Plant Biostimulants: Rationale, State of the Art and Evolution Recently, the agricultural sector is facing concomitant challenges of rising the productivity to feed the growing global population and increasing the resources use efficiency, while reducing the environmental impact on the ecosystems and human health. In fact, fertilizers and pesticides play a crucial role in agriculture, representing a powerful tool for growers to increase yield and guarantee continuous productivity throughout the seasons under both optimal and suboptimal conditions. In the last three decades, several technological innovations have been proposed to enhance the sustainability of agricultural production systems, through a significant reduction of synthetic agrochemicals like pesticides and fertilizers. A promising and environmental-friendly innovation would be the use of natural plant biostimulants (PBs) that enhance flowering, plant growth, fruit set, crop productivity, and nutrient use efficiency (NUE), and are able also to improve the tolerance against a wide range of abiotic stressors ( Colla and Rouphael, 2015 ). PBs were initially defined by excluding some functionalities like fertilizers or plant protection products. In 1997, in Grounds Maintenance web-journal, Zhang and Schmidt from the Department of Crop and Soil Environmental Sciences of the Virginia Polytechnic Institute and State University, defined PBs as “materials that, in minute quantities, promote plant growth”. By using the statement “minute quantities” for describing PBs, the authors implicitly wanted to discriminate biostimulants from nutrients and soil amendments, which also promote plant growth, but are clearly applied in larger quantities. The PBs mentioned in this web article were two important categories such as humic acids and seaweed extracts, and their action on plants was proposed to be essentially hormonal. In 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 (2012) 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 (2012) 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. Three years later in the frame of a special issue on “Biostimulants in Horticulture” conducted by Colla and Rouphael (2015) , a new definition was proposed by du Jardin (2015) , 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 (2015) 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 their special issue Colla and Rouphael (2015) proposed 6 non-microbial and 3 microbial categories of PBs: (i) chitosan ( Pichyangkura and Chadchawan, 2015 ), (ii) humic and fulvic acids ( Canellas et?al., 2015 ), (iii) protein hydrolysates ( Colla et?al., 2015 ), (iv) phosphites ( Gómez-Merino and Trejo-Téllez, 2015 ), (v) seaweed extracts ( Battacharyya et?al., 2015 ), (vi) silicon ( Savvas and Ntatsi, 2015 ), (vii) arbuscular mycorrhizal fungi (AMF; Rouphael et?al., 2015 ), (viii) plant growth-promoting rhizobacteria (PGPR; Ruzzi and Aroca, 2015 ), and (ix) Trichoderma spp. ( López-Bucio et?al., 2015 ). The definition of PBs has been rigorously debated over the last decade, and recently under the new Regulation (EU) 2019/1009, which led to the following: “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
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