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Biostimulants, revisited

By Stanley Kostka, PhD, and Michael Fidanza, PhD

Our industry contains a vast array of biostimulant products claiming positive effects on plant or soil health, particularly under stress conditions. But what are biostimulants?

What is a biostimulant?

Within the context of sports turf, the earliest definition of a biostimulant appeared in a scientific paper by Zhang and Schmidt at Virginia Tech in 2000, and focused on the effects of a hormone containing product on drought tolerance in tall fescue and creeping bentgrass. They defined biostimulants simply as ” . . . materials that, in minute quantities, promote plant growth.”

Over the past 17 years, different definitions have been proposed in North America and in the European Union. From a regulatory perspective, an accepted definition does not exist, in North America or in Europe. In the absence of an official legal definition, these products are often regulated as soil amendments or as fertilizers. How these products are defined will ultimately control how they are regulated on the state and national levels.

In 2012, the European Biostimulants Industry Council proposed the following: plant biostimulants contain substance(s) and/or micro-organisms whose function when applied to plants or to the rhizosphere is to stimulate natural processes to enhance/benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and crop quality (http://www.biostimulants.eu). This definition is consistent with proposals from the Biostimulants Coalition in North America (http://www.biostimulantcoalition.org). Considerable debate is ongoing, with a group even suggesting that such products must be exclusively of “biological origin” to be considered biostimulants.

What do biostimulants do?

Biostimulants foster plant growth and development throughout the crop life cycle from seed germination to plant maturity in a number of demonstrated ways, including but not limited to:

  • Improving the efficiency of the plant’s metabolism to induce yield increases and enhanced crop quality
  • Increasing plant tolerance to and recovery from abiotic stresses
  • Facilitating nutrient assimilation, translocation and use
  • Enhancing quality attributes
  • Regulating and improving plant water balance
  • Enhancing certain physicochemical properties of the soil and fostering the development of complementary soil microorganisms.

There are several generally accepted categories of plant biostimulants. These include humic and fulvic acids, protein hydrolysates and N-containing compounds, seaweed extracts and other botanicals, chitosan and other biopolymers, inorganic chemicals, beneficial fungi, and beneficial bacteria.

Humic and fulvic acids. Humic substances (HS) are the largest constituents of soil organic matter (upwards of 60%) and are responsible for many chemical interactions via their ability to interact with metal ions, oxides, hydroxides, mineral and organic compounds and to dissolve, mobilize and transport metals and organics and influence nutrient availability. HS enhance root, leaf and shoot growth may also stimulate the germination. However, distinguishing between the direct and indirect effects of these substances is challenging. In fact, some of their positive effects may be ascribed to a general improvement of soil fertility, leading to higher nutrient availability for plants. HS have been shown to contain auxin and an “auxin-like” activity of humic substances has been proposed, but support for this hypothesis is fragmentary at this time. The source of the HS, the environmental conditions, and the manner of application all influence observed effects.

HS are derived from a variety of sources each having different chemical properties. They may be extracted from naturally occurring organic matter (such as peat or muck soils), from composted organic matter or vermicomposts, or from leonardite, a form of lignite (brown coal). Alternatively, agricultural wastes and byproducts may be processed (reacted under controlled conditions) to produce compounds referred to as “humic-like” substances.

Protein hydrolysates and other N-containing compounds.

This class is composed of amino acids and short chain peptides derived by chemical or enzymatic hydrolysis of plant or animal by-products, fermentation metabolites by microorganism, or chemical modification. Included in this group are nitrogenous compounds such as polyamines, betaines, and non-protein amino acids.

These compounds can have diverse activity in treated plants or soils. Amino acids and short chain peptides have been reported to increase N uptake and its assimilation. Hydrolysates of complex proteins and tissues have also been reported to have hormonal-like activity. Certain amino acids (for example glycine) have been reported to have chelating effects resulting in better access to and bioavailability of micronutrients. Other responses from amino acid and short chain peptide applications, for example phenylalanine, include increased production of antioxidants, key components in dealing with abiotic stress (heat, light, or salinity). Proline is reported to be involved in plant responses to water stress. When applied to soil, microflora (bacteria and fungi) can readily access and assimilate the amino acids, resulting in increased soil microbial biomass, which is considered to be an important component of soil health.

