Originally developed in the 1980s as a by-product of tissue culture biotechnical research, plant biostimulants are non-hormonal mixtures of antioxidants, vitamins, amino acids, metabolism enhancers, co-enzymes, and other bioactive materials that when exogenously applied to plants stimulate growth, increase stress resistance, promote metabolic activity in the rhizosphere, and facilitate optimal yield. These compounds create a self-reinforcing feedback loop, enabling plants to achieve their full genetic capacity and increasing yield in both high and moderate stress environments. Mitigating environmental stressors and understanding the corresponding response pathways is imperative amid climate change and efforts to increase food production. With a high probability, amplification of extreme weather events and seasonal temperature fluctuations in tandem with exponential population growth will strain agricultural resources over the next several decades and stress amelioration will become an economic priority. Even today, environmental stressors reduce agricultural production by an estimated 78% (Boyer, 1982), and abiotic stress alone reduces crop yields by about 50% (Hasanuzzaman et al., 2012). Understanding biostimulants, how their main bioactive components function as stress mitigants, and their larger implications will be vital for adept stress management as agriculture evolves amid climate change and elevated consciousness of the sector’s high environmental impact. Overall, biostimulants hold the potential to increase yields, reduce inorganic fertilizer pollution, and mitigate the adverse impact of climate change on global agriculture.
A mixture of stress-mitigating vitamins and coenzymes for higher plants, plant biostimulants have emerged as a rapidly growing field over the past 40 years, with increasingly demonstrated commercial applications. Russo and Berlyn (1990) defined biostimulants as non-fertilizer compounds that when exogenously applied to plants in small quantities, increase stress resistance, promote optimal yield, and reduce required fertilizer (Berlyn and Beck, 1980; Berlyn et al., 1990; Russo and Berlyn, 1990; Russo and Berlyn, 1992; Russo and Berlyn, 1994; Berlyn and Sivaramakrishnan, 1996). Applied either foliarly or via a root drench, these metabolic enhancers directly promote plant growth as well as stimulate the rhizosphere to a plant’s benefit. However, other working definitions of biostimulants exist, some of which include plant hormones, other growth regulators, non-essential trace minerals, and micronutrients. The original interpretation of the concept is used herein.
In small quantities, non-hormonal biostimulants have profound effects on plant growth and total biomass production. Plants are seldomly situated in an ideal environment in which they can achieve their full autotrophic capacity and maximum yield. Even in seemingly optimal environments, stressors are omnipresent, even if they are undetectable (Foyer et al., 1994). By stimulating key metabolic growth processes, biostimulants mitigate these stressors’ affect and regulate the response pathways at the molecular level. Also referred to as organic growth enhancers, biostimulants have been demonstrated to increase crop yield, overall plant biomass, plant vigor (Russo and Berlyn, 1990), crop nutritional value (Borsook and Berlyn, 2015), polyphenol content (Berlyn, Shields, and Young, 1995), plant vascular system development, and stress resistance. The effects are self-reinforcing and create a positive feedback loop: Increased root biomass promotes higher nutrient and water uptake, which, in tandem with the elevated hydraulic conductance from increased xylem development, reduces water and nutrient stress levels in leaves, which also benefit from the innate antioxidant capacity of the compounds. The reduced stress in the leaves and the resulting elevated photosynthetic capacity increase the carbon to nitrogen ratio, carbon compounds available for growth, and overall biomass, which in turn invigorates root growth. This is achieved with naturally occurring, non-hormonal ingredients.
These compounds achieve this stress reduction predominately via their antioxidant capacity. Under high light fluxes, plants experience photooxidative stress, which is when the light-dependent photosynthesis reactions generate reactive oxygen species (ROS) when the chlorophyll absorbs more energy than it can physically use in photosynthesis. When the photoelectron chain is bottlenecked due to a dearth of NADP or oxidized electron acceptors, the additional electron, usually from photosystem II, reduces an oxygen molecule. The resulting oxygen species, which are energetic molecules with unpaired electrons, can damage lipids, DNA, RNA, proteins, and is exceptionally harmful to chlorophyll. All stressors effect the electron transport chain or its supporting process to a certain degree and will result in ROS. Examples of ROS include singlet oxygen (1O2), hydroxyl radical (HO.), hydrogen peroxide (H2O2), and superoxide (O2−), one of the most detrimental. The resulting damage is often irreparable and can result in mutagenesis and cell death. Reactive oxygen species are created by metabolic processes even under optimal conditions and have been shown to function in signaling (Shin et al., 2012). In response, plants have evolved antioxidant responses to detoxify these harmful compounds through both enzymatic and non-enzymatic processes that scavenge and neutralize ROS. An enzymatic process, superoxide dismutase (SOD) is an exceptionally powerful antioxidant that to reduces ROS into hydrogen peroxide. Hydrogen peroxide is then converted into water and oxygen in either the peroxisome, which is an organelle containing catalase (CAT), or via the ascorbic acid glutathione chain. Since chloroplasts do not contain catalase, it relies on the latter process. Non-enzymatic processes predominately include lower molecular mass antioxidants, which include ascorbic acid, flavonoids, and glutathione (Huang et al., 2019).
The biostimulant hypothesis is founded on the observation stimulating such pathways can increase detoxification capacity and overall productivity (Berlyn and Beck, 1980). Through identifying the limiting metabolic processes on plant growth, exogenously applying select compounds can relieve the limiting factor and release the bottleneck. This concept has been coined the ‘Limiting-stress-elimination hypothesis’ (Berlyn and Atta-Boateng 2019), building off the fundamental concepts of Liebig’s Law of the Minimum. Generally, biostimulants have proved their ability to effectively address plant stress and the evolving needs of modern agriculture.
As global temperatures rise and the climate becomes more extreme, solutions for dealing with plant stress will become an agricultural necessity. Today, an estimated 45% of agricultural lands responsible for feeding 38% of the population are subject to continuous or frequent drought (Bot et al., 2000). Dudal (1976) estimated that 90% of arable land experiences major environmental stresses. Sanghera et al. (2011) claims that low temperature damage alone costs global agriculture over $2 billion annually in lost production. These statistics have real human implications, as elevated environmental stress threatens food security for entire populations. We believe that the Berlyn Lab’s biostimulant concept can also play an integral part in this transformation because current commercial biostimulant products lack the most bioactive compounds in our formula. Moreover, the substances in the Berlyn formula act synergistically: When applied in isolation, the ingredients have a nominal effect, but together in balanced proportions, the biostimulant’s components unlock a fuller growth potential.
Overall, biostimulants have the potential to increase global crop yield, reduce inorganic fertilizer pollution, and mitigate the effects of climate change. To this end, biostimulants are a powerful tool to ensure the continued prosperity in agriculture and to promote more effective stewardship of the environment.