INFLUENCE OF THE FORM OF NITROGEN ON AMMONIUM, AMINO-ACIDS AND N-ASSIMILATING ENZYME-ACTIVITY IN MAIZE GENOTYPES

An understanding of nitrogen (N) interactions with plant growth and me tabolism is important in order to increase the efficiency of fertilize r amendments for crop production. This study was initiated to evaluate the effect of the form of N and buffering with calcium carbonate (CaC O3) on the activity of ammonium (NH4) assimilating enzymes in a maize hybrid, a GDH-deficient mutant, and the N6 normal inbred the mutant wa s derived from. Plants were grown in a growth chamber in sand culture with 10 mM of the different N forms (nitrate (NO3), NH4, ammonium nitr ate (NH4NO3), and NH4 plus CaCO3). Hybrid plants grown in the unbuffer ed NH4 were much smaller than those grown in NO3 as the only source of N, but had comparable growth with each source of nitrogen if the pH w as controlled. In contrast, a glutamate dehydrogenase-deficient mutant (GDH) inbred and its normal Control grew almost as well on both NH4 t reatments as on NO3. GDH activity was higher in plants grown with NH4 than NO3, and it was almost nil in tissue of the GDH deficient mutant. Glutamine synthetase (GS) activity was lower in the unbuffered NH4 tr eatment (acid pH) and increased significantly when CaCO3 was added to the NH4. Ammonium, as a sole source of N, induced a higher concentrati on of free ammonium and most free amino acids in plant tissues than NO 3.