Soil is a complex environment inhabited by varied soil microbial communities interacting with each other solubilizing and mineralizing organic matter and different chemical compound secreted into the soil and making it available to the plants. Beneﬁcial microbial interactions with roots influence plant mineral nutrient availability.
A plant biostimulant is a substance or microorganisms when applied to plants enhance nutrition efficiency and tolerance to abiotic stress regardless of its nutrients content (du Jardin 2015). The proposed definition is “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 response” (du Jardin 2012). Biostimulant can also be defined as “A formulated product that improves plant productivity by a mechanism of action that is not the sole consequence of the presence of known essential plant nutrients, plant hormones, plant growth regulators, or plant protective compounds” (Yakhin et al., 2017). Biostimulants are not fertilizers, they do not contain nutrient intended to be delivered to plants. The first in-depth analysis of plant biostimulant with emphasis on biostimulant systemization and categorization was made on the basis of biochemical, physiological function, mode of action and origin by du Jardin (2012, 2015).
Many biostimulants improve photosynthetic efficiency and plant defense against pathogens. Plant growth-promoting rhizobacteria (PGPR) can be effective in biostimulation of plant growth (Calvo et al., 2014). The dual function of microorganisms (biostimulant and biocontrol) can be seen when applied to plants. There are eight categories of biostimulants (du Jardin 2012):
- Humic substances
- Complex organic materials
- Beneficial chemical elements
- Inorganic salts (such as phosphite)
- Sea weed extracts
- Chitin and chitosan derivatives
- Free amino acids and other N-containing substances.
The subcategories proposed by the Biostimulant Coalition are: antioxidants, amino acids materials, biomolecule, enzymatic extracts, fulvic acid materials, humic acid materials, microbial inoculants, microbial soil amendments, mycorrhizal fungi, PGPRs, phytohormones, seaweed extract materials (du Jardin 2015).
On the basis of their source and content, biostimulants are classified into three major groups (Kauffman et al., 2007). These group are humic substances, hormone containing products (HCP) and amino acid containing group.
Humic substances (HS):
Humic substances (HS) are natural constituents of the soil organic matter, resulting from the decomposition of plant, animal and microbial residues, as well as from the metabolic activity of soil microbes using these as substrates. Humic substances exhibit stimulatory effect on plant growth. It improves soil fertility by improving soil structure and porosity through soil aggregation (Bronick and Lal 2005), increase water holding capacity and bioavailability of nutrient. HS are collection of heterogeneous compounds, categorized according to their molecular weights and solubility into humins, humic acids and fulvic acids (du Jardin 2015). Low molecular size fraction reaches the plasmalemma of higher plant cells and is taken up by the plants while high molecular size fraction is not absorbed but only interacts with cell wall. Humic matter displays a hormone like activity. It is not clear whether the hormonal activity is linked to the chemical structure of HS (functional group of humic molecule) or it depends on hormones of microbial origin entrapped into them (Nardi et al., 2002).
Hormone containing products (HCP):
Hormone containing products such as seaweed extracts, contain identifiable amounts of active plant growth substances such as auxins, cytokinins, or their derivatives (Kauffman et al., 2007). Seaweeds include green, brown and red macroalgae. Liquid extract of seaweed when applied to seeds or added to soil stimulate plant growth. Extract derived from Polysiphonia, Ulva and Cladophora can be nontoxic biostimulants of plant growth Godlewska et al. (2016).
Extracts derived from algae contain:
- Polysaccharides such as galactan, fucoidan, alginate, and laminarin
- Proteins example lectins
- Polyunsaturated fatty acids (PUFAs)
- Pigments such as chlorophylls, carotenoids, and phycobiliproteins
- Polyphenols example phenolic acids, flavonoids, cinnamic acid, isoflavones, benzoic acid, quercetin and lignans
- Minerals such as potassium, magnesium, calcium and sodium
- Plant growth hormones such as cytokinins, auxins, gibberellins, and abscisic acid (Chojnacka 2012).
Seaweed extracts act as biostimulants, enhancing seed germination and establishment, improving plant growth, yield, flower set and fruit production, increasing resistance to biotic and abiotic stresses, and improving postharvest shelf life ( Rayorath et al., 2008; Khan et al., 2009; Craigie 2011).
Amino acid containing products (AACP):
Protein based products can be divided into two categories:
i Protein hydrolysate: Protein hydrolysate formulation can be used as a biostimulant to improve crop productivity. It consists of a mixture of peptides and amino acids of animals and plant origin. Protein hydrolysates can improve crop tolerance to abiotic stresses (Ertani et al., 2013).
ii Individual amino acids (glutamate, glutamine, proline and glycine betaine): Amino acid can easily be absorbed by plant root and through foliage (Nacry et. al.,2013 and Stiegler et al., 2013).
Application of plant derived protein hydrolysate containing amino acids and small peptides elicited a hormone like activity and enhanced nitrogen uptake (Colla et al., 2014). Amino acid and peptide mixtures are obtained by chemical and enzymatic hydrolysis from plant sources (crop residue) and animal wastes (e.g. collagen, epithelial tissues) (du Jardin 2012 and Calvo et al., 2014). Protein hydrolysates and specific amino acids including proline, betaine, their derivatives induce plant defense responses and increase plant tolerance to abiotic stresses (salinity, drought, temperature etc.) and oxidative conditions (Ashraf and Foolad 2007; Chen and Murata 2008)
Amendment of organic matter into the soil is beneficial, it conserve nutrients, reduce environmental pollution and improves soil physico-chemical structure. Organic farming promote the use of biocontrol agent and biofertilizer. The biostimulants may be used to attain better germination, improved growth and yield and can be understood only when the interaction between plant, biostimulant and environment is addressed.
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