Possible causes of the physiological decline in soybean nitrogen fixation in the presence of nitrate
1997
Arrese-Igor, C. | Minchin, F.R. | Gordon, A.J. | Nath, A.K.
Nodulated soybean plants (Glycine max (L.) Merr. cv. Clarke) were supplied with 10 mol m-3 nitrate at the vegetative stage. This treatment caused a rapid decline in nitrogen fixation (acetylene reduction) activity and a consequent decline in ureides in the xylem sap. However, there was virtually no effect on the nitrogenase complex, according to Western blots against components 1 and 2. The effect on nitrogen fixation was matched by a decrease in nitrogenase-linked respiration and increases in nodule oxygen diffusion resistance and the carbon cost of nitrogen fixation. The addition of nitrate had little effect on protein content from either nodule plant or bacteroid fractions. Activities of nitrate reductase (NR) and nitrite reductase (NiR) from either the plant fraction or the bacteroids were affected in different ways during 8 d of NO3- supply. Nodule plant NR and bacteroid NiR were not affected. However, nodule plant NiR increased 5-fold within 2 d of supplying NO3-. Bacteroid NR only increased after 6 d. These results could be interpreted in terms of a restricted nitrate access into the infected region of nodules. However, denitrification was detected within 2 d of nitrate supply in soybean nodules. The results are discussed in relation to possible causes of the nitrate-induced decline in nitrogenase activity.
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