Abiotic stresses encompass a range of non-living environmental factors that can detrimentally affect plant growth and development. These stresses include extreme temperatures, water scarcity or excess, soil salinity, nutrient imbalances, light intensity variations, wind, air pollution, and heavy metal contamination. For instance, heat stress can lead to dehydration and leaf scorching, while drought stress can result in stomatal closure and reduced growth. Soil salinity disrupts osmotic balance and nutrient uptake, and nutrient deficiencies or toxicities impair plant metabolism and productivity. In response to these challenges, biostimulants and biofertilizers have emerged as promising tools to enhance plant resilience to abiotic stresses.
Abiotic stresses are non-living environmental factors that can negatively impact plant growth, development, and productivity. Below, we will look deeper into these abiotic stress issues.
Temperature Stress
Heat Stress: High temperatures can lead to heat stress, causing dehydration, wilting, leaf scorching, and even plant death. Heat stress can disrupt chlorophyll production, photosynthesis, protein denaturation, and metabolic processes.
Cold Stress: Frost and freezing temperatures can damage plant tissues, leading to cell rupture, impaired water uptake, and reduced photosynthesis. Cold stress can also affect nutrient uptake and metabolism.
Water Stress
Drought Stress: Insufficient water availability in the soil can lead to drought stress, causing stomatal closure, reduced cell expansion, wilting, and decreased growth. Drought stress can impair nutrient uptake and increase susceptibility to diseases and pests.
Flooding Stress: Excessive water in the soil can restrict oxygen availability to plant roots, leading to hypoxia and root damage. Flooding stress can cause nutrient leaching, root rot, and growth inhibition. Flooding can also lead to anaerobic and compacted soils, removing valuable oxygen in the soil and choking out roots and nutrient flow to plants.
Salinity Stress
Soil Salinity: High concentrations of salts in the soil can disrupt osmotic balance, leading to water stress and ion toxicity in plants. Salinity stress can impair nutrient uptake, inhibit root growth, and cause leaf necrosis and chlorosis.
Salt Spray: Coastal plants may experience salt spray from ocean winds, leading to salt deposition on leaves. Salt spray can damage leaf tissues, impair photosynthesis, and increase susceptibility to pathogens (biotic stress).
Nutrient Stress
Nutrient Deficiency: Inadequate essential nutrients such as nitrogen, phosphorus, potassium, hydrogen, oxygen, and micronutrients can limit plant growth and development. Nutrient deficiencies can cause chlorosis, stunted growth, and reduced yield.
Nutrient Toxicity: Excessive concentrations of certain nutrients, such as heavy metals or salts, can lead to nutrient toxicity in plants. Toxic levels of nutrients can disrupt cellular processes, inhibit enzyme activity, and cause oxidative stress.
Light Stress
Excessive Light: Intense sunlight can cause photoinhibition, leading to damage to the photosynthetic apparatus, the production of reactive oxygen species (ROS), and oxidative stress in plants.
Low Light: Insufficient light intensity can limit photosynthesis, reducing growth and yield. Plants may exhibit elongated stems, pale leaves, and reduced chlorophyll content under low-light conditions.
Other Abiotic Stressors
Wind Stress: Strong winds can cause mechanical damage to plant tissues, breakage of stems and branches, and disruption of transpiration and gas exchange.
Air Pollution: Pollutants such as ozone, sulfur dioxide, and nitrogen oxides can damage plant tissues, impair photosynthesis, and increase disease susceptibility.
Heavy Metal Contamination: Elevated levels of heavy metals in the soil can inhibit plant growth, disrupt nutrient uptake, and cause physiological disorders.
Micro and Nanoplastics: These small plastics can get into the stoma and block nutrient flow and CO2, which plants need to survive. (See: Microplastics and Nanoplastics: Ubiquitous Pollutants, Bioaccumulators, and Toxins).
Biostimulants promote plant growth and stress tolerance by stimulating physiological processes. As Patrick du Jardin described in his 2015 paper, Plant biostimulants: Definition, concept, main categories and regulation, “A plant biostimulant is any substance or microorganism applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrients content.” (du Jardin, 2015) They can include humic and fulvic acids, seaweed extracts, amino acids, and microbial-based products. For example, humic substances improve soil structure and water retention, while seaweed extracts contain plant growth-promoting compounds like cytokinins and auxins. On the other hand, biofertilizers are formulations containing beneficial microorganisms such as nitrogen-fixing bacteria, phosphate solubilizers, and mycorrhizal fungi. These microorganisms enhance nutrient availability, improve soil health, and facilitate nutrient uptake by plants.
