Distribution of nitrate reductase activity in Vicia faba: Effect of nitrate and plant genotype

The short term effect of NO₃⁻ (12 mM) on nitrate reductase (NR. EC 1.6.6.1) activity has been studied in the roots, nodules and leaves of different genotypes of Vicia faba L. at the end of vegetative growth. Root and leaf NR activity responded positively to NO₃⁻ while nodule activity, where detected, proved to he strongly inhibited. The withdraw of this NO₃⁻ from the solution consistently reduced activity in the roots and leaves but surprising, promoted a significant increase in nodule activity, which matched or surpassed that of control plants On the other hand, nodules developed in the presence of 8 mM NO₃⁻ expressed an on average 141% higher level of NR activity than did controls. This effect was observed even in nodules with negligible control activity. In any case, a naturally occurring mutant (VF17) lacking root and nodule NR activity is described. The results indicate that in V. faba. the effects of NO₃⁻ and plant genotype on NR activity depended on plant organ and time of NO₃⁻ application, hut the distribution of NO₃⁻ reduction through the plain was mainly dependent on plant genotype, and to a lesser extent on NO: supply and plant age.