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.

Carbohydrate Supply and N2 Fixation in Soybean

“…In the period from 3 to 26 h after girdling, the soluble sugar levels were relatively stable, but the starch pools continued to decline to 47 and 54% of the initial pool sizes in nodule and roots, respectively. These results are similar to those reported in a recent study (25) (26) increased photosynthesis by increasing the pCO2 surrounding the leaves and found that nodule specific activity was not affected. Since leaf starch pools accumulate at a rate such that export rates from the leaf are constant under high pCO2 (8), it may be concluded, again, that translocation rate is limiting nodule activity.…”

“…3), the absolute size of these pools cannot be used as a reliable indicator of the carbohydrate limitation of the nodule. Previous workers (17,22,25) have claimed that nodules are not carbohydrate limited, since carbohydrate reserves remained in the nodule under conditions in which nodule activity was declining. These studies have not considered the size of the nodule starch/sucrose pool relative to the nodule respiratory need, the distribution of these reserves between nodule cortical, infected and uninfected cells, or the possibility that nodule starch reserves might not be mobilized at a rate sufficient to maintain maximal rates ofnodule metabolism.…”

“…Nodule cyclitols and Phb pools have an extremely show turnover rate (11,12,16,28) and are therefore unlikely to play a role in the support of nodule respiration. However, nodule starch and sugar levels decline under conditions of carbohydrate limitation (15,22,25) and are therefore possible candidates. Rainbird et al (15) estimated that the depletion of nodule starch and sugar was sufficient to account for 50% of the nodule respiration over a night period.…”

“…However, nodule starch and sugar levels decline under conditions of carbohydrate limitation (15,22,25) …”

Effects of Carbohydrate on the Internal Oxygen Concentration, Oxygen Uptake, and Nitrogenase Activity in Detached Pea Nodules

“…activity and nodule carbohydrate levels were observed after shoot excision in French bean (22), defoliation of clover (3), and stem girdling of soybean (21). The organic acids succinate and fumarate were most effective in stimulating ARA, although neither of the substrates completely prevented the decline in ARA with time (Table I).…”