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 proline metabolism in soybean (Glycine max [L.] Merr.) nodules. Nitrogenase (C2H2 reducing) activity was inhibited 90% or more as a result of drought stress. This inhibition was substantially reversed after a 4 h recovery period. Pyrroline-5-carboxylate reductase activity in extracts of drought-stressed nodules from 25-d-old plants was 55% higher than in unstressed nodules, but the same activity in preparations from 55-d-old plants was similar to that of control plants. Extracts of recovering nodules on plants of both ages had activities near those of controls. Drought stress increased the activity of the pentose phosphate pathway by about 65% in extracts of nodules from 55-d-old plants, but there was no effect in extracts of nodules from younger plants (25-d-old). Proline dehydrogenase activity was 3.7 and 1.6 times higher in bacteroids isolated from nodules taken from 25- and 55-d-old stressed plants, respectively, than in comparable control bacteroids. This activity remained high in bacteroids from both sets of recovering nodules. The amount of proline in extracts from stressed nodules was 3- to 4-fold higher than in unstressed nodules, despite increased proline dehydrogenase activity and remained high in nodules collected 4 h after rewatering. This increase was observed in both cytoplasmic and bacteroid fractions. The possible physiological significance of these results is discussed.