Effects of nitrogen rate and genotype on seed protein and amino acid content in canola

Canola (Brassica napus L.) meal is widely used in animal feed as a protein source, and its quality relies on protein and amino acid content. However, little information is available regarding amino acid regulation in canola seed with nitrogen (N) application. The present study is aimed to evaluate the effect of N rate and genotype on canola seed amino acid concentrations under field conditions. A split-plot design comprising four N rates (0, 120, 240 and 360 kg N/ha) and three genotypes differing in seed protein content were used in 2010/11 and 2011/12. The results showed that increasing N rate decreased seed oil content linearly but increased seed protein content in all of the genotypes. The total amino acid concentration and absolute concentrations of individual amino acids in canola seed also improved significantly with the N rates in all of the genotypes. Regarding the proportions of amino acids, a group that included glutamic acid (Glu), proline (Pro) and arginine (Arg) dominated and occupied > 0·30 compared with other amino acids. The ratio of amino acids in this group increased by 8·3% with 360 kg N/ha compared with the control. However, the proportions of the other amino acids showed negative responses to the N rates. The results of regression analysis of the responses of individual amino acids to N rate indicated that Glu, Pro and Arg had a greater improvement potential with application of N fertilizer, as revealed by higher slopes in the linear equations compared with the other amino acids. Additionally, the concentrations of sulphur-containing amino acids, methionine and cysteine, were also a potential target for improving with N application because these are always deficient in major crops. In conclusion, N application cannot only improve seed protein content but also enhance deposition of amino acids such as Glu, Pro and Arg.