ALBERT

Notes: Nodulated soybeans (Glycine max L. Merr, cv. Maple Arrow) were exposed to various physiological and environmental treatments to determine the relationship between nodule adenylate pools and the degree of O2 limitation of nitrogenase. Adenylate energy charge (AEC = [ATP + 0.5 ADP]/[ATP + ADP + AMP]) and ATP/ADP ratios declined under conditions of decreased (10%) external pO2 but increased in nodules exposed to elevated (30%) external pO2. Nitrogenase activity was inhibited by both pO2 treatments, but recovered towards initial levels within 45 min. AEC also returned to initial levels during this period. To account for these and related data in the literature, it was hypothesized that 1) legume nodules regulate infected cell O2 concentration (Oi) to maintain adenylate pools at levels which limit respiratory metabolism: 2) treatments which decrease Oi alter the adenylate pools and further limit nodule metabolism; 3) treatments which increase Oi to levels in excess of a narrow range alter the adenylate pools and activate biochemical pathways which are not conducive to nitrogenase activity. In a preliminary test of these hypotheses, changes in AEC and ATP/ADP ratio were studied in nodules in which nitrogenase activity was inhibited by stem girdling, nitrate fertilization and exposure to an Ar:O2 atmosphere. All three treatments caused an increased O2 limitation of nodule respiration and nitrogenase activity. However, decreases in AEC were observed only in the stem girdling and nitrate fertilization treatment: Ar:O2 exposure had no effect on whole nodule AEC. While this result challenged the hypotheses suggesting a central role for adenylates in the regulation of O2-limited metabolism, it was noted that the Ar:O2 treatment would differ from the other treatments in that it would have a specific effect on the ATP demands for NH3 assimilation in the plant fraction. Since AEC and ATP/ADP ratio would be affected by both the rate of ATP synthesis (potentially an O2-limited process) and the demand for ATP, changes in these parameters in the whole nodule may not be a reliable indicator of adenylate-mediated O2 limitation. Futher studies are needed to examine in vivo changes in adenylate pools in the plant and bacteroid fractions in nodules which vary in their degree of O2-limited metabolism.