The definition and concept of plant biostimulants is still evolving, which is partly a reflection of the diversity of inputs that can be considered to be biostimulants . Plant biostimulants, or agricultural biostimulants, include diverse substances and microorganisms that enhance plant growth.
This article describes the emerging definitions of biostimulants and reviews the literature on five categories of biostimulants:
i. microbial inoculants: Microbial inoculants are typically classified as biocontrol agents (also called biopesticides) or biofertilizers. Microbial inoculants that act as biofertilizers are considered as biostimulants in the present review. Enhanced plant growth and yield by microbial inoculants has been linked in some cases to enhanced nutrient uptake and improved nutrient status of the plant. Also, The production of plant growth-regulators by many bacterial species and their effect on plant growth was reported more than 30 years ago.
ii. humic acids: Humic substances are end products of microbial decomposition and chemical degradation of dead biota in soils and are considered to be the most abundant naturally occurring organic molecules on earth and the major components of soil organic matter. The activity of humic substances is related to their structural characteristics. Many humic acids and humic substances have been shown to elicit diverse morphological changes in plants, leading to changes in plant growth.
iii. fulvic acids: The scientific literature contains reports that specify humic or fulvic acid as well as reports that refer generically to humic substances. Fulvic acids have been reported to enhance some aspect of growth of a similar range of plant species as humic acids. The most commonly recorded manifestations of plant growth promotion were measures of root growth. Also, assessments of the effects of fulvic acids on yield or fruit quality have been reported in a few tests conducted to maturity under greenhouse or field conditions. And also the capacity of fulvic acid to enhance uptake of nutrients has been reported in diverse systems.
iv. protein hydrolysates and amino acids: Plant growth stimulation and enhanced tolerance to biotic and abiotic stresses have been reported following application of a variety of protein-based products. These plant stimulatory effects appear to be distinct from the nutritional effect of an additional nitrogen source. Protein hydrolysates have been shown to stimulate carbon and nitrogen metabolism and to increase nitrogen assimilation. Also there is considerable evidence that protein hydrolysates and specific amino acids including proline, betaine, their derivatives and precursors can induce plant defense responses and increase plant tolerance to a variety of abiotic stresses, including salinity, drought, temperature and oxidative conditions.
v. seaweed extracts: Seaweed has been used for millennia, either directly or following composting as a soil amendment to enhance soil fertility and crop productivity. Seaweed extracts also 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.
The large number of publications cited for each category of biostimulants demonstrates that there is growing scientific evidence supporting the use of biostimulants as agricultural inputs on diverse plant species. The cited literature also reveals some commonalities in plant responses to different biostimulants, such as increased root growth, enhanced nutrient uptake, and stress tolerance.