Biochemical Nitrogen Fixation Studies. II Comparative Respiration of Nodules and Roots, Including Non-Legume Roots

The results of respiration studies with detached nodules, legume roots, and non-legume roots from several species of plants, carried out according to standard manometric procedures, are as follows: 1. Rates of respiration of legume nodules and small legume and non-legume roots maintained in a nutrient solution in air averaged nearly the same, the QO2values being about 2.2. Rates of respiration of the roots of various species of plants showed no marked differences for roots of comparable age, size, and condition. 2. Respiratory quotients of small legume and non-legume roots maintained in air were usually slightly less than 1.0 and hence were definitely lower than for nodules, although large roots may also show values considerably above 1.0 because of slow oxygen penetration. 3. Rates of respiration per unit of dry matter of legume nodules in pure oxygen were more than twice as great as in air, whereas values for the small legume and non-legume roots were usually not markedly affected by the additional oxygen. Nodules therefore have an inherent capacity for respiring at a more rapid rate than root tissues, provided the inner cells can get the necessary oxygen. 4. Small legume and non-legume roots are more deficient in available energy supply than are nodules, a mean increase in QO2of about 40 per cent due to glucose additions to roots being obtained, compared with 12-14 per cent with nodules. 5. Rates of respiration per unit of nitrogen of legume and non-legume roots maintained in air were three to four times as great as for nodules. Even in pure oxygen the QO2(N) values of roots were on the average about twice as great as for nodules. 6. These data are in harmony with the idea, so frequently suggested by morphological studies, that the nodule consists of plant cells largely filled with comparatively inactive bacteria. The bacteria oxidize only a very small portion of the total carbohydrate photosynthesized by the host plant.