Water-deficit effects on carbon and nitrogen metabolism of pea nodules

Two experiments were carried out to investigate the effects of water-deficit stress on carbon and nitrogen metabolism of Pisum sativum nodules. In the first experiment, leaf Psi(w) was allowed to reach -1.0 MPa over a period of 14 d whilst in the second experiment -1.5 MPa was reached during the same time period. Nodule activities of phosphoenol pyruvate carboxylase, glutamine synthetase, alkaline invertase, pyruvate decarboxylase, alcohol dehydrogenase, uridine pyrophosphorylase, and malate dehydrogenase activities were not affected by water-deficit stress. In the first experiment (-1.0 MPa), sucrose synthase (SS), an enzyme which hydrolyses sucrose to support nodule metabolism, declined by 50% in activity and about 25% in content, according to Western immunoblot data. In the second experiment(-1.5 MPa), SS activity decreased by 75% together with glutamate synthase and aspartate aminotransferase which declined by 60% and 40%, respectively. Coincident with the decline of these activities, a dramatic increase in the nodule content of sucrose and a slight increase in the levels of total free amino acids were found. It has been recently suggested that the decline in SS activity and, therefore, a reduced potential to metabolize sucrose may be an important factor contributing to the overall response of soybean nodules to water stress. These results suggest that this observation may be also correct for temperate legumes with indeterminate nodules. However, in this latter case, the activity of some enzymes involved in nitrogen assimilation (glutamate synthase and aspartate aminotransferase) were also affected by water-deficit stress.