Biostimulants and biofertilizers address abiotic stresses through various mechanisms. For instance, they can improve soil structure and water-holding capacity, enhance nutrient availability and uptake, stimulate root growth, and activate plant defense mechanisms against stressors. Biostimulants and biofertilizers help mitigate the adverse effects of abiotic stresses on crop productivity and quality by promoting plant vigor and stress resilience.
Effective Microorganisms, sold globally as EM•1, is the microbial component of bokashi. In making bokashi, EM•1 is added to stimulate the fermentation of organic materials (dried plants, food waste, manures, etc.). By itself, EM•1 is a biostimulant, with its groups of microbes—lactic acid bacteria, yeasts, and photosynthetic bacteria—protecting plants from several abiotic stresses, including drought, as shown in the research by Kalaji et al. below.
Kalaji et al ran their 2018 drought tolerance study on Arabidopsis thaliana grown in a controlled greenhouse, a type of mustard that is cabbage, broccoli, turnip, and canola with plants that were inoculated with Effective Microorganisms. The main parameters they used were chlorophyll production and photosynthesis rates. They concluded application of EM in the irrigation water was able to improve chlorophyll production by as much as 90% compared to the ontrol and the EM has a positive influence on chlorophyll production and photosynthesis rates under drought conditions. (Kalaji et al., 2018).
Bokashi is a biofertilizer and biostimulant, as it contains the EM•1 microbes, nutrients, and organic matter that can relieve plants of abiotic stress. There are two ways bokashi can be made: aerobic and anaerobic. The aerobic method tends to create heat and release ammonia and requires active turning, similar to thermophilic composting. Unlike thermophilic composting, bokashi can be used on plants once the heat stops being produced. Compost requires a curing process of ninety days or more. The anaerobic method of making bokashi is similar to making silage. However, it uses 30% to 35% moisture instead of 60% to 70%. Both silage and an agricultural anaerobic bokashi take two to three months of fermentation. Once fermentation is complete, bokashi is added to fields at a rate of 2 to five tons per acre and can provide up to 100% of the nutrients needed to grow crops, depending on the state of the soil. A paper from 2023 by Kruker et al., summarizes several other papers conducted on bokashi as a biofertilizer.
The combination of the bokashi-type biofertilizer with substrates is beneficial for the cultivation of vegetables, as it increases the efficiency of nutrient use by activating enzymes that stimulate the absorption and chelation of certain elements [65]. Through the mineralization process of the organic substances present in these mixtures, several nutrients and organic molecules are released into the soil solution, promoting plant growth [69]. Collectively, these mechanisms contribute to biomass production and can also influence the content of macronutrients within plant tissues. … Some of the reference studies that applied bokashi to the soil reported an increase in the crop yields of sweet corn [71], tomato [72], Pogostemon cablin [73], and Origanum vulgare [74]. A similar study evaluated the effect of organic fertilizers, organic compost, biodynamic compost, laminar compost with manure, and laminar compost with bokashi on beet production and concluded that all forms of fertilization were satisfactory in terms of nutrient supply and beet production [75]. Several studies with the use of bokashi in different doses and formulations have shown positive results and increased yields of brassicas and other vegetables [76,77,78]. Among the benefits of using organic fertilizers are the increase in the amount and diversity of microflora, which favor the cycling of nutrients, thus improving the chemical fertility conditions of the soil, in addition to acting in the control of pests and diseases such as cruciferous hernia, caused by Plasmodiophora brassicae Wor [79]. The use of 4 Mg ha−1 bokashi biofertilizer resulted in greater vegetative development of cabbage plants, with an average productivity of 80.36 Mg ha−1 [79]. It evaluated the doses of bokashi 0, 2.5, 5.0, 7.5, and 10.0 Mg ha−1 in a test with summer broccoli, and it was possible to verify that the doses of bokashi linearly increased the characteristics of plant height, number of leaves per plant, stem and head diameter, and average head mass up to the maximum dose used, which corresponded to 10 Mg ha−1 [80]. The cabbage crop is responsive to organic fertilization with bokashi, and the application of a dose of 10.0 Mg ha−1 of the biofertilizer resulted in plants with superior characteristics, providing higher crop productivity [30]. (Kruker et al., 2023)
Example Programs
An example of a biofertilizer and biostimulant program would be as follows:
An agricultural bokashi containing manures, plant materials, and EM•1 is applied at 2 to 5 tons per acre a week or two before planting.
