ABSTRACIPhysiological and biochemical markers of metabolic competence were assayed in bacteroids isolated from root nodules of control, dark-stressed, and recovered plants of Glycine max Merr. cv 'Woodworth.' Nitrogenasedependent acetylene reduction by the whole plant decreased to 8% of control rates after 4 days of dark stress and could not be detected in plants dark stressed for 8 days. However, in bacteroids isolated anaerobically, almost 50% of initial acetylene reduction activity remained after 4 days of dark stress but was totally lost after 8 days of dark stress. Bacteroid acetylene reduction activity recovered faster than whole plant acetylene reduction activity when plants were dark stressed for 8 days and returned to a normal light regimen. Significant changes were not measured in bacteroid respiration, protein content, sodium dodecyl sulfate-polyacrylamide gel electrophoresis protein profiles, or in bacteroid proteolytic activity throughout the experiment. Immunoblots of bacteroid extracts revealed the presence of nitrogenase component II in control, 4-day dark-stressed, and 8-day dark-stressed plants that were Studies of the effects of imposed stress on nodulated plants have provided much information on the changes taking place in whole nodules. There is rapid attenuation of nitrogenase activity (AR activity), with subsequent declines in total nodule protein and Lb levels (3,7,18,20,22,29). However, corresponding data on bacteroid status are incomplete. In legumes such as peas, which possess indeterminate nodules, bacteroid function generally responds in a similar manner to stress as the host plant (3, 4). In plants with determinate nodules, such as soybean, existing evidence suggests that bacteroids are stable during both stressinduced and natural senescence (9,17,19,25,26).We initiated this study to measure bacteroid stability during senescence induced by dark stress and subsequent recovery in root nodules of soybean plants. MATERIALS
