DROUGHT STRESS, PERMEABILITY TO O-2 DIFFUSION, AND THE RESPIRATORY KINETICS OF SOYBEAN ROOT-NODULES

In legume nodules, treatments such as detopping or nitrate fertilizati on inhibit nodule metabolism and N-2 fixation by decreasing the nodule 's permeability to O-2 diffusion, thereby decreasing the infected cell O-2 concentration (Oi) and increasing the degree to which nodule meta bolism is limited by O-2 availability. In the present study we used no dule oximetry to assess and compare the role of O-2 limitation in soyb ean (Glycine max L. Merr) nodules inhibited by either drought or detop ping. Compared to detopping, drought caused only minor decreases in Oi , and when the external O-2 concentration was increased to raise Oi, t he infected cell respiration rate in the drought-stressed plants was n ot stimulated as much as it was in the nodules of the detopped plants. Unlike those in detopped plants, nodules exposed to moderate drought stress displayed an O-2-sufficient respiration rate that was significa ntly lower than that in control nodules. Despite possible side effects of oximetry in altering nodule metabolism, these results provided dir ect evidence that, compared to detopping, O-2 limitation plays a minor role in the inhibition of nodule metabolism during drought stress and changes in nodule permeability are the effect, not the cause, of a dr ought-induced inhibition of nodule metabolism and the O-2-suffiecient rate of respiration.