In recent years, in the process of exploring new products of plant protection, biostimulant has gradually gained the attention of the research community and the industry. Plant biostimulants are substances or microorganisms used to promote plant growth and development, alleviate abiotic stress and improve crop quality. It mainly includes humic acid, chitin and its derivatives, seaweed extract, etc. With the increasing use of biostimulant in agricultural production, it has been called plant growth promoter, biological active agent, plant growth substance, soil conditioner, growth regulator, etc. The term biostimulant has only recently attracted people's attention, but its concept has not been unified in academic circles.
1 Definition and function of biostimulants
In 2012, the European Biostimulants Industry Consortium (EBIC) defined plant biostimulant as a substance containing certain components and/or microorganisms that regulate physiological processes in plants when applied to their leaves or rhizosphere. Such as beneficial to the absorption of nutrients, resistance to abiotic stress and improve crop quality, etc.
The target of biostimulant is the crops themselves and the soil environment, mainly through a variety of ways to affect the whole life cycle of crops from seed germination to harvest maturity. Compared with traditional chemical pesticides and fertilizers, the functions of biological stimulants are different as follows.(1) promote plant growth by enhancing the absorption and transport of nutrients;(2) plant resistance and stress resistance can be improved by enhancing plant immunity;(3) regulate and improve the water balance in plants;(4) improve the physical and chemical properties of the soil, protect and improve the soil, and promote the growth of soil beneficial microorganisms;(5) improve the quality of agricultural products (sugar and color) and extend the storage time, etc.
The function of plant biostimulant is similar to that of plant growth regulator in traditional concept, but it is different. Compared with plant growth regulators, the sources of biostimulant are more extensive and the functions are more varied. Biostimulant is not only used on plants, but also can act on soil and soil microorganisms. More importantly, biological hormone does not change the original metabolic pathway of plants by improving the metabolic process of plants.
2 The main types of biostimulants
Currently, the recognized plant biostimulant mainly includes the following five categories: humic acid, seaweed extract, proteolytic substance and amino acid, chitin, chitosan and its derivatives and microbial agents.
Humic acid is an important component of soil organic matter, which is formed in soil, animal manure, low-rank coal (peat, lignite, weathered coal, etc.), as well as agricultural by-products and waste disposal. Humic acids, as plant biostimulants, have a variety of physiological functions, such as enhancing the absorption of nutrients, improving the rhizosphere environment of plants, improving soil structure and fertility, accelerating the metabolism of plants, promoting plant growth, improving plant stress resistance and reducing the occurrence of diseases and pests. Humic acid, as a biostimulant, promotes plant growth mainly by strengthening root development. Humic acid can also enhance plant resistance to a variety of biological stresses.
By regulating the metabolism and physiological function of crops, seaweed extract can promote the growth of crop roots, increase the biomass and thus increase the yield of crops, alleviate diseases and pests, prevent freezing and drought and other abiotic adversity, and also improve the quality of crops. Algal extracts promote plant growth by improving the rhizosphere soil environment of plants. Seaweed polysaccharides can also affect the distribution of rhizosphere microbial flora of plants, stimulate the beneficial bacteria in the soil to better function on the plant body, and inhibit the proliferation of pathogenic microorganisms. Similar to humic acid, alginate extract also contains a large number of polyanionic compounds, which can chelate heavy metal cations in soil and play a role in remediation of heavy metal contaminated soil.
Proteolysis is mainly the product of enzymatic, chemical or thermohydrolysis product of plant and animal origin, as well as amino acids, peptides, protein mixtures and some nitrogen-containing compounds (e.g., betaine, polyamines, non-protein amino acids) obtained by hydrolysis of industrial and agricultural by-products. By absorbing and transporting proteolytic amino acids and small peptides, plant roots regulate plant metabolism and physiological and biochemical reactions, promote seed germination and root development, enhance nutrient absorption, improve plant stress resistance and thereby increase crop yield.
Chitin is a component of the shell of Marine crustaceans and the cell walls of many fungi. It is the linear polysaccharide formed by N-acetylglucosamine through the β-1,4 glycosidic bond. Chitosan is the product of chitin deacetylation, while chitosan oligosaccharide is the degradation product of chitosan. Chitin, chitosan and its derivatives, as biological stimulants, can improve nutrient absorption by increasing plant cell permeability, promote root development, improve plant photosynthesis, regulate crop growth and induce plant disease resistance. In addition, chitosan can also inhibit the growth of pathogenic bacteria in soil, and effectively improve the structure of soil aggregates, so as to improve the yield and quality of crops.
Microbial fungicides are a kind of beneficial fungi and bacteria rich in specific microorganisms living, which can increase the supply of plant nutrients or promote plant growth, increase yield, improve the quality of agricultural products and agricultural ecological environment through the life activities of microorganisms contained in them. The application of microbial agents can significantly increase the microbial biomass in the soil, and the increased activity of these microorganisms can promote the enhancement of soil enzyme activity and the decomposition and release of soil insoluble mineral nutrients. At the same time, these microorganisms can also secrete plant hormones to promote crop growth. In addition, beneficial bacteria can form dominant populations around the root system of crops, inhibit or antagonize the growth and reproduction of harmful pathogens, and reduce the degree of crop diseases.
