Six Ways Biostimulants Build Resistant and Resilient Plants
With plants being susceptible to many stresses — many of them unpredictable — growers can turn to biostimulants to mitigate all kinds of stressors, as Natalia Diaz-de-Cerio explains.
During their growth, plants are at the mercy of many potential abiotic stressors, including extremes in temperature, salinity issues, inadequate water supply, heavy metal contamination, and nutrient availability. Even with environmental controls, mineral fertilizers, and automated irrigation, cultivation is a high-risk activity where alterations of the plant environment have a rapid onset, can be unpredictable, and will quickly wreak havoc on a crop.
What Are Plant Biostimulants?
Plant biostimulants are a diverse group of substances made from organically or microbially derived ingredients that can improve plant growth when applied in small quantities. Numerous trials in crops including tomatoes, strawberries, soybean, cucumbers, wheat, and cannabis have demonstrated the ability of biostimulants to mitigate a range of abiotic stress. Biostimulants can improve a plant’s resistance to stress which is measured by how well it tolerates changes in environmental conditions, and they can also improve a plant’s resilience measured by its ability to recover from stress once it is removed.
Most biostimulants are derived from seaweed, chitin (the structural component in insect exoskeletons, crustacean shells, and fungal cell walls), microorganisms (bacteria, fungi, yeast, and microalgae), humic substances, and protein hydrolysates (amino acids derived from agricultural plant and animal by-products). More recently, studies have also shown beneficial biostimulant-like activity from non-essential elements, including aluminium, cobalt, and silica.
Biostimulant Benefit #1: Stress Mitigation
Environmental stress can disrupt the natural balance in plants that governs the generation and accumulation of oxygen radicals and their derivatives known as reactive oxygen species (ROS). These compounds govern cellular signaling and transmit information to a plant regarding changes in its environment. Excess concentrations of ROS cause oxidative stress which can damage DNA, alter cell enzyme activity, and disrupt carbohydrate, protein, and lipid metabolism. Abiotic stress can therefore compromise proper crop physiology due to cell misfunction or death. This can leave plants susceptible to pest and pathogen infection and reduces essential metabolic processes like photosynthesis, adversely affecting the overall quality and marketable yields of a crop.
Applying biostimulants before stress can delay the onset of stress symptoms such as wilting, while applying them after stress can accelerate how quickly a plant recovers from exposure to stress and continues to grow. In some cases, biostimulant application can entirely mitigate abiotic stress-derived impacts on yields.
Different biostimulants target plant resistance and resilience to stress through distinct mechanisms resulting in various forms of adaptation. This makes them interesting tools for pre-conditioning a plant’s defense mechanisms — a process known as priming. This results in a faster, stronger, and more effective response to abiotic stress.
Biostimulant Benefit #2: Eliciting the Antioxidant Defense System
The antioxidant defense system in plants is made up of enzymatic and non-enzymatic processes which work to reduce the build-up of ROS that occurs during stress by either removing them (ROS scavenging) or controlling their production.
Chitosan derived from chitin, is an elicitor meaning it can activate growth-stimulating and stress-mitigating pathways in plants and increase the concentration of defensive secondary metabolites. Its application can increase the activity of ROS-neutralizing antioxidant enzymes like superoxide dismutase and catalase, and non-enzymatic antioxidants including phenolic acids, alkaloids, and flavonoids. These physiological changes help plants mitigate the impacts of salt, temperature, and water stress.
Protein hydrolysates can also induce protective effects in plants by regulating defense phytohormones like jasmonic and salicylic acid which activate the antioxidant system to sustain growth under stress. This reduces the potential cell damage caused by ROS and can also signal the accumulation of amino acid osmolytes like proline and betaine which help maintain plant cell volume. Trials have shown protein hydrolysate-derived biostimulants can maintain plant functioning under temperature, water, salinity, and heavy-metal stress.
Stress defense pathways are also activated by symbiotic growth-promoting rhizobacteria (PGPR) through the release of volatile organic compounds. These signal the plant to produce molecules known as osmoprotectants that can allow them to survive extreme changes to the water potential of their cells.
Biostimulant Benefit #3: Improved Water Use Efficiency
Drought tolerance is the ability of plants to resist dehydration through a complex variety of dynamic adaptations. These can be physiological changes that include the antioxidant defense system but also consist of morphological and anatomical changes including alterations to root architecture, reductions in leaf surface area and stomatal adjustment.
Humic substances like humic and fulvic acids make up 60 percent of the organic matter in the world’s soil and are formed by the chemical and biological transformation of plant and animal matter. Their application targets root morphology and can increase root surface area, stimulate the growth of lateral roots and intensify the activities of root cells to sustain nutrient uptake and intracellular pH regulation. A larger root system benefits plants in drought and saline environments by increasing the explorative potential of the roots while a more complex and efficient root architecture increases water and nutrient use efficiency and promotes rapid growth once the stress is alleviated.
