Drought-Induced effects and recovery of nitrate assimilation and nodule activity in cowpea plants inoculated with Bradyrhizobium spp. under moderate nitrate level

This study was carried out to establish comparative effects of drought and recovery on the nitrate assimilation and nodule activity related to N2 fixation in cowpea plants [Vigna unguiculata L. (Walp.)] previously inoculated with Bradyrhizobium spp. BR-3256 (CB-756) strain in the presence of 5 mol m-3 NO-3. Twenty-eight-day-old nodulated plants were submitted to water deprivation during 4 consecutive days and afterwards resupplied with nutrient solution during 2 days. The water deprivation caused a rapid increase in the nitrate content in root and a marked reduction in leaf nitrate reductase (NR) activity. In contrast nodule NR activity was slightly increased by water deprivation. Concomitantly, in nodules of water stressed plants, leghemoglobin and glutamine synthetase (GS) activity declined and a progressive reduction in ureide-N concentration in xylem sap was observed. Leaf-NR activity increased rapidly after rehydration while leaf nitrate content declined. In contrast both GS activity and soluble protein content in the nodule continued to decline in rewatered plants. In addition the concentration of leghemoglobin recovered well, while the xylem ureide-N content experienced a slight increase after rehydration. Despite the nitrate assimilation in leaves and the nodule activity had been both severely affected by water stress, the rapid recovery of nitrate reductase activity suggests that the nitrate assimilation process is less sensitive to drought/rehydration cycle when cowpea plants are nodulated in presence of moderate nitrate level.

Water stress response on the enzymatic activity in cowpea nodules

A greenhouse experiment was carried out aiming to study the effect of water stress on metabolic activity of cowpea nodules at different plant development stages. Cowpea plants were grown in pots with yellow latosol soil under three different matric potentials treatments: -7.0 (control-S1), -70.0 (S2) and <-85.0 KPa (S3). The experimental design was randomized blocks with sub-divided plots, each plot containing a different degree of water stress, divided in sub-plots for the four different developmental stages: E1 (0-15), E2 (15-30), E3 (20-35) and E4 (30-45) days after emmergence. Water stress treatments were applied by monitoring soil water potential using a set of porous cups. The effect of water stress was most harmful to cowpea when it was applied at E2 than at other symbiotic process stages. Shoot/root ratio decreased from 2.61 to 2.14 when matric potential treatment was <-85.0 and -70.0 KPa respectively. There was a reduction in the glutamine synthetase activity and phosphoenolpyruvate carboxilase activity with increased stress, while glutamine synthase activity was the enzyme most sensitive to water stress. Glutamate dehydrogenase activity increased in more negative matric potential, indicating that this enzyme is sufficiently activitye under water stress.

Multiple forms of glutamine synthetase in Bacillus brevis AG 4.

Glutamine synthetase in Bacillus brevis AG 4, a Gram-positive spore forming bacteria, has been found to exist in multiple molecular forms. It was purified to electrophoretic homogeneity by single-step Blue Sepharose affinity chromatography. The native enzyme has a molecular weight of 600,000 with subunits of 50,000. The enzyme samples purified from different stages of growth differed in Mg2+ sensitivity and other kinetic properties. Four different enzyme samples selected on the basis of Mg2+ sensitivity showed distinct mobilities at pH 6.3 on PAGE using discontinuous buffer system. A correlation amongst Mg2+ sensitivity, electrophoretic mobility, and kinetic properties was highly suggestive of multiple forms of glutamine synthetase in Bacillus brevis arising due to modification.