In recent years, the application of natural substances on crops has been intensified in order to increase the resistance and yield of the soybean crop. Among these products are included plant biostimulants that may contain algae extracts, amino acids and plant regulators in their composition. However, there is little information on the isolated effect of each of these constituents. The objective of this research was to evaluate the effect of the application of isolated amino acids on the antioxidant metabolism of the soybean crop. Experiments were carried out in a greenhouse and in the field with the application of the amino acids glutamate, phenylalanine, cysteine, glycine in seed treatment and foliar application at V4 growth stage. Antioxidant metabolism constituents evaluated were superoxide dismutase, catalase, peroxidase, hydrogen peroxide content, proline and lipid peroxidation. In addition, resistance enzymes as polyphenol oxidase and phenylalanine ammonia-lyase (PAL) were evaluated. In both experiments, the use of cysteine, only in seed treatment and in both seed treatment and foliar application increased the activity of the enzyme PAL and catalase. Also in both experiments, the use of phenylalanine increased the activity of the enzyme PAL when the application was carried out as foliar application or both in seed treatment and foliar application. In the field experiment, the application of glutamate led to an increase in the activity of the catalase and PAL enzymes for seed treatment and foliar application. The use of the set of amino acids was only efficient in foliar application, which led to a greater activity of the enzymes peroxidase, PAL and polyphenol oxidase. The other enzymes as well as lipid peroxidation and hydrogen peroxide presented different results according to the experiment. Therefore, glutamate, cysteine, phenylalanine and glycine can act as signaling amino acids in soybean plants, since small doses are enough to increase the activity of the antioxidant enzymes.

On-farm soybean yield has increased considerably in the last 50 years in southern Brazil, but there is still little information about how selection and breeding for yield increase has changed the agronomic attributes of cultivars. The objectives of this study were to evaluate the changes in soybean yield, seed oil and protein concentration, and changes in plant attributes that might be associated with yield improvement of 26 soybean cultivars released over the past 50 years in southern Brazil, sown simultaneously in a common field environment for two growing seasons. The average rate of yield gain was 45.9 kg ha⁻¹ yr⁻¹ (2.1% ha⁻¹ yr⁻¹), mainly due increased seed number per area and harvest index. Over year of cultivar release, cultivars became less susceptible to lodging, as well as plant mortality reduced. Meanwhile, the seed oil concentration increased, and seed protein concentration decreased, which could have negative consequences for soybeans use and requires further attention for breeding of future cultivars. Breeders have successfully contributed to the annual rate of soybean yield increase in southern Brazil. By our results, as well as the official on-farm production data, there is no evidence of soybean yield reaching a plateau in the near future in southern Brazil.

Nitrogen (N) is the most extracted and exported element by the soybean crop. In high yield tropical environments with irrigation, little is known about N accumulation in different soybean plant organs as well as the N balance. The objective of this study was to characterize soybean growth, N accumulation in plant organs, N balance, and N gap in a high yield tropical environment. This study was performed in a homogeneous field, in a soil with low organic matter, with 20 kg ha^-1 of N, under furrow fertilization. Evaluations were performed ten times, temporally distributed from emergence to senescence. The soybean cultivar used was ‘RK7518 IPRO’ and was sown with row spacing of 0.45 m and a seeding rate of 300,000 plants ha^-1. Plant N partition, N from the biological N fixation (BNF), grain yield, crop harvest index (HI), N harvest index (NHI) with and without root contribution were evaluated. Also, at the grain filling stage the N gap was evaluated from the soil by difference between whole plant accumulated N and the amount of N from BNF. The average grain yield was 6,470 kg ha^-1 and leads to a negative partial balance of N of -33.4 and -42.8 kg_[N] ha^-1 with and without roots, respectively. The N gap from the soil was 231.7 kg_[N] ha^-1. It is recommended to adopt techniques that increase the efficiency of BNF and the soil N accumulation to balance these production systems in the medium to long term.