⦁ 1933: the term biostimulant is linked to the concept of “biogenic stimulant” theory attributed to Prof. V.P. Filatov. Filatov proposed that biological materials derived from various organisms, including plants, that have been exposed to stressors could affect metabolic and energetic processes in humans, animals, and plants.
⦁ 1956: Blagoveshchensky further developed these ideas with specific reference to their application for plants, considering biogenic stimulants as “organic acids with stimulating effects due to their dibasic properties which can enhance the enzymatic activity in plants.” Filatov’s concept, was, however, not limited to these compounds alone.
⦁ 1994: Herve’s pioneering review provides the first real conceptual approach to biostimulants. Herve suggests the development of novel “bio-rational products” should proceed on the basis of a systemic approach founded in chemical synthesis, biochemistry, and biotechnology as applied to real plant physiological, agricultural, and ecological constraints. He suggests these products should function at low doses, be ecologically benign and have reproducible benefits in agricultural plant cultivation.
⦁ 1997: the first definition of the word biostimulants appeared in a web journal dedicated to turf maintenance professionals, called Ground Maintenance. In this web journal Zhang and Schmidt from the Department of Crop and Soil Environmental Sciences of the Virginia Polytechnic Institute and State University defined biostimulants as “materials that, in minute quantities, promote plant growth”. By using the words “minute quantities” for describing biostimulants, the authors aimed at distinguishing biostimulants from nutrients and soil amendments, which also promote plant growth but are applied in larger quantities. The biostimulants mentioned by this web article were humic acids and seaweed extracts. Later peer-reviewed papers by the same authors on the same or similar research did not necessarily use the term biostimulant. For instance, a paper describing the use of humic acids and seaweed extracts for increasing drought tolerance of turfgrass did not use the term biostimulant at all. The paper focused on the hormone-like activities of these compounds and the term ‘hormone-containing products’ was used instead of biostimulants. This choice could also be explained by the regulation in the United States, where the Environmental Protection Agency (‘EPA’) exempts “vitamin-hormone horticulture products” from registration under certain conditions. Zhang and Schmidt explained the biostimulation action by hormonal effects and, secondly, by protection against abiotic stress by antioxidants. The term “metabolic enhancers” was also used in later papers.
⦁ 2007: in the scientific literature, the word biostimulant was first defined by Kauffman et al. in a peer-reviewed paper, with modifications: “biostimulants are materials, other than fertilisers, that promote plant growth when applied in low quantities.” Worth mentioning is the addition of the words “other than fertilisers”, which is in line with the description of Zhang and Schmidt, but which was not explicitly included in their original definition. Kauffman et al. attempt to summarize what biostimulants are, by introducing a classification: “Biostimulants are available in a variety of formulations and with varying ingredients but are generally classified into three major groups on the basis of their source and content. These groups include humic substances (HS), hormone containing products (HCP), and amino acid containing products (AACP). HCPs, such as seaweed extracts, contain identifiable amounts of active plant growth substances such as auxins, cytokinins, or their derivatives”.
⦁ 2012: the European Commission has assigned an ad hoc study on plant biostimulants to evaluate the substances and materials involved, which was published by du Jardin as: “The Science of Plant Biostimulants – A bibliographic Analysis”. Based on the scientific literature (250 scientific articles using the term ‘biostimulant’ in their titles and/or abstracts), the following definition was proposed: “Plant biostimulants are substances and materials, with the exception of nutrients and pesticides, which, when applied to plant, seeds or growing substrates in specific formulations, have the capacity to modify physiological processes of plants in a way that provides potential benefits to growth, development and/or stress responses”. du Jardin concluded that PBs are very heterogeneous materials, and proposed in his study eight categories of substances that acts as biostimulants: humic substances, complex organic materials (obtained from agro-industrial and urban waste products, sewage sludge extracts, composts, and manure), beneficial chemical elements (Al, Co, Na, Se, and Si), inorganic salts including phosphite, seaweed extracts (brown, red, and green macroalgae), chitin and chitosan derivates, antitranspirants (kaolin and polyacrylamide), and free amino acids and N-containing substances (peptides, polyamines, and betaines); but did not include any microbial biostimulants.
⦁ 2015: in the frame of a special issue on “Biostimulants in Horticulture” conducted by Colla and Rouphael, a new definition was proposed by du Jardin, which was supported by scientific evidence about the mode of action, nature and types of effects of PBs on agricultural and horticultural crops. PBs were defined by du Jardin as follows: “A plant biostimulant is any substance or microorganism applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrient content”. This definition could be completed by “By extension plant biostimulants also designate commercial products containing mixtures of such substances and/or microorganisms”. In the same year, in a special issue Colla and Rouphael proposed 6 non-microbial and 3 microbial categories of PBs: (i) chitosan, (ii) humic and fulvic acids, (iii) protein hydrolysates, (iv) phosphites, (v) seaweed extracts, (vi) silicon, (vii) arbuscular mycorrhizal fungi (AMF), (viii) plant growth-promoting rhizobacteria (PGPR), and (ix) Trichoderma spp.
⦁ 2019: the new EU regulation (EU) 2019/1009 known as Fertilising Product Regulation, formally recognized and provided for the first time an official definition for plant biostimulants: “A plant biostimulant shall be an EU fertilising product the function of which is to stimulate plant nutrition processes independently of the product’s nutrient content with the sole aim of improving one or more of the following characteristics of the plant or the plant rhizosphere: i) nutrient use efficiency, ii) tolerance to abiotic stress, iii) quality traits, or iv) availability of confined nutrients in the soil or rhizosphere”. Based on this definition, PBs are specified on the basis of agricultural functions claims and include diverse bioactive natural substances: (i) humic and fulvic acids, (ii) animal and vegetal protein hydrolysates, (iii) macroalgae seaweeds extracts, and (iv) silicon, as well as beneficial microorganisms: (i) arbuscular mycorrhizal fungi (AMF) and (ii) N-fixing bacteria of strains belonging to the genera Rhizobium, Azotobacter, and Azospirillum.