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. al the end of vegetative growth. Root and leaf NR activity responded positively to NO3-, while nodule activity, where detected, proved to be strongly inhibited. The withdrawal of this NO3- from the solution consistently reduced activity in the roots and leaves but, surprisingly, 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, but the distribution of NO3- reduction through the plant was mainly dependent on plant genotype, and to a lesser extent on NO3- supply and plant age.