Microalgae as Plant Bio-Stimulants
Strategies concerning the future of agriculture are of great importance considering the increasing demand for population growth. In general, agricultural research is focused on increasing yields. Bio-stimulants are promoters, which improve productivity and product quality by increasing the efficiency of plant nutrient uptake.
Introduction to Bio-Stimulants
Typically, bio-stimulants are defined as compounds that produce higher crop yields through metabolic stimulation or nutrient optimization. Agricultural bio-stimulants include a wide variety of compounds such as substances and molecules, microorganisms, plant growth-promoting inter-rhizosphere bacteria, trace elements, enzymes, plant growth regulators, as well as plant and algal extracts.
Fig.1 Schematic showing the key to commercialization of microalgae-based bio-stimulants. (Kapoore, R. V., et al., 2021, Biotechnology Advances)
Bio-Stimulant Effects Caused by Microalgae
By applying different concentrations of algae-produced plant bio-stimulants to different parts of the plant, different promotion effects will be produced.
Plant Foliar Growth Promoted by Algae
It is found that foliar spraying of the cyanobacteria Anabaena vaginicola or Nostoc calcicola on cucumber, pumpkin, and tomato plants significantly will increase the dry weight, fresh weight, root length, height, and leaf number of these three plants compared to the control group.
Plant Root Development Promoted by Algal Bio-Stimulants
Bio-stimulants from algae can also affect root development. For example, Chlorella kessleri extracts, which are rich in phytohormones such as auxin and gibberellins (GAs), improve germination, seedling growth parameters, leaf area, pigment content, and sodium and potassium accumulation in roots and shoots in Vicia faba compared to the control.
Reducing the Effects of Salt Stress on Crops Through Algae
Excess salt is toxic to plants and causes ionic imbalance and osmotic stress. Application of extracts of Dunaliella salina and Phaeodactylum tricornutum reduced salt stress during germination of pepper plants while enhancing root growth and increasing germination rate. Compared to control samples, microalgae significantly reduced superoxide radical production and significantly increased the presence of antioxidant enzymes, implying that salt stress altered oxidative metabolism.
Improving Crop Germination, Flowering, and Fruit Yield
The different sites and concentrations of application can have a significant impact on the activity of plant bio-stimulants. For example, cell extracts of Scenedesmus dimorphus are applied to Roma tomato plants. When applied at doses greater than 0.75 g/mL, seed priming will cause germination to occur two days earlier and lateral root development to be greater, while foliar application at a concentration of 3.75 g/mL, it will increase plant height, number of flowers, number of per number of branches, and cause early fruit development.
Diverse Bio-Stimulants Produced by Algae
Algae metabolites contain a variety of different bio-stimulants.
Phytohormones - small naturally occurring signaling molecules produced at low concentrations that act as chemical messengers to regulate and stimulate the overall growth and development of land plants.
Protein Hydrolysates and Amino Acids - a major class of plant bio-stimulants with direct roles in regulating plant carbon and nitrogen metabolism, as well as exerting auxin and gibberellin activity in plants to improve overall growth and crop yield.
Polysaccharides - involved in many metabolic pathways of plants and therefore can be a good source of potential bio-stimulants for overall crop improvement and protection against biotic and abiotic stresses.
Antioxidants - including vitamins C and E, carotenoids, chlorophyll, and phenolics, which can be used to improve plant growth.
Allelochemicals – with potential uses as equivalents of chemicals to inhibit microorganisms, insects, or weeds.
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Reference
- Kapoore, R. V., Wood, E. E., & Llewellyn, C. A. (2021). Algae biostimulants: A critical look at microalgal biostimulants for sustainable agricultural practices. Biotechnology Advances, 49, 107754.
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