Depending on the species used and method of extraction, seaweed biostimulants contain various beneficial compounds like phytohormones, amino acids, polysaccharides, vitamins, and minerals. In trials with various crops, seaweed products have delayed wilting, decreased water use, improved heat, and salinity tolerance and increased the leaf water content and recovery of drought-stricken plants compared to controls. They do this by regulating phytohormones like abscisic acid (ABA) which enable precise control of stomata delaying their closure during drought and thereby increasing gas exchange and photosynthesis.
An opposite form of resistance comes from the anti-transpirant properties of chitin which can rapidly induce stomatal closure helping to reduce transpiration during hot and water-deficit conditions and conserve valuable water stores. One study on peppers found chitosan application reduced their water use by 26-43 percent without any reductions in final yields.
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Biostimulant Benefit #4: Enhanced Nutrient-Use Efficiency and Availability with Biostimulants
Experiments with plants grown in nutrient-deficient conditions suggest an important role for biostimulants in improving the growth of crops in such instances but also in minimizing the fertilizer inputs and reducing the overall cost of high-intensity cropping systems. Related to poor fertility is salinity stress which causes ion imbalances in plant cells, restricting nutrient absorption and translocation, and reducing cell water potential. Alterations of metabolic processes including photosynthesis, respiration, and phytohormone regulation can all contribute to oxidative stress.
Protein hydrolysate biostimulants can increase the expression of genes that code for the amino acid transporters and enzymes involved in nitrogen assimilation. They can also target root apparatus to facilitate the uptake and translocation of essential macronutrients and micronutrients, particularly iron, zinc, manganese, and copper. Under sub-optimal temperature and nutrient conditions, protein hydrolysates can also alter the salicylic acid content of the rootzone stimulating its growth and improving its architecture.
Microbial root symbionts also improve root morphology through the release of root-growth promoting phytohormones like indoleacetic acid (IAA) which has been linked to salt-stress alleviation. The inoculation of tomato plants with Azotobacter chroococcum, a nitrogen-fixing bacteria, increased their growth under the combined stress of salinity and sub-optimal nitrogen concentrations via improved nitrogen assimilation at the root zone.
Biostimulant Benefit #5: Improvements of Photosynthetic Components with Biostimulants
Chlorophyll is one of the major components of chloroplasts which are the site of most ROS production and where photosynthesis occurs. During abiotic stress, the concentration of chlorophyll is often measured as an indicator of plant metabolic and energetic function and used to estimate tolerance to stress.
Seaweed biostimulants can prime plants to mitigate drought stress by increasing chlorophyll generation and reducing its degradation when applied before stress. This results in a net increase in chlorophyll content and sustained photosynthesis which improves nutrient availability and water-holding capacity, thereby mitigating the possible effects of water deficit on yields. Two separate trials showed seaweed extracts increased the recovery rate of treated soybean by maintaining increased gaseous exchange once drought stress was removed.
Biostimulant Benefit #6: Enhancement of the Substrate Environment
The application of all the biostimulant categories previously discussed can indirectly influence plant resistance and resilience by improving the conditions of the rootzone environment, particularly concerning how they influence the population and activities of symbiotic microbial communities.
Bacteria species like Azotobacter, Bacillus, Enterobacter, and Pseudomonas play a key role in nitrogen fixation and phosphorus solubilization and produce siderophores, molecules that bind and transport iron. The phytohormones they produce also promote stress priming via improvements of root structure and function.
Mycorrhizal fungi co-evolved with plants and form symbiotic relationships with 90 percent of plant species. They enhance plant-substrate interactions by also increasing the surface area of roots and their penetration through growing media thereby improving the distribution of water and nutrients on root hairs before their absorption. Another fungal species, Trichoderma, produce hormones like auxins and gibberellins, and enzymes and antioxidants that make plants more tolerant to stress by priming their defense pathways prior to stress and rapidly activating them during the onset of stress.
Humic substances, chitin, seaweed, and protein hydrolysate-based biostimulants all shape microbial soil communities by providing a valuable food source in the form of carbon and nitrogen. This increases their activity resulting in a net increase in many of the associated plant-microbe interactions and processes.
The Importance of In-House Testing
Although it might seem tempting to combine different biostimulants and apply them all to your plants, their prudent use is advised. Before introducing any new additive into your fertilization regime, you should trial the effects of a single product to determine its efficacy.
One of the greatest difficulties in biostimulant research is repeating the results of another study using different crops and growing conditions. Biostimulants can be expensive so testing one product at a time allows you to more clearly investigate if it is worth adding to your nutrient programme.
Research also continues to try to provide clarity on the most effective dosages necessary to harness the full benefits of some biostimulants since these also depend on the crop being treated and the stress that is targeted. In the case of protein hydrolysates, humic substances, and chitosan, growth-depressing effects have also been found by research trialing multiple dosages. Read the recommended application rates for every product and resist the temptation to apply products at concentrations above those stated.
Maximum Yield uses high-quality sources to support the facts within our content including peer-reviewed studies, academic research institutions, professional organizations, and governmental organizations.
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