Ammonium can stimulate nitrate and nitrite reductase in the absence of nitrate in Clematis vitalba

Nitrogen assimilation was studied in the deciduous, perennial climber Clema tis vitalba, When solely supplied with NO3- in a hydroponic system, growth and N-assimilation characteristics were similar to those reported for a ran ge of other species. When solely supplied with NH4+, however, nitrate reduc tase (NR) activity dramatically increased in shoot tissue, and particularly leaf tissue, to up to three times the maximum level achieved in NO3- suppl ied plants, NO3- was not detected in plant material that had been solely su pplied with NH4+, there was no NO3- contamination of the hydroponic system, and the NH4+-induced activity did not occur in tobacco or barley grown und er similar conditions. Western Blot analysis revealed that the induction of NR activity, either by NO3- or NH4+, was matched by NR and nitrite reducta se protein synthesis, but this was not the case for the ammonium assimilati on enzyme glutamine synthetase, Exposure of leaf disks to N revealed that N O3- assimilation was induced in leaves directly by NO3- and NH4+ but not gl utamine, Our results suggest that the NH4+-induced potential for NO3- assim ilation occurs when externally sourced NH4+ is assimilated in the absence o f any NO3- assimilation. These data show that the potential for nitrate ass imilation in C. vitalba is induced by a nitrogenous compound in the absence of its substrate and suggest that NO3- assimilation in C, vitalba may have a significant role beyond the supply of reduced N for growth.