Proline Accumulation, Nitrogenase (C2H2 reducing) Activity and Activities of Enzymes related to Proline Metabolism in Drought-Stressed Soybean Nodules

We examined the effect of drought stress on proline accumulation,nitrogenase activity and activities of enzymes related to prolinemetabolism in soybean (Glycine max L.] Merr.) nodules. Nitrogenase(C₂H₂ reducing) activity was inhibited 90% or more as a resultof drought stress. This inhibition was substantially reversedafter a 4 h recovery period. Pyrroline-5-carboxylate reductaseactivity in extracts of drought-stressed nodules from 25-d-oldplants was 55% higher than in unstressed nodules, but the sameactivity in preparations from 55-d-old plants was similar tothat of control plants. Extracts of recovering nodules on plantsof both ages had activities near those of controls. Droughtstress increased the activity of the pentose phosphate pathwayby about 65% in extracts of nodules from 55-d-old plants, butthere was no effect in extracts of nodules from younger plants(25-d-old). Proline dehydrogenase activity was 3.7 and 1.6 timeshigher in bacteroids isolated from nodules taken from 25- and55-d-old stressed plants, respectively, than in comparable controlbacteroids. This activity remained high in bacteroids from bothsets of recovering nodules. The amount of proline in extractsfrom stressed nodules was 3- to 4-fold higher than in unstressednodules, despite increased proline dehydrogenase activity andremained high in nodules collected 4 h after rewatering. Thisincrease was observed in both cytoplasmic and bacteroid fractions.The possible physiological significance of these results isdiscussed. Key words: Proline metabolism, pentose phosphate pathway, drought stress, soybean nodules