Alternatively, amino acids have also been shown to be readily available N-sources for rapid root and foliar uptake in plants, including turfgrass, suggesting that they also have potential as fertility management options or to maximize plant use of the existing soil nutrients. This last application is intriguing, especially for recovery of turf with a compromised root system or in environments where N leaching concerns may preclude other N sources.

Seaweed extracts. Seaweed extracts (SWE) are commonly derived from brown, green, or red macroalgae and have been used in turf since the 1950s. Extracts of brown seaweeds are widely used in in turf due to their well-documented effects on growth promotion and mitigation of abiotic stresses, including salinity, extreme temperatures, nutrient deficiency and drought.

A range of manufacturing processes including, alkaline or acid hydrolysis, cellular disruption under pressure, or fermentation followed by various separation technologies are used to produce SWE. As a consequence, considerable heterogeneity in extract constituents and stability may exist between SWE produced using different extraction processes.

The chemical constituents of SWE include polysaccharides, fatty acids, vitamins, elicitors, phytohormone-like compounds, and mineral nutrients. When applied to soils, SWE polysaccharides behave as gels increasing water retention and potentially influencing aeration. By modifying soil water, SWE also impart positive effects on soil microbial populations, especially plant growth-promoting rhizobacteria (PGPR) that provide the plant with phytohormones, facilitate nutrient uptake, or are sources of biopesticidal metabolites or activators of plant defense responses such as laminaran, fucoidan, algnate, and ulvans.

SWE also have direct plant effects associated with phytohormone like properties of the extract (cytokinin-like activity) or regulatory effects on biosynthetic regulation on hormone synthesis at the cellular level. Up regulation of antioxidants in response to water deficit and elevated temperatures is well documented in SWE treated turf, enabling treated plants to better withstand periods of high evaporative demand, limited water inputs and heat stress.

Chitosan and other biopolymers. Chitin is the second most important natural polymer in the world. The main sources exploited are shells of marine crustaceans, particularly shrimp and crabs. Reacting chitin with an alkaline substance such as sodium hydroxide produces chitosan (CHT); a linear polysaccharide composed of randomly distributed β- (1→4)-linked D-glucosamine and N-acetyl-D-glucosamine.

CHT has been documented to stimulate plant growth, to protect the safety of edible products (as a preservative), and to induce biological responses to abiotic and biotic stresses. CHT interacts with a wide range of cellular components ranging from DNA to plasma membranes to cell walls where binding to specific receptor sites for defense gene activation resulting in some systems in increased protection against pathogens, but more broadly as enhancing tolerance to abiotic stress (drought, cold, and salinity) resulting in better plant performance and crop quality.

Activity is based on CHT structure and concentration and on the plant species and its developmental stage. Most research on CHT has focused on application as an elicitor of stress response signaling, for example, stomatal closure, a means of management of transpiration and water use. There is considerable research ongoing to understand the mode of action of chitin and chitosan polysaccharides in both stress tolerance and suppression of pathogens. Complex polysaccharides from SWE have similar effects.

Inorganic compounds. Inorganic compounds encompass elements and many of their salts. Certain elements, including aluminum (Al), cobalt (Co), sodium (Na), selenium (se), and silica (Si) are recognized as “beneficial elements.” While essential for certain plants, they are not required by most species but can play a role in management of abiotic and biotic stresses. These beneficial elements may enhance resistance to attack by certain insect pests and pathogens, and more broadly to abiotic stresses such as drought, salinity, and nutrient toxicity or deficiency.

Mineral salts including silicates, phosphites and phosphates, but also bicarbonates, sulphates, nitrates, may provide protection against fungi via direct fungicidal action or indirect protection by stimulating plant defenses. Some due to their effect on plant physiological processes influence quality and yield in the absence of biotic stress. For example, silicates (salts of silica) are involved in strengthening plant cell walls and have also been reported to have disease suppressive effects. Phosphite (Phi) or its conjugate phosphorous acid, a reduced form of Phi and inorganic salt, has been used as a pesticide, supplemental fertilizer, and as a plant biostimulant. As a plant biostimulant, Phi improved nutrient uptake and assimilation, abiotic stress tolerance, plant quality, and root growth. In horticultural crops, Phi has been reported to increase yield and nutritional value. Their action on the physiology of the plant, on stress response, and on yield explains why these inorganic compounds are sometimes referred to as biostimulants.

