Hormones are of central importance for the regulation of metabolic processes and plant development in a complex system of interacting hormones and cofactors, the functions of which are closely intertwined and mutually dependent . bio-stimulants developed from humic substances, complex organic materials, seaweeds, antitranspirants, free amino acids , and crude extracts of lower and higher plants have been frequently demonstrated to have an effect on plant hormonal status . While hormone-like compounds may be present in bio-stimulants, it is also possible that de novo synthesis of hormones may be induced by such preparations in treated plants and amino acids, glycosides, polysaccharides and organic acids are contained in many bio-stimulants and may act as precursors or activators of endogenous plant hormones . Hormones or hormone-like effects could therefore be responsible for the action of natural bio-stimulants derived from microorganisms, algae, higher plants, animal, and humate based raw material . Information on currently available bio-stimulants gives some insight into the possible biochemical and molecular genetic effects of bio-stimulants derived from different natural raw materials . Many published reports are available suggesting various bio-stimulants improve plant productivity through increased assimilation of N, C, and S , improved photosynthesis,hydroponic nft channel improved stress responses, Thered senescence, and enhanced ion transport . bio-stimulants are also reported to increase free amino acids, protein, carbohydrates, phenolic compounds, pigment levels, and various enzymes .
The protective effect of many bio-stimulants against biotic and abiotic stresses has been associated with a reduction of stress-induced reactive oxygen species, activation of the antioxidant defense system of plants, or increased levels of phenolic compounds . While it is clear that many biologically derived bio-stimulants contain small molecular weight compounds that are involved in signaling events and may directly influence plant metabolic processes, it remains unclear how an exogenous soil or foliar application of an uncharacterized product can have predictable and beneficial responses in plants. It is well-known, for example, that application of exogenous plant hormones or compounds that disrupt hormone function can have markedly negative effects on plants and that optimization of PGR materials and their applicaitons requires precise information on dosage and timing. Application of bio-stimulants for which the dosage and efficacy of the functional compounds is unknown, cannot, therefore, be expected to result in predictable plant responses and identification of molecules with effects on plant metabolic processes is not, in of itself, a sufficient explanation for the function of a bio-stimulant. It is also uncertain why the application of a bio-stimulant with purported function as a PGR, signaling molecule or other discrete compound would be superior to, or more easily controlled, than a direct application of the purifified product itself. Modern crop production requires a balance of high and consistent productivity with maximum safety for consumers, agricultural workers, and the environment . While some bio-stimulants have been analyzed with regard to unwanted side effects including negative impact on the natural environment most bio-stimulants have not been fully characterized but have been regarded as generally recognized as safe on the basis of the biological origin of their constituents . Generally, bio-stimulants are assumed to be biodegradable, non-toxic, nonpolluting and non-hazardous to various organisms.
While this may be a rational conclusion for many formulations derived from biological materials such as seaweed extracts and their components , higher plants , chitin and chitosan it is not clear that this is a valid assumption for microbial products or products that would not normally be present in agricultural fields. bio-stimulants have been utilized as bio-remediants and have been shown to improve ATP levels and phosphatase and urease activity , and hence increase the rate of degradation of xenobiotics in the soil and to enhance beneficial soil microbial communities under semi-arid climates . bio-stimulants may also help reduce the amount of potentially risky agrochemicals including reducing the use of fertilizers and pesticides . Most compounds contained in bio-stimulants are natural constituents of terrestrial and aquatic ecosystems and metabolites of plant and microbial origin and as such most are generally regarded as safe, particularly at the low rates at which they are typically applied. Thus, it has been proposed that bio-stimulants can be positioned as eco-friendly products for sustainable agriculture . In many countries, however, bio-stimulants are not subject to rigorous toxicological screening and there remains the potential for the persistence of human pathogens in materials of animal origin and for the synthesis of novel compounds of unknown function or toxicology during the manufacturing process. Even though there have been relatively few rigorous demonstrations of the benefit of bio-stimulants, and to a large extent the mode of action of these products remains uncertain, the industry for bio-stimulants is substantial and rapidly growing. Though many recent “market” studies show that the market for these products is growing at a remarkable rate, the validity of these analyses must be considered with care as they frequently do not provide an explicit definition of term “bio-stimulants.” The value of the European bio-stimulants market ranged from e200 to e400 million in 2011, e500 million in 2013 and may grow to more than e800 million in 2018 with annual growth potential in 10% and more . France, Italy, Spain are the leading EU countries in the production of bio-stimulants .
