THE ROLE OF OXYGEN IN THE REGULATION OF NITROGENASE ACTIVITY IN DROUGHT-STRESSED SOYBEAN NODULES

The aim of this study was to investigate the mechanism of nitrogenase inhibition in drought-stressed soybean (Glycine max L.) nodules to det ermine whether this stress was similar to other inhibitory treatments (e.g. detopping) known to cause an O-2 limitation of nodule metabolism . Nodulated soybean plants were either detopped or subjected to mild, moderate, or severe drought stress by growth in different media and by withholding water for different periods. All treatments caused a decl ine in nitrogenase activity, and in the drought-stressed nodules, the decline was correlated with more negative nodule water potentials. Inc reases in rhizosphere O-2 concentration stimulated nitrogenase activit y much more in detopped plants than in drought-stressed plants, reflec ting a greater degree of O-2 limitation with the detopped treatment th an with the drought-stressed treatment. These results indicated that d rought stress differs from many other inhibitory treatments, such as d etopping, in that its primary cause is not a decrease in nodule permea bility and a greater O-2 limitation of nodule metabolism. Rather, drou ght stress seems to cause a decrease in the maximum O-2-sufficient rat e of nodule respiration or nitrogenase activity, and the changes in no dule permeability reported to occur in drought-stressed nodules may be a response to elevated O-2 concentrations in the infected cell that m ay occur as nodule respiration declines.