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

Nitrogen assimilation was studied in the deciduous, perennial climber Clematis vitalba. When solely supplied with NO₃/^- in a hydroponic system, growth and N-assimilation characteristics were similar to those reported for a range of other species. When solely supplied with NH₄/⁺, however, nitrate reductase (NR) activity dramatically increased in shoot tissue, and particularly leaf tissue, to up to three times the maximum level achieved in NO₃^- supplied plants. NO₃/^- was not detected in plant material that had been solely supplied with NH₄/⁺, there was no NO₃/^- contamination of the hydroponic system, and the NH₄⁺-induced activity did not occur in tobacco or barley grown under similar conditions. Western Blot analysis revealed that the induction of NR activity, either by NO₃/^- or NH₄/⁺, was matched by NR and nitrite reductase protein synthesis, but this was not the case for the ammonium assimilation enzyme glutamine synthetase. Exposure of leaf disks to N revealed that NO₃/^- assimilation was induced in leaves directly by NO₃/^- and NH⁴/⁺ but not glutamine. Our results suggest that the NH₄/⁺-induced potential for NO₃/^- assimilation occurs when externally sourced NH₄/⁺ is assimilated in the absence of any NO₃/^- assimilation. These data show that the potential for nitrate assimilation 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.