GENOTYPIC VARIABILITY OF NITROGEN-METABOLISM ENZYMES IN NODULATED ROOTS OF VICIA-FABA

Twenty-nine genotypes of Vicia faba were grown in the presence or abse nce of NO3- to study variability in root and nodule nitrate reductase (NR, EC 1.6.6.1) and nodule cytosolic glutamine synthetase (GS, EC 6.3 .1.2) activities. These V. faba lines apparently lack constitutive roo t NR activity (NRA), whereas inducible activity was detected in all of them except line VF17. Although a marked genotypic variability was fo und for this activity (F-value = 400), the major factor affecting its expression was nitrate (95% of the total effect observed). Most of the lines, however, showed appreciable rates of constitutive NRA in their nodules, that further declined with 4 mM NO3-. High genotypic variabi lity was also found for the level of GS activity from the plant fracti on of V. faba nodules. This activity, little affected by 2 mM NO3-, wa s generally declined by 4 mM for a week at the end of the experiment. No correlation was found between the activities measured here and meas urements of symbiotic performance. Nodule NR, however, correlated inve rsely with both GS and nodule protein content.