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Articles

Ion Balance, Uptake, and Transport Processes in N₂-Fixing and Nitrate- and Urea-Dependent Soybean Plants ¹

United States Department of Agriculture, Agricultural Research Service, Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27650, Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27650

The objective of this study was to examine the influence of N₂ fixation and NO₃^–-N and urea-N assimilation on ion balance, uptake, and transport processes in soybean (Glycine max L. Merr.).

Inoculated plants were grown in Perlite supplied daily with nutrient solutions which contained zero-N, 10 and 20 millimolar NO₃^–-N, and 10 and 20 millimolar urea-N, and they were sampled 41, 76, and 151 days after transplanting. Total uptake of inorganic cations and anions was determined by analysis of tissue for K⁺, Ca²⁺, Mg²⁺, Na⁺, total N from NO₃^–, total S, H₂PO₄^–, and Cl^–. Differences in total inorganic cations (C) and inorganic anions (A) in plant tissue were used to estimate total carboxylate content.

Internal OH^– generation resulting from excess cation uptake (net H⁺ excretion) by the roots accounted for more than 89% of the carboxylate accumulation in N₂- and urea-fed plants, while OH^– generation resulting from SO₄^2– reduction accounted for less than 11%. Shoots contained over 89% of the total plant carboxylate content. Malate balanced about 75% of the excess inorganic cationic charge of the xylem sap; allantoate and aspartate balanced most of the remaining charge. These results indicate that carboxylates (primarily malate) are synthesized in roots of N₂- and urea-fed plants and transported to the shoots in the xylem to maintain charge balance. The high malate concentration resulted in the C/N weight ratio of xylem sap from N₂-fed plants being >2.0, even though 83% of the N was transported as allantoin and allantoic acid which have a C/N ratio of 1.0. The data emphasize that C and N content of N compounds should not be the sole basis for calculating the C/N weight ratio of xylem sap.

The C-to-A uptake ratio for plants supplied 10 millimolar NO₃^– ranged from 1.24 to 1.57 during development, indicating that internal OH^– was generated both by excess cation uptake and by NO₃^– and SO₄^2– reduction. The C-to-A uptake ratio for 20 millimolar NO₃^– -fed plants ranged from 0.86 to 0.96 during development, indicating a small net OH^– efflux from the roots for support of excess anion uptake. On a seasonal basis, only 15% of the OH^– generated during NO₃^– and SO₄^2– reduction was associated with OH^– efflux (excess anion uptake), while 85% was associated with carboxylate accumulation. The malate concentration in xylem sap from plants supplied 20 millimolar NO₃^– was only one-third that of N₂- and urea-fed plants; however, it did balance 75% of the excess inorganic cationic charge. Potassium, recycling to accommodate excess anion uptake by 20 millimolar NO₃-fed plants, was calculated to involve at most 17% of the total K⁺ absorbed during the 41- to 76-day growth interval.

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