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Bacteroids Are Stable during Dark-Induced Senescence of Soybean Root Nodules ^1,2

Department of Agricultural Biochemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68583-0718

Physiological 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.' Nitrogenase-dependent 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 allowed to recover under a normal light regimen, but not in 8-day dark-stressed plants. Our data indicate that dark stress does not greatly affect bacteroid metabolism or induce bacteroid senescence.

⁴ Present address: USAID/GUYANA, Agency for International Development, Washington, DC 20523.

¹ Supported in part by the United States Department of Agriculture under grant 83-CRCR-1-1293 and a grant from the Nebraska Soybean Development, Utilization and Marketing Board.

² Published as paper No. 8010, Journal Series, Nebraska Agriculture Research Division.

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