Ammonium assimilation in different plant species as affected by nitrogen form and pH control in solution culture

Abstract

Plant growth, ammonium assimilating enzyme activity, and free ammonium in tissue of different plant species were compared in the presence of either NO3 or NH4 forms of N at two pH levels in solution culture. Tomato and corn growth, but not rice growth, was reduced when NH4 was the sole form of N in the solution culture. Ammonium toxicity was more severe at low than higher pH. Glutamate dehydrogenase activity in roots of the three plant species was higher in the presence of NH4 than NO3 nitrogen. Asparagine synthetase activity was very low in roots of the three species regardless of the N form supplied. Glutamine synthetase activity in roots and shoots of rice was much higher than in tissue of tomato and corn. Glutamine synthetase activity in rice increased sharply in the presence of NH4, and much less free NH4 was detected in green tissues compared to tomato plants. Glutamine synthetase appears to be a key factor to detoxify NH4 in the leaves of rice.