POSSIBLE CAUSES OF THE PHYSIOLOGICAL DECLINE IN SOYBEAN NITROGEN-FIXATION IN THE PRESENCE OF NITRATE

Nodulated soybean plants (Glycine max (L.) Merr, cv. Clarke) were supp lied with 10 mol m(-3) nitrate at the vegetative stage. This treatment caused a rapid decline in nitrogen fixation (acetylene reduction) act ivity and a consequent decline in ureides in the xylem sap. However, t here was virtually no effect on the nitrogenase complex, according to Western blots against components 1 and 2. The effect on nitrogen fixat ion was matched by a decrease in nitrogenase-linked respiration and in creases in nodule oxygen diffusion resistance and the carbon cost of n itrogen fixation. The addition of nitrate had little effect on protein content from either nodule plant or bacteroid fractions. Activities o f nitrate reductase (NR) and nitrite reductase (NIR) from either the p lant fraction or the bacteroids were affected in different ways during 8 d of NO3- supply. Nodule plant NR and bacteroid NiR were not affect ed. However, nodule plant NIR increased 5-fold within 2 d of supplying NO3-. Bacteroid NR only increased after 6 d. These results could be i nterpreted in terms of a restricted nitrate access into the infected r egion of nodules. However, denitrification was detected within 2 d of nitrate supply in soybean nodules. The results are discussed in relati on to possible causes of the nitrate-induced decline in nitrogenase ac tivity.