Proteins and Amino Acids in Grain of Maize Grown with Various Levels of Applied N

Cereals provide a major source of protein in food chains. Yet, particularly as compared with associated yield responses, relatively little information is available about possible effects of N supply on protein quality in cereal grains. Maize (Zea mays L.) was grown in a series of field plots which received various rata of applied N as ammonium sulfate. The site was in an area mapped as Yolo (Typic Xerorthents) fine sandy loam, and rates of N application of 0, 90, 180, and 360 kg N/ha were used on quadruplicate plots. The entire plot area had been fertilized with K, and sufficient water was applied during the season to meet evapotranspiration needs. Total N in the grain was determined by a Kjeldahl method, amino acids with an amino acid analyzer after subjecting samples to conditions which would hydrolyze protein, and protein fractions were extracted using a modified Osborne-Mendel procedure. Yields of grain were nearly doubled to approximately 10,700 kg grain/ha by application of N, with no significant difference between yield means for the 180 and 360 kg N/ha rates. Concentrations of crude protein (N ✕ 6.25) ranged from about 6% in grain from plots receiving no added N or 90 kg N/ha, to nearly 10% (mean of concentrations in grain from plots receiving 180 and 360 kg N/ha, which did not differ significantly). The concentrations in the grain protein of tryptophan, lysine, glycine, arginine and threonine were decreased, and the concentrations of alanine, phenylalanine, tyrosine, glutamic acid and leucine increased by applications of N. The first increment of N (90 kg/ha), which had the greatest effect on grain yield, did not have significant effects on the concentrations of tryptophan, lysine or alanine in the protein. The differences in concentrations of the amino acids in the grain from the different N treatments were in part, but not wholly, accountable for by differences in the concentrations of the prolamine (zein) protein fraction. Thus, for example, serine, which is present in relatively high concentration in zein, should have been present at higher concentration in the grain from the N-fertilized mainze. Valine, histidine and methionine, on the other hand, might be expected to be present at lower concentrations with this treatment. The yields of grain crude protein were approximately 2-fold greater with applications of 180 or 360 kg N/ha than with no applied N. Treatment effects on yields of amino acids ranged from nil in the case of tryptophan to about four-fold greater amounts of leucine in grain from plots receiving the two highest rates of N.