Abstract: In this work, we aimed to study the effect of foliar application of rare earth elements on protein content and amino acid composition of soybeans and to evaluate the nutritional quality of soybean protein. In this pot experiment, LaCl3, CeCl3 and LaCl3 + CeCl3 solutions of three different concentrations were sprayed on soybean leaves in the seedling and early flowering stages. Ripe seeds were harvested for the determination of protein content using a near-infrared grain analyzer. An automatic amino acid analyzer was used to determine amino acid compositions. The protein nutritional quality of soybean seeds was evaluated by calculating amino acid score (AAS), chemical score (CS), score of ratio coefficient of amino acid (SRCAA) and essential amino acid index (EAAI). The results showed that foliar application of rare earth elements could significantly increase the content of protein, with a maximum increment of 5.17% as compared with control group. The ratio of essential to total amino acids in the different experimental groups was between 32.89% and 35.17%, which was significantly higher in all the experimental groups except for La10 than in the control group. According to AAS values, the first limiting amino acid in the La10 group was valine, while the first limiting amino acids in all other groups were methionine and cystine. The AAS, SRCAA, and EAAI in the 60 mg/L CeCl3 treatment group were 94.05, 84.08 and 82.46, respectively, higher than those in all other groups. On the other hand, the nutritional quality of soybean protein decreased at higher concentration of rare earth elements. The above results show that in the seedling and early flowering stages, foliar application of rare earth elements at the appropriate concentrations can improve the nutritional quality of soybean seeds.

Key words: lanthanum, cerium, soybean, amino acid, nutritional evaluation