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Nitrogen Metabolism of Soybeans

I. Effect of Tungstate on Nitrate Utilization, Nodulation, and Growth

United States Department of Agriculture, Science and Education Administration, Federal Research, and Department of Agronomy, Illinois Agricultural Experiment Station, Urbana, Illinois 61801

The effects of N source (6 mM nitrogen as NO₃^– or urea) and tungstate (0, 100, 200, 300, and 400 µM Na₂ WO₄) on nitrate metabolism, nodulation, and growth of soybean (Glycine max [L.] Merr.) plants were evaluated. Nitrate reductase activity and, to a lesser extent, NO₃^– content of leaf tissue decreased with the addition of tungstate to the nutrient growth medium. Concomitantly, nodule mass and acetylene reduction activity of NO₃^–-grown plants increased with addition of tungstate to the nutrient solution. In contrast, nodule mass and acetylene reduction activity of urea-grown plants decreased with increased nutrient tungstate levels. The acetylene reduction activity of nodulated roots of NO₃^–-grown plants was less than 10% of the activity of nodulated roots of urea-grown plants when no tungstate was added. At 300 and 400 µM tungstate levels, acetylene reduction activity of nodulated roots of NO₃^–-grown plants exceeded the activity of comparable urea-grown plants.

Addition of tungstate to the nutrient solution decreased plant growth, regardless of the N source, although the effect was more pronounced with NO₃^– nutrition. The increased nodulation and decreased nitrate reductase activity noted with plants grown in the presence of tungstate and a high (6 mM) external supply of NO₃^– suggests that NO₃^– does not directly inhibit nodulation but rather affects nodulation indirectly through subsequent metabolism of NO₃^–.