Variations in response to ammonium and nitrate by different plant species including corn (Zea mays L.) quires further definition. Corn was grown in nutrient solutions under greenhouse conditions to investigate (1) uptake of ammonium and nitrate from the solutions and (2) the effect of ammonium and nitrate on growth, nitrate reductase activity, and accumulation of total N and nitrate in leaves, stems, and roots at four stages of development (21, 28, 40, and 47 days after planting). Uptake was determined by following the disappearance of ammonium-N and nitrate-N from nutrient solutions in which N was supplied in various combinations and rates. The nutrient solution treatments were as follows (ppm NO3--N/ ppm NH4+-N): 0/50, 0/100, 0/150, 50/0, 100/0, 150/0, 25/75, 50/50 and 75/25.
Fresh and dry weights per plant generally increased more rapidly when plants were provided 100 ppm of N as a combination of nitrate-N and ammonium-N (25/75, 50/50, or 75/25) than wherein 100 ppm of either nitrate-N (100/0) or ammonium-N (0/100) was supplied alone.
When both nitrate-N and ammonium-N were supplied, uptake rates of the two forms were similar. Uptake of nitrate was not retarded by the presence of ammonium, but assimilation of nitrate into organic N was retarded by ammonium. When both forms of N were absorbed, ammonium was used preferentially for synthesis of amino acids and protein.
Nitrate reductase activity was localized principally in the leaf blades and tended to be highest in plants provided a combination of nitrate-N and ammonium-N. Total N in the leaf and root tissues generally was higher in plants grown on ammonium alone or in combination with nitrate than in plants grown on nitrate alone. More nitrate accumulated in stem tissue than in leaf or root tissues. Amino acid composition of leaf tissue was not affected by the N treatments.