In North America, the bio-stimulant market was valued at $0.27 billion in 20131 , and is expected to grow at a growth rate of 12.4% annually, to reach $0.69 billion by 2018, the USA is the largest producer and consumer of bio-stimulants in the region . In 2014, the USA market was assessed at $313.0 million and is projected to reach $605.1 million by 20192 , at a CAGR of 14.1% . The bio-stimulants market in the Asia-Pacific was valued at $0.25 billion in 2013, and is expected to grow at a CAGR of 12.9% annually, to reach $0.47 billion by 2018. China and India are key countries playing a significant role. The Southeast Asian & Australasian bio-stimulants market was valued at $233.8 million in 2015, and is projected to reach $451.8 million by 2021. The market in Latin America was valued at $0.16 billion in 2013, and is expected to grow at a CAGR of 14.4% annually, to reach $0.32 billion by 2018 . This market is mostly concentrated in Brazil and Argentina. The regional market shares of the global bio-stimulants market6 are: EU—41.7%, North America—21.5%, the Asia-Pacific region—20%, Latin America—12.9%. Globally, it bio-stimulants were valued at $1402.15 million in 2014 and are projected to have aCAGR of 12.5% reaching $2524.02 million by 2019, largely as a consequence of growing interest in organic products. Wu summised that “the global bio-stimulants market is projected to reach $2.91 billion by 2021, with a CAGR of 10.4% from 2016 to 2021. In terms of area of application, the bio-stimulants market is projected to reach 24.9 million hectares by 2021 and is projected to grow at a CAGR of 11.7% from 2016 to 2021” .” The bio-stimulant industry faces many problems and challenges. Until recently bio-stimulant products based on natural raw materials and particularly waste stream has mainly been developed based on observational and less commonly,nft growing system empirical data. While many contemporary bio-stimulants have been shown to be effective in practice, very few bio-stimulants can claim to understand the mechanisms or modes of action . Furthermore, while bio-stimulants can be categorized by source of origin, this is frequently inadequate as very substantial differences can exist between products even within a common feed stock origin. The challenge to bio-stimulant science is further exacerbated since composition and content of active substances in the original plant raw material can be affected by many factors including the location and growing conditions, season, species, variety, organ, and the phase of growth . Similarly, the response of the target crop can be expected to vary across crops and environments. One solution to this problem is to derive the raw materials for the bio-stimulant under highly regulated conditions. This approach has been successfully implemented by leading seaweed producers and fermentation based products that have developed harvesting and manufacturing processes that ensure uniformity of product performance through time. The development of a product with uniformity of response is not, however, a guarantee that the product is optimized for biological efficacy. To address these issues, developments in -omics approaches will be critical in accelerating the discovery of mode of action of bioactive compounds and optimizing their use. Metabolomics, phenomics and agronomics represent the integration of gene expression, protein interactions, and other regulatory processes as they impact on plant productivity and thus are more appropriate tools for discovery in this field than mRNA, transcripts, or proteins analyzed in isolation . Integrative, multidisciplinary approaches using tools from transcriptomics in conjunction with metabolomics and biochemical analysis are necessary to establish the mechanism of action and to identify the active components in the extracts . The difficulty in identifying modes of action and subsequent standardization of composition of multi-component bio-stimulants based on natural raw materials will continue to hamper the use, certification and registration of bio-stimulants. The solution to this problem will require the collaborative efforts of specialists from different fields: chemists, biologists, plant physiologists, industrial manufacture, sales and distribution and those with expertise in practical agricultural production .
Products with a single active substance represent a simpler construct in which the physiological effects and mechanism of action can be more readily determined and hence certification and registration is simpler. The multi-component composition of many preparations, however, are much more difficult to characterize , though they may offer novel insight into biological synergy , multi-functionality and emergence which may be crucial to product efficacy . In the absence of a functional rationale for every constituent in a multi-component bio-stimulant, it is likely that there will be molecules present that may positively or negatively influence plant productivity. Currently, it is almost impossible using available chemical-synthetic, and genetic engineering approaches to reproduce the full suite of molecules and complexes of biologically active substances that are present in most bio-stimulants. Many have noted the state confusion in the field of bio-stimulants and this has resulted in the opinion that much of the bio-stimulant market is not based on science or efficacy and that many products are little more than recycled waste products sold on the basis of pseudoscience and marketing. Indeed, research on several bio-stimulant products has shown them to be ineffective or to contain inactive, unstable or inconsistent properties with several showing negative effects compared when contrasted with well-designed controls . For example, foliar and root application of a product containing amino acids from animal origin have been reported to cause severe plant-growth depression and negative effects on Fe nutrition while a second product containing amino acids from plant origin stimulated plant growth . In another report that tested several bio-stimulant products it was concluded that “none of the bio-stimulant products tested achieved a sufficient degree of pathogen control to warrant replacement of or supplementation with conventional synthetic fungicides” , and there have been demonstrated positive and negative impacts and overall questions of the economic feasibility of the use of humic substances for increasing crop yields . Since biological systems are inherently complex, and given that most bio-stimulant products have not been characterized and have received relatively little replicated and rigorous independent validation, it is perhaps not surprising that many products are ineffective or highly variable in response. Nevertheless, there are a significant number of rigorous independent reports of benefits from some bio-stimulant formulations and market growth data demonstrates that there is a good deal of support for these products within agricultural producer communities. That such market growth has occured, even in the absence of a known “mechanism of function” suggests that there are aspects of plantmetabolism and productivity constraints that are not understood but are potentially important if we are to achieve the goal of increased global food production. The market euphoria that is taking place in the bio-stimulant industry recognizes these unknowns and bio-stimulants are viewed by many innovators and investors as a mechanism to conduct broadscale, if unfocussed, discovery of novel biologically derived molecules. Much as the exploration of marine organisms, and plants and microbes from diverse ecosystems has led to the discovery of novel pharmaceuticals, so too the development of bio-stimulants from the broad range of source materials, holds significant promise of discovery.