A comparison of inhibition of French‐bean and soybean nitrogen fixation by nitrate, 1% oxygen or direct assimilate deprivation

Abstract

Inhibition by NO−3 of acetylene reduction in bean (Phaseolus vulgaris L. cv. Contender) and soybean (Glycine max L. cv. Amsoy 71) was measured in parallel with nodule carbohydrate and nitrate metabolism. In bean the onset of inhibition of C2H2 reduction (6 h) coincided with decreased import of assimilates and a lowering of carbohydrate pools (sucrose, glucose and starch). Nitrate reductase (EC 1.6.6.1) activity was induced in all plant organs after 3 h but no nitrite was detected in the nodules. In soybean, nodule carbohydrate concentrations and import of assimilates into the nodules increased markedly between 6 to 24 h after supply of nitrate when the nitrogenase (EC 1.7.99.2) was progressively inhibited. High nitrate reductase activity was observed in the nodules and nitrites accumulated because of insufficient nitrite reductase activity. The nitrate‐induced inhibition of nitrogenase was compared with the inhibition observed with low oxygen around the roots (1% O2) or with direct assimilate deprivation (girdling or decapitation). Soybean and bean appeared equally sensitive to these treatments as regards to acetylene reduction. The results are discussed in relation to the current hypotheses explaining nitrate‐induced inhibition of dinitrogen fixation: assimilate deprivation or nitrite poisoning. Present data are in favour of the first for bean and of the second for soybean.