Drought Stress Response on Some Key Enzymes of Cowpea (Vigna unguiculata L. Walp.) Nodule Metabolism

A greenhouse experiment was carried out aiming to evaluate the response to drought stress of cowpea nodule enzymatic activities during different plant developmental stages leading to biological N₂ fixation. Stress was applied by controlling soil’s water-potential through a porous cup. Cowpea plants cv IPA 205 were grown in pots with yellow latosol soil under three different matric potential (ψ_m) treatments. Even with high evaporative demand and limited soil water availability, cowpea could not induce an extremely low leaf water potential (ψ_w). Sap ureides concentration in cowpea declined during the drought stress period. There was a decline in enzyme activity in the metabolic pathways concerned with N₂ fixation: NADH-dependent glutamate synthase (EC 1.4.1.14), glutamine synthetase (EC 6.3.1.2) and phosphoenolpyruvate carboxylase (EC 4.1.1.31). In contrast, an increase in glutamate dehydrogenase (EC 1.4.1.4) was observed as the ψ_m declined. Metabolism associated with N₂ assimilation was impaired every time that the ψ_w was reduced below −0.73 MPa as had happened in the stressed treatments. The stress applied by the porous cup was gradual and the plant recovered its turgor, avoiding permanent deleterious alterations in the cellular metabolism, even from a limited cowpea-growth ψ_m.