Biochemical Nitrogen Fixation Studies. I. Evidence for Limited Oxygen Supply Within the Nodule

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Respiration studies were undertaken with a view to obtaining fundamental information concerning the metabolic processes of legume nodules, inasmuch as the knowledge gained should facilitate investigation of the process of nitrogen fixation. The results obtained by standard manometric procedures with detached nodules from eight species of legumes may be summarized as follows: 1. The rates of respiration of nodules maintained in various oxygen-nitrogen mixtures increased as the oxygen percentage increased. The QO2 values varied between 0.62 and 6.25 with an average of about 2.2 in air, and were about 2.4 times as great in pure oxygen, with maxima for the eight species varying between 4.3 for soybean and 8.9 for hairy vetch. 2. The respiratory quotients increased with increase in size of nodule and with decrease in concentration of oxygen. In air the values varied between 1.0 and 2.0, depending largely on the size of the nodules. In pure oxygen, excluding very large nodules, the values averaged 1.03. 3. The addition of glucose to the nutrient medium produced a mean increase in QO2 of about 12 per cent in air and 14 per cent in 100 per cent oxygen. It increased the R.Q. at the lower concentrations of oxygen but not appreciably in pure oxygen. 4. These results with detached nodules are interpreted as indicating that with most nodules maintained in air, or with large nodules at oxygen concentrations even as high as 100 per cent, the interior of the nodules is under anaerobic or partially anaerobic conditions. The evidence further indicates that under natural conditions in the soil the oxygen supply inside most nodules is also limited. 5. Crushed nodules gave higher QO2 values when maintained in air than did whole nodules, but usually lower values than whole nodules in pure oxygen. This method of increasing the aeration of the inner cells of the nodules was not very satisfactory. 6. The nodules of all the species showed rather similar responses, the variations between species being not much greater than that within species.