A review of the role of amino acids in organic agriculture

Uncontrolled use of chemical fertilizers not only causes economic problems but also imposes significant damages to the environment. The accumulation of pollutants in soil, water resources, plants, and the food chain of both humans and animals has adverse effects. Furthermore, climate change globally has led to various stresses on agricultural crops.

It seems that growth stimulants play a crucial role in reducing the consumption of chemical substances in agriculture and enhancing plant resistance to various stresses. These compounds include amino acids, humic substances, seaweed extracts, and other similar substances. Zargreen organic liquid fertilizer is an example of such compounds, containing various types of amino acids. It improves and increases the stability of crop production, enhances plant resistance against abiotic stresses, and ultimately improves the quality of the produced crops.

Keywords: Amino acids, organic, Zargreen, stress, growth stimulant.

Organic agriculture, known by various names such as biological agriculture, biodynamic agriculture, agroecological farming, permaculture, and natural farming, is a type of agriculture that is regulated and based on specific ecological criteria. The primary goal of organic agriculture is to achieve sustainable production without the need for maximum yields. In this approach, human factors and farming communities are considered as an inseparable system. According to global statistics on organic agriculture published by the International Federation of Organic Agriculture Movements (IFOAM) and the Research Institute of Organic Agriculture (FiBL) in 2015, the largest areas under organic cultivation are found in Oceania (4.32%), Europe (25.29%), Latin America (18.21%), Asia (8.57%), North America (0.28%), and Africa (0.53%). In Iran, the reported area under organic cultivation was zero in 2005, but it increased to 43,332 hectares in 2011 (Khoshkhoi, 2016).

In recent years, experts in agriculture have focused on improving the quality, sustainability of cultivation systems, and reducing production costs by decreasing input use. This has led to increased attention to plant growth stimulants in sustainable agriculture. The term “plant growth stimulant” was first used by horticulturists to describe substances that promote plant growth but are not classified as nutrients, soil improvers, or pesticides. Plant growth stimulants can enhance growth and development throughout the plant’s life cycle, from seed germination to maturity. These stimulants can include improved metabolic efficiency to enhance performance and product quality, increased plant resistance to abiotic stress, facilitation of nutrient uptake, transport and utilization, increased water use efficiency, improved physical and chemical properties of soil, and enhanced growth of soil microorganisms. They are usually applied to plants along with conventional fertilizers to increase their efficiency (Heckman, 1994). Many active compounds found in plant growth stimulants exist in very low concentrations, and sometimes, the levels fall below the detection limit of measurement methods. However, they can have significant biological effects. The complexity of extracts and the wide range of molecules present in these stimulants make the identification of active compounds challenging (Guinan et al., 2013).

In recent years, the use of plant growth stimulants has been increasing worldwide. The Association of Plant Growth Regulator Industries in Europe reported that in 2012, these stimulants were used on 6.2 million hectares of land on this continent. The reasons for the increased use of these stimulants include agricultural and environmental policies aiming for sustainable agriculture, simultaneously increasing resource efficiency and performance. Another reason is the high investment of commercial companies (between 3% and 10% of annual turnover) in research and development of these materials (Garnett et al., 2013). Plant growth stimulants include humic substances, seaweed extracts, amino acids, and other nitrogen-containing compounds, microbial inoculants, mineral substances like essential elements, inorganic salts such as phosphates, antitranspirants, vitamins, chitin, chitosan, and poly- or oligosaccharides (Du Jardin, 2012).

Liquid organic fertilizer Zargreen is based on plant derivatives and is environmentally friendly. With various free amino acids (6%), it provides essential nutrients for plants such as nitrogen (3%), phosphorus (2.5%), and potassium (2%). With high organic matter content (30%) and organic carbon (11%), Zargreen is considered one of the organic fertilizers (Zargreen, 2016).

Amino acids and peptide compounds are produced through enzymatic and chemical hydrolysis of proteinaceous materials, agricultural and industrial waste, plant sources (crop residues), and animal residues. Studies have shown that by tagging amino acids, it has been demonstrated that plant roots are capable of absorbing amino acids and peptides (Nardi et al., 2016).

