Biostimulants : nature, function, regulation, uses

Before presenting biostimulants, we need to ask ourselves why a plant would need biostimulants to thrive. To answer this question, it's important to remember a plant's nutritional requirements. To ensure its growth and development, a plant needs oxygen for respiration, carbon dioxide and solar energy for photosynthesis, which provides it with carbon, water and nutrients from the soil. Classically, a distinction is made between major nutrients: nitrogen, phosphorus and potassium, which are essential for the synthesis of plant cell constituents, and trace elements: copper, iron, magnesium, manganese, sulfur, zinc, etc., which contribute to the plant's proper