Seeds are treated with Activated EM (AEM) at 8 fluid ounces per 100 pounds of seed. The seeds are added to a feeder. The AEM is poured into the feeder. Using a pole, the materials are mixed as thoroughly as possible.
AEM is applied at 5 gallons (20 liters) per acre at planting.
AEM applications vary by crop type:
For cotton, corn, soy, and wheat, 5 gallons (20 L) of AEM are applied per acre about 1 month after the plants sprout. This can be applied with an irrigation system, a sprayer, or via airplane.
For vegetable crops, 1 to 5 gallons per acre per week can be applied through an irrigation system or sprayer.
For tree and vine crops, 40 gallons (160 L) per acre of AEM are applied per season. Divide the total gallons by the number of irrigations.
Apply 5 gallons (20 L) per acre per cut for alfalfa and hay.
For gardening, try the following:
Bokashi
Make your bokashi in the fall. It can contain manure, food waste, and chopped plant waste.
Apply 2.2 ounces of bokashi per square foot of soil in the garden to prepare the soil. Incorporate it into the soil with an iron rake. You may apply more bokashi throughout the growing season.
EM
Make a quart or gallon (1 liter or 4 liters) of AEM.
Add 5 or 6 fluid ounces (148 ml or 177 ml) per gallon (~4 liters) of water to the area where seeds were freshly planted. Continue this process as needed until the plants have three leaves.
Water your plants with 1 teaspoon (5 ml) per gallon (4 liters) of water once per week until harvest. This can be done on the entire yard, including grass and any other plants on the property.
In the fall, leave the plant material to protect the soil and perennials. Using a hose-end sprayer, spray 5 or 6 fluid ounces (148 ml or 177 ml) per gallon (~4 liters) of water onto the garden area. Depending on size and thickness, the microbes will slowly break down the plant material.
Biofertilizers and biostimulants can be used wherever plants are grown. They provide bioavailable nutrients and protect plants against various abiotic stresses. Several application programs exist, like the one above, for biostimulants and biofertilizers to optimize their efficacy in mitigating abiotic stresses. Farmers and gardeners can incorporate these products into their soil management practices through foliar sprays, seed treatments, root drenches, and soil amendments like compost and minerals. Biostimulants and biofertilizers can replace conventional fertilizers to achieve synergistic effects on plant growth and stress tolerance. Utilizing biostimulants and biofertilizers is a regenerative approach to improving crop resilience to abiotic stresses and enhancing agricultural sustainability.
References
du Jardin, P. (2015). Plant biostimulants: Definition, concept, main categories and regulation. Sci. Hortic. 196, 3–14. doi: 10.1016/j.scienta.2015.09.021. https://www.sciencedirect.com/science/article/pii/S0304423815301850?via%3Dihub
Kalaji, H. M., Pathom-Aree, W., Lotfi, R., Balaji, P., Elshery, N., Górska, E. B., Swiatek, M., Horaczek, T., Mojski, J., Kociel, H., Rytel, M. R., and Yoosathaporn, S. (2018). Effect of microbial consortia on photosynthetic efficiency of arabidopsis thaliana under drought stress. Chiang Mai University Journal of Natural Sciences. https://www.researchgate.net/publication/327307510_Effect_of_Microbial_Consortia_on_Photosynthetic_Efficiency_of_Arabidopsis_thaliana_under_Drought_Stress
Kruker, G., Guidi, E. S., Santos, J. M., Mafra, Á. L., & Almeida, J. A. (2023). Quality of Bokashi-Type Biofertilizer Formulations and Its Application in the Production of Vegetables in an Ecological System. Horticulturae, 9(12), 1314. https://doi.org/10.3390/horticulturae9121314