N in all the tissues; root, stem, and leaf. Assimilation of NH4-N and NO3-N decreased by the —K and —Mg treatments. Leaf NRA of wheat plants decreased in the —K and —Ca plants. Higher leaf NRA was found when K was given again to the —K plants than when the plants were continuously grown in K-free media. Replacement of NO3 − with NH4 + as the nitrogen source caused a decline of leaf NRA, while the supply of both NH4 −N and NO3-N slightly affected the leaf NRA.