GENOTYPIC DIFFERENCES IN NITROGEN ASSIMILATION IN VICIA-FABA - EFFECT OF NITRATE

Five inbred lines and a commercial cultivar of field-bean (Vicia faba) were inoculated with a strain of Rhizobium leguminosarum by. viciae and grown, with or without nitrate, under axenic conditions in a growth chamber. There were significant (p < 0.01) differences between lines in N yield, both in N-free solution (N2 fixed, fully symbiotic) and with added NO3-. Increase in the N yield of NO3--fed plants compared with fully symbiotic plants also varied between genotypes, with line VF109 being the least responsive to NO3-. This may indicate nitrate tolerance in relation to N2 fixation. The indication was supported by the observations that nodule development and nodule activity (acetylene reduction) were less inhibited in VF109 in the presence of 8 mM NO3- than in the other lines of Vicia faba. Glutamine synthetase and glutamate synthase (GOGAT) activities appeared to be related to genotypic differences in symbiotic efficiency. This finding suggests that assays of nodule glutamine synthetase and NADH-glutamate synthase might be used in Vicia faba breeding programs to indicate capacity for symbiotic N2 fixation, particularly in the presence of NO3-.