PROTEOLYSIS AND NITROGEN-FIXATION IN FABA-BEAN (VICIA-FABA) NODULES UNDER WATER-STRESS

Nodulated faba-beans (Vicia faba L. var. minor) exhibiting high rates of N2 fixation (133-mu-mol C2H4 g-1 dry weight h-1), were subjected to water restriction. A loss of C2H2 reduction due to water stress was always associated with a decline of the leghemoglobin content for each of the 4 decreasing values of psi-nod. Electron micrographs showed ultrastructural alterations of the fixing tissue, which affected both partners and increased with the severity of water stress. In the nodule cytosol, the alkaline proteolysis approximately doubled when psi-nod decreased from -0.55 MPa to -1.55 MPa. Concomitantly, an increase of the nodule intracellular pH from 6.3 to 7.0 was observed. Proteolysis was due to serine proteases, exhibiting a pH-optimum of 8 and which actively degraded purified leghemoglobin in vitro (K(m) = 100-mu-M). The degradation of leghemoglobin during water stress may contribute to the loss of C2H2 reduction and may affect the pattern of recovery upon rewatering.