Fungi. Fungi and plants have co-evolved since terrestrial plants first appeared on earth. Beneficial fungi such as mycorrhizae are fungi that form a symbiotic relationship with plants, and 90% of all plants species have a symbiotic relationship with fungi. Mycorrhizae colonize roots and provide increased water and nutrient absorption while the plant feeds the fungus with carbohydrates from root exudates. Research on various crops has shown that arbuscular mycorrhizal fungi can act as “biofertilizers” (interacting with the plant’s rhizosphere to absorb and translocate mineral nutrients), “bioregulators” (interacting with the host plant to influence plant development), and “bioprotectors” (inducing a plant’s tolerance to abiotic and biotic stresses).

There are many mycorrhizal and soil inoculant-type products available today that claim to improve plant growth and plant health. Success of these products rely on the ability of the fungal organism to be delivered to the rootzone and effectively colonize and establish a relationship with existing roots. With sports turf management, it means not only applying the fungal organisms but also creating rootzone conditions that favor their successful growth and development. Current research on the new frontier of soil microbiomes and metagenomics may yet reveal mycorrhizae and other fungi that can effectively colonize and successfully populate the rootzone to the benefit of the plant.

Bacteria. Bacteria can interact with plant roots in many ways, from a mutualistic partnership where bacteria and plants live in direct contact to the benefit of both, to parasitism and infection. Bacterial niches can form in the soil rhizosphere and rhizoplane and even into the interior of plant cells. These bacteria/plant interactions can be temporary to permanent. Bacteria can play a role in soil and plant biogeochemical reactions, increased nutrient availability and nutrient use efficiency, induced disease resistance, improved tolerance to abiotic stress, and possibly more. Some examples of bacterial species used in biostimulant products include Rhizobium, Bacillus, Pseudomonas, and others, including mutualistic rhizospheric plant growth promoting rhizobacteria.

Current research agrees that growing health roots produce exudates that essentially select for the “good” bacteria and other microorganisms favored by the plant. The challenge has always been this: how to get introduced bacteria to colonize and populate the soil and rootzone quickly and effectively. In the past, inconsistent results of these microbial products have been attributed to the formulation. These products may also require special handling such as refrigeration, or a fermentation process, or other specialized methods of storage, production, and even delivery. The good news is that many biological and specialty chemical companies are investing heavily into research on improving the formulation and delivery of these microbial biostimulant-type products, and they are regarded as plant “probiotics” that will contributed to plant health and immunity.

Where do we go from here?

Many biostimulant products are formulated from multiple components and/or are of undefined composition. While activity may be measured from such formulations, is the observed plant or soil response a consequence of a single or a multiplicity of components? In studies conducted in growth chambers and in greenhouses, effects of single components can be measured; however, measuring such effects in the field is often more challenging. One or more complex formulations may need to be applied to observe a visual plant response. The challenge then becomes to explain what components were responsible for the observed effects.

Moderating stress is a key component to building a healthy, resilient turf stand. As stress builds a number of plant processes may be compromised: efficiency of light and carbon capture declines; destructive reactive oxygen species (free radicals) increase in shoots and roots; roots take a “double-hit” because shoots stop allocating energy to roots and may pull energy from roots; and root decline (coupled with pressure from secondary plant pathogens) precedes shoot decline. Biostimulant-containing products provide management options to maintain turf performance under stress.

Should a biostimulant product become a valuable part of your turf management program? This all depends on what exactly a turf manager wants to accomplish (i.e., better rooting, better heat or drought stress, traffic tolerance, turf recovery, disease prevention, better color or visual quality, better playability, and more). Be sure to critically evaluate your turf for the response you want (i.e., better rooting, improved stress tolerance, etc.). Also be sure to review all available information, and consider that the use of a biostimulant or any product should be predicated on results from independent and replicated third party research.

Stanley Kostka, PhD, is an agronomist based in Cherry Hill, NJ and a Visiting Scholar at The Pennsylvania State University, Berks Campus, Reading, PA, stan.kostka@gmail.com, @kostkastan. Michael Fidanza, PhD, is Professor of Plant and Soil Sciences at Penn State Berks, maf100@psu.edu, @MikeFidanza.

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