Results presented in scientific sources demonstrate the combined effects of using these substances and their potential for improving performance and nutrient absorption. In tomato plants, these effects have been observed in terms of increased height, flower and fruit numbers per plant, and enhanced fruit yield (number or weight of fruits) (Parrado et al., 2008). Studies on amino acids have also shown that these compounds may play a role in regulating nitrogen uptake (reducing nitrates, ammonium influx, and transporter expression) by plant roots, as observed in barley plants (Miller et al., 2007). Plants can utilize free amino acids according to their growth conditions, environmental stress, and gene expression for protein or enzyme production by ribosomes. However, peptide compounds, poly-peptides, and polyamines often induce osmotic pressure decline in the cytoplasm, which can have reverse effects when used excessively in foliar application. Hence, it is crucial to use more free amino acids in such cases. Plants respond to environmental stresses by adapting and establishing physiological mechanisms to cope with the stressors. Besides biochemical processes, plants combat stress by producing various enzymes. With stress induction, genes are expressed to produce the required enzymes, and ribosomes use free amino acids to carry out this process. Cationic metal can act as a co-factor to activate enzymes. The enzymes produced alter the physiological behavior of the cell, providing the means to overcome the stress. Thus, thermal, light, free radicals, and other stresses can be controlled through this mechanism, and the presence of free amino acids is essential for stress relief in plants (Ghaibi, 2018). Glycine betaine and proline are amino acids that act as osmoprotectants, stabilizing proteins, enzymes, and membranes against non-physiological effects of high salt concentration and extreme temperatures. The application and accumulation of glycine betaine and proline have increased plant resistance to non-biological stresses in maize, barley, soybean, alfalfa, and rice, as they reduce the negative effects of reactive oxygen species. Other amino acids also play a role in resistance to non-biological stresses. Glutamate or ornithine (proline precursor) can be effective in enhancing resistance to salinity stress (Liang et al., 2013).

One of the modern methods for controlling plant pests and diseases, especially for organic crop production, is the use of natural or green materials and compounds of microbial and plant origin. In this context, the role and importance of natural plant compounds in controlling various plant diseases, including fungal, bacterial, viral, and nematode diseases, are prominent and significant. Therefore, many countries utilize new technologies to develop and formulate non-chemical pesticides, including plant-based pesticides, for integrated control of important plant diseases (Hassanzadeh, 2005). Hosseini and colleagues (2014) investigated the effect of amino acid application on inducing resistance in citrus trees against bacterial canker disease. The tested trees were treated with amino acids, including L-arginine, L-methionine, and L-ornithine, and after 48 hours, they were inoculated with Xanthomonas citri subsp. Citri bacteria. Based on the phenotypic results, antioxidant enzyme activity, and molecular study of stressed plants, the resistance of plants treated with the amino acid methionine significantly increased, and it also led to a reduction in the severity of the disease.

During their growth, plants encounter various environmental stresses, each of which can have different effects on the growth, metabolism, and performance of the plant, depending on the plant species’ sensitivity and growth stage. Drought is one of the most important environmental factors that significantly reduce the growth and performance of many agricultural, horticultural, and medicinal plants, especially in arid and semi-arid regions, causing significant damage to plants worldwide every year. One of the methods to reduce the adverse effects of drought stress is the breeding of drought-tolerant crop plants. However, improving the tolerance of crop plants to environmental stresses requires an understanding of the physiological and genetic mechanisms controlling plant growth and development in different stages. One of the common responses of plants to drought stress is the synthesis and accumulation of low-molecular-weight compounds called osmoprotectants. These compounds reduce the intracellular osmotic potential and help maintain cellular turgor. Non-organic ions, organic ions, soluble carbohydrates including polyols (sugars, alcohols), amino acids (such as proline), and quaternary ammonium compounds like glycine betaine are among the osmoprotectants that accumulate in plant cells under moisture stress conditions (Kadkhodayi et al., 2014). To investigate the application of the amino acid glycine in reducing the effects of water deficiency stress in the plant (Hyssopus officinalis L.), an experiment was conducted. The experimental treatments included irrigation regimes at three levels of irrigation (25%, 50%, and 75% of available water depletion from the soil) and foliar spray treatments at two levels of distilled water (control) and glycine amino acid during two stages: 1- at the vegetative growth and flowering, and 2- at the flowering stage. The results showed that the time of foliar spray had a significant effect on canopy cover area, leaf chlorophyll content, and flower fresh weight. Severe water stress (75% of available water depletion) caused a decrease in canopy cover area by 14%, the number of flowering branches by 36%, flower fresh weight by 32%, relative leaf water content by 5.0%, and leaf chlorophyll by 3.3% compared to the control. However, foliar application of glycine amino acid led to an increase in canopy cover area by 72.8%, flower fresh weight by 23.7%, relative leaf water content by 56.6%, and leaf chlorophyll by 44.3% compared to the control. Overall, the use of glycine amino acid in Hyssopus officinalis effectively mitigated the adverse effects of water stress (Hosseini et al., 2018). In another study, the effects of proline application on drought tolerance in faba bean plants were investigated. Irrigation was performed at two levels, 50 mm evaporation pan class A (normal conditions), and 100 mm evaporation pan class A (water stress), along with the application of proline at seven levels. The application of proline had a positive effect on the measured traits under both normal and water stress conditions. The highest seed yield (1008 g/m²) was obtained in the treatment with irrigation after 50 mm evaporation and proline foliar spray at the 6-leaf and flowering stages, while the lowest seed yield (489 g/m²) was observed in the treatment with irrigation after 100 mm evaporation and without proline application (Ardabili et al., 2013). Natural liquid fertilizer Zargreen, containing various amino acids and organic compounds, plays an important role in inducing plant resistance under water-deficient conditions and increasing both quantitative and qualitative performance of agricultural products.

