Ammonium influence on nitrogen assimilating enzymes and protein accumulation in suspension cultures of Paul's Scarlet rose

The influence of NH₄⁺ on protein accumulation was examined by growing suspension cultures of Rosa cv. Paul's Scarlet on two defined media. Both contained 1920 μmol of NO₃^? but only one contained 72.8 μmol of NH₄⁺. At the conclusion of a 14-day growth period, cultures grown with NH₄⁺ possessed twice as much protein as cultures grown without NH₄⁺. The influence of NH₄⁺ did not appear to be a substrate effect, since the amount of NH₄⁺ provided accounted for only 10% of the nitrogen recovered in protein. The provision of NH₄⁺ in the starting medium increased the activity (μmol substrate. h^?1· g^?1 fr wt) of glutamate dehydrogenase and glutamate synthase, and reduced the activity of glutamine synthetase. A comparison of the total activity per culture for each of these enzymes with the rate of nitrogen incorporation into protein showed that the enzymatic potential of glutamine synthetase and glutamate dehydrogenase greatly exceeded the actual in vivo rate of nitrogen assimilation through the respective pathways. Thus it was concluded that the availability of either of these enzymes does not limit nitrogen assimilation in rose cells and the fluctuations in their level brought about by NH₄⁺ was of no physiological importance. The activity of glutamate synthase per culture approximated the rate of nitrogen incorporation into protein during early stages of growth, and for that reason may have limited nitrogen assimilation or caused a diversion of nitrogen through the alternative pathway to glutamate catalyzed by glutamate dehydrogenase.