Nitrogen Metabolism of Soybeans

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Abstract
The effects of N source (6 mM N as NO3- or urea) and tungstate (0, 100, 200, 300 and 400 .mu.M Na2WO4) on nitrate metabolism, nodulation and growth of soybean (Glycine Max [L.] Merr.) plants were evaluated. Nitrate reductase activity and 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 .mu.M 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-.