Biometric, Nutritional and Productive Responses in Soybean Crops Induced by Foliar Application of Amino Acids

1. Introduction

Amino acids can be defined as organic compounds composed of an amine group (NH₂) and a carboxylic functional group (COOH), which serve to form proteins and act as precursors of substances that regulate plant metabolismo (Teixeira et al., 2017; Gluhić, 2020). They are involved in metabolic pathways, processes, and important plant functions, such as physiological and biochemical pathways, influencing numerous physiological processes (Winter et al., 2015; Amir et al., 2018; Yang et al., 2018; Urbano-Gámez et al., 2020; Chen et al., 2022).

The use of amino acids in agriculture is relatively recent and aims to enhance the efficiency of agricultural production. The application of these substances gains importance as the genetic potential of crops is increased (Ramos et al., 2024), enabling resilience to biotic and abiotic stresses, supporting the process and maintenance of photosynthesis, among other benefits. When amino acids are absorbed by plants, they stimulate chlorophyll formation, indoleacetic acid (IAA) production, vitamin synthesis, and the activation of numerous enzymatic processes. Additionally, they have positive effects on plant growth and yield yield (Wang et al., 2014; Mondal et al., 2015; Sadak; Abdelhmid; Schmidhalter, 2022).

Glycine is a precursor for chlorophyll synthesis and an important metal chelator; arginine plays a role in root development, increases nutrient solubility and absorption, and is the main amino acid translocated in the phloem(Sadak, Abdelhmid, Schmidhalter, 2022). Studies show that proline, lysine, methionine, and glutamic acid are essential for pollen germination and subsequent fruiting (Teixeira et al., 2022). Some amino acids are involved in lignin synthesis, helping to reduce water loss in plants and mitigating damage caused by water deficit in cultivation (Gins et al., 2017).

Sadak, Abdelhmi, and Schmidhalter (2022), demonstrated that the application of amino acids significantly increases growth variables, fresh weight of seedlings, and productive yield. Santos Costa et al. (2019), Gluhić (2020), and Liu et al. (2008), showed in their studies that foliar application of amino acids increases nutrient levels, such as nitrogen, in the aerial parts of plants. Abo Sedera et al. (2010) reported that the use of amino acids in strawberries significantly increased nitrogen, phosphorus, and potassium content in plant tissues compared to the control treatment.

This study tested the hypothesis that amino acids improve productive performance, biometric growth variables, and macro- and micronutrient levels in plants. The aim of this study was to evaluate the biometric, nutritional, and productive parameters of soybean plants subjected to amino acid application.

2. Materials and Methods

The experiment was conducted under field conditions in an area designated for agricultural experimentation, with sowing carried out on November 10, 2019, in Rio Verde, State of Goiás, Brazil (17°44'20.88''S and 50°57'55.79''W, at an altitude of 860 m) using a no-tillage system.

The soil in the experimental area is classified as dystrophic Red Latosol (LVd) (Santos et al., 2018). Soil preparation included subsoiling and two subsequent leveling operations after the application of lime. Correction and planting fertilization were carried out based on soil analysis (Table 1) and according to the recommendations of Sousa and Lobato (2004).

The quantities and fertilizers used for both soil correction and planting are described in Table 2.

The phytosanitary management was carried out through foliar application of chemical products for the control of diseases, pests, and weeds (Table 3).

The statistical design was a randomized block design with four replications and 20 treatments. The experimental plots consisted of 8 rows spaced 0.45 m apart and 10 m in length, with 2 m borders between plots and 0.90 m between blocks. Different amino acids were tested (Table 4).

The soybean sowing was carried out using a 5-row Jumil mechanical planter, equipped with a double-disc furrow opener mechanism, with a plant population of 280,000 plants per hectare (Glycine max (L.) Merrill – cultivar BRASMAX BÔNUS 8579 IPRO).

The dose of 20 g ha⁻¹ was determined in a previous study. The treatments were applied at two phenological stages (V4 + R1), the main period when farmers carry out phytosanitary management. Applications were made using a backpack sprayer with CO₂ pressurization, equipped with a 2-meter boom and four TT 110.02 spray nozzles (0.45 m between nozzles), applying a spray volume equivalent to 100 L ha⁻¹. Environmental conditions were continuously monitored to ensure favorable conditions, with an average temperature of 25ºC, average relative humidity of 60%, and average wind speed of 2.5 km h⁻¹.