3 Regulation of biostimulants in European and American markets
At present, Europe and the United States have the largest market of biological hormone products, and the research and development and application of biological hormone focus in Europe and North America. In EU legislation, agricultural products fall into two broad categories: (1) nutrients (fertilizers) that provide nutrients to plants, that are controlled by EC No.2003/2003; (2)As active substances, safety agents, synergist, pesticides, herbicides, hormone substances and other plant protectants, controlled by the EU plant protectants act (EC No.1107/2009). Before the revision of the European Union fertilizer law, only mineral fertilizers were classified as "EC fertilizers" under the control of the EU fertilizer law (EC No.2003/2003), organic fertilizers, organic mineral fertilizers, cultivation substrates, soil improvers, and biological stimulants are all based on the EU consensus legal document (EC No.764/2008), regulated by national legislation and the domestic market. Unlike fertilizers and plant protectants, biostimulants have complex chemical properties, some of which are synthetic, some of which are organic and inorganic natural forms. The biostimulants cannot be classified as chemical fertilizer simply, because its main function is not to transfer nutrients to plants, but promoting nutrient absorption and soil nutrition. Different from plant protectants, biostimulants do not directly protect plants, but enhance plant disease resistance by enhancing plant regulatory effect. Not all biostimulants have the function of protecting plants from disease invasion. In order to establish a unified and standardized biostimulants management system, the European commission expanded and revised the EU fertilizer law (EC No. 2003/2003) in 2012-2013, and established a legal framework on plant biostimulants and agricultural fertilizer additives in 2013-2014. In March 2016, the EU published a revision to its fertilizer law, which included biological stimulants in fertilizer management. The revised EU fertilizer law covers mineral fertilizers, organic fertilizers, cultivation substrates, soil improvers, biological stimulants and agricultural fertilizer additives. The benefits include: (1)To ensure the safety of fertilizers and related products circulating in the market; (2)To ensure the safety of fertilizers and related products circulating in the market; (3) to ensure that farmers buy and use the efficacy of biostimulants;(4) simplify the process of new products launching, reduce the management burden of the competent authorities.
In the current EU system for the management of agricultural products, chemicals and plant protectants are regulated in accordance with REACH and EC No.1107/2009 for registration administration, while food and feed additives are evaluated and registered by EFSA. None of the above EU regulations applies to biostimulants, which are mainly used on crops such as fruits and vegetables and contain a significant proportion of microbiological products. REACH does not consider the potential risks of food-related products and does not include microorganisms in its management system, which focuses on food and feed additives. In order to comply with the current EU and national regulatory system, the registration of biostimulants is required. First of all, the applicant submits a registration information of the biostimulants to the EUnion regulatory authority (ECHA or EFSA), including: safety assessment data, such as phys-chem, healthy & ecological toxicity, pathogenicity, infectious information;(2) risk assessment information, involving risk characteristics, exposure assessment, risk characteristics, etc.;(3) product feature description information, including species, category, chemical name, application range, etc. ECHA or EFSA receives the application dossier, reviews the information and gives a comprehensive conclusion. Finally, ECHA or EFSA approves registration and manage the application dossier.
The improved farm production act was included into federal law in 2018. It is the first federal law of plant biostimulants. The act defines plant biostimulants as "substances or microorganisms used in seed, plant or rhizosphere processing to promote the absorption of nutrients, improve the efficiency of nutrient utilization, enhance the resistance of plants to abiotic stress, and achieve the effect of increasing production and quality." The act requires the heads of the department of agriculture, environmental protection agency, state governments and related agencies to report to congress on legislative recommendations, including appropriate definitions of plant biostimulants. The aim of the submission of the report is to facilitate the development of a legal system for plant biostimulants, ensuring that evaluation, registrations, national labelling, and product supply processes are conducted efficiently and appropriately. The definition of a product in the law is largely consistent with the current EU definition.
4 The development status of biostimulants in China
Biostimulants has long been used in China. It is different from hormone, and there are significant differences between them. It is not appropriate to use the existing concept of hormones to explain biostimulants. In fact, biostimulants is not a new thing, such as humic acid, microbial fertilizer, etc.. In our country, there is a long history of application, but not clearly defined and classified until now. Although the definition is still vague, but the plant has physiological activity, non-fertilizer, non-pesticide, beneficial to plant growth and other views have been recognized by the industry of the biostimulants. By the end of 2017, the global market value of biostimulants was around 1.3 billion USD. Among them, the market of Chinese biostimulants is about 200 million USD. It is estimated that by 2020, the global market value of biological hormone products will reach 2 billion to 3 billion USD, with an annual growth rate of more than 10%. In the next three to five years, the market value of biostimulants in China will reach 400 million to 500 million USD. China is likely to become the largest market for the application of biostimulants in future. Biostimulants can solve problems that fertilizers and pesticides cannot, especially when plants encounter physiological diseases or meteorological disasters caused by abiotic stresses. The role of biostimulants in assisting chemical fertilizer and pesticide to reduce quantity and increase efficiency is also cannot be underestimated.