Nowadays, chemical fertilizers are widely used as the most economical tool to achieve maximum production per unit area in agriculture. However, the use of chemical fertilizers, especially phosphate fertilizers containing heavy metal impurities like cadmium, may have adverse effects on plant uptake and transfer. Due to the high price of wheat, farmers often use large amounts of chemical fertilizers and excessive water to maximize agricultural yields. Chemical fertilizers, especially phosphate fertilizers, contain heavy metals that can contaminate the soil, reduce microbial activities, and potentially be absorbed by plants, eventually entering the food chain of humans and animals (Pormoghaddas & Zafarzadeh, 2016). To investigate the effect of reducing the use of chemical fertilizers on the quality of tea leaf compositions, research was conducted. The results showed that reducing the use of chemical fertilizers could increase the potassium content and soil organic matter, polyphenols, and water extracts in tea leaves. However, the application of chemical fertilizers in tea gardens led to a significant reduction in caffeine content (Gao et al., 2020). Studies were also conducted to improve the uniformity of cabbage production and prevent nitrate accumulation resulting from the use of mineral nitrogen by using amino acids. For this purpose, a combination containing 11.3% amino acids, 4% nitrogen, and 25% plant-based organic materials was used. The application of amino acids prevented nitrate accumulation in plants treated with ammonium nitrate. Moreover, amino acid foliar spraying significantly increased the antioxidant capacity compared to the control. Therefore, the use of amino acid compounds as an alternative to mineral fertilizers to improve cabbage production uniformity, minimize nitrate in plants, without negatively affecting other nutritional components or performance, is of great importance (Tsouvaltzis et al., 2014). Lin and colleagues (2019) investigated the long-term effects of chemical and organic fertilizers on the properties of tea and rhizosphere soil in tea gardens. The results showed that organic fertilizer treatment significantly reduced the content of copper, lead, and cadmium in the rhizosphere soil. Furthermore, the results indicated that the use of organic fertilizer led to a significant reduction in the content of arsenic, cadmium, and lead in tea leaves. Additionally, organic fertilizer significantly increased the content of amino acids in tea. Consequently, the use of organic fertilizer significantly increased the relative abundance of microbial compositions and beneficial bacteria in the rhizosphere, improved the quality of tea, and reduced heavy metal content in the rhizosphere soil and tea leaves. Zargreen liquid organic fertilizer, as a plant growth stimulator, can play a significant role in improving crop production and its sustainability by enhancing the efficiency of chemical fertilizers and preventing environmental pollution, while also increasing plant resistance to abiotic stresses.

Food security alongside environmental preservation has become a crucial global issue in recent decades. Determining the optimal fertilizer levels to achieve high yields is one of the key objectives of nutritional research. The application of growth stimulants sustains intensive production systems, attributed to the improvement of soil quality and potentially releasing nitrogen as needed by plants. Recent investigations indicate a notable increase in research on the application of these substances in both research centers and universities. Growth and performance characteristics of plants have received more attention, with fewer studies focusing on physiological attributes. Nevertheless, a wide variety of agricultural, horticultural, and medicinal plant products have been observed with the application of various humic, fulvic, and amino acids, as well as their combined treatments.

This article demonstrates that plant growth stimulants can lead to improved crop production and sustainability by reducing the use of chemical fertilizers and preventing environmental pollution. Simultaneously, they enhance plant resistance to abiotic stresses and improve the internal and external quality of the produced crops. The industrial research group, Farhangian Zarnam, has taken steps in this field by producing Zargreen liquid organic fertilizer while adhering to the essential principle of supplying natural fertilizers for agricultural production and scientifically managing the production and consumption of various fertilizers to increase yield and improve the quality of agricultural products while promoting the production of healthy products.

Authors:
1. Ali Nezhadranjbar
2. Arash Ershadi
3. Mehdi Jafari Asl
4. Mehdi Amini

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A review of the role of amino acids in organic agriculture

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