The following variables were evaluated and quantified in the central rows of each plot: plant height, stem diameter, number of internodes, plant height, number of leaves, number of branches, and number of pods, measured at the R6 phenological stage. Stem diameter was determined with the aid of a digital caliper at the base of the plants, and expressed in millimeters. The number of internodes, leaves, branches, and pods were obtained through counting. Plant height was measured using a tape measure, positioned from the base of the plant to the tip of the last trifoliate. For the biometric data, two plants per experimental plot were collected and quantified.

At the time of soybean flowering (R2), the third fully developed trifoliate from the apex of the plant was collected from about 20 plants per plot. These trifoliates were collected for the determination of nutritional content (N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn) following the methodology described in Malavolta et al. (1997). To do so, the leaves were dried in a forced-air oven at an average temperature of 65ºC until they reached a constant weight, and then ground in a Willey mill for the determination of nutritional content.

At the end of the cycle, the plants were desiccated, and subsequently, the 100-grain weight and grain yield were quantified. The grain yield was determined by harvesting and threshing the plants. Based on the degree of maturation, the moisture content of the total grain mass was determined and corrected to 13% moisture, and the values were extrapolated to kg ha⁻¹.

The data were subjected to analysis of variance (p<0.05), and significant cases were further analyzed using the Scott-Knott mean test (p<0.05), using the statistical software SISVAR® (Ferreira, 2014).

3. Results and Discussion

The variables plant height and soybean grain yield showed effects due to the foliar application of amino acids. However, the parameters related to the foliar content of macro- and micronutrients were not influenced by the foliar application of amino acids (Table 5).

The results observed by Dörr et al. (2018) support the findings of this study, as the authors did not observe significant variations in biometric parameters such as the number of internodes, leaves, branches, and pods. Amino acids are not considered as nutrient sources for plants, as the goal of their application is not to supply the essential substances needed for plant growth, but rather to activate the physiological metabolism of the plants, playing an important anti-stress (Hammad; ALI, 2014; Mondal et al., 2015; Dörr et al., 2019).

The foliar application of isolated amino acids in soybean plants promoted increases in plant height. The use of Aspartic Acid, Arginine, Cysteine, Cystine, Taurine, Tyrosine, Threonine, Tryptophan, and Valine resulted in an average increase of 20.63% (0.13 m) compared to the Control (Figure 1).

Means followed by the same letter in the columns do not differ among themselves (Scott-Knott at 5%).

Cystine, Phenylalanine, Isoleucine, Leucine, Methionine, Ornithine, Proline, Taurine, Threonine, and Valine promoted an average increase of 22.77% (691.9 kg ha⁻¹) in soybean grain yield, compared to the Control and the other amino acids (Figure 2).

Cystine, Phenylalanine, Isoleucine, Leucine, Methionine, Ornithine, Proline, Taurine, Threonine, and Valine were more efficient than the other amino acids, as well as the Control treatment.

Amino acids are precursors of substances that regulate plant metabolism and the production of enzymes such as superoxide dismutase, catalase, and peroxidase (Sharma; Bhardwaj, 2014; Gluhić et al., 2020).

Santos et al. (2014) observed that bio stimulants containing amino acids promoted increases in grain yield, both in seed and foliar applications. The results found by Teixeira et al. (2022) corroborate those of this study, as they observed increases in coffee yield when amino acids were applied foliarly.

Amino acids are directly related to the increased stabilization of cell membranes and nucleic acids, the maintenance of the ideal cellular redox potential, as well as the removal of reactive oxygen species (ROS). In addition, they promote a decrease in cytoplasmic acidosis and help maintain the NADP+/NADPH ratio compatible with cellular metabolismo (Alhasawi et al., 2015; Nazar et al., 2015).

Considering the potential for improvement in the vegetative and productive performance of soybean, the use of amino acids should be further investigated, especially under environmental conditions more susceptible to water deficit. It is also important to highlight that the use of amino acids is a relatively low-cost technology, easy to apply, and non-polluting, making it sustainable from both an economic and environmental perspective.

4. Conclusions

The foliar application of amino acids improves the productive performance and plant height of soybean.

The foliar application of Aspartic Acid, Arginine, Cysteine, Cystine, Taurine, Tyrosine, Threonine, Tryptophan, and Valine induced responses in plant height.

The use of Cystine, Phenylalanine, Isoleucine, Leucine, Methionine, Ornithine, Proline, Taurine, Threonine, and Valine promoted increases in grain productivity.

The foliar application of amino acids does not promote changes in the macro and micronutrient content in soybean leaves.

Based on the results, the conclusion is that for better physiological performance, a biofertilizer formulation should contain Cystine, Taurine, Threonine, Tryptophan, and Valine, as these amino acids showed positive effects on the variables of soybean plant height and grain productivity.