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 NO 3 – in a hydroponic system, growth and N‐assimilation characteristics were similar to those reported for a range of other species. When solely supplied with NH 4 + , however, nitrate reductase (NR) activity dramatically increased in shoot tissue, and particularly leaf tissue, to up to three times the maximum level achieved in NO 3 – supplied plants. NO 3 – was not detected in plant material that had been solely supplied with NH 4 + , there was no NO 3 – contamination of the hydroponic system, and the NH 4 + ‐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 NO 3 – or NH 4 + , 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 NO 3 – assimilation was induced in leaves directly by NO 3 – and NH 4 + but not glutamine. Our results suggest that the NH 4 + ‐induced potential for NO 3 – assimilation occurs when externally sourced NH 4 + is assimilated in the absence of any NO 3 – 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 NO 3 – assimilation in C. vitalba may have a significant role beyond the supply of reduced N for growth.