THE RELATIONSHIP BETWEEN NODULE ADENYLATES AND THE REGULATION OF NITROGENASE ACTIVITY BY O-2 IN SOYBEAN

Nodulated soybeans (Glycine mat L. Merr, cv. Maple Arrow) were exposed to various physiological and environmental treatments to determine th e relationship between nodule adenylate pools and the degree of O-2, l imitation of nitrogenase. Adenylate energy charge (AEC = [ATP + 0.5 AD P]/[ATP + ADP + AMP]) and ATP/ADP ratios declined under conditions of decreased (10%) external pO(2) but increased in nodules exposed to ele vated (30%) extemal pO(2). Nitrogenase activity was inhibited by both pO(2) 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 O-2 concentration (Oi) to maint ain adenylate pools at levels which limit respiratory metabolism; 2) t reatments which decrease Oi alter the adenylate pools and further limi t nodule metabolism 3) treatments which increase Oi to levels in exces s of a narrow range alter the adenylate pools and activate biochemical pathways which are not conducive to nitrogenase activity. In a prelim inary 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:O-2 atmosphere. All three treatments caused an increased O-2 limitation of nodule res piration and nitrogenase activity. However, decreases in AEC were obse rved only in the stem girdling and nitrate fertilization treatment; Ar :O-2 exposure had no effect on whole nodule AEC. While this result cha llenged the hypotheses suggesting a central role for adenylates in the regulation of O-2-limited metabolism, it was noted that the Ar:O-2 tr eatment 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 ra te of ATP synthesis (potentially an O-2-limited process) and the deman d for ATP, changes in these parameters in the whole nodule may not be a reliable indicator of adenylate-mediated O-2 limitation. Futher stud ies are needed to examine in vivo changes in adenylate pools in the pl ant and bacteroid fractions in nodules which vary in their degree of O -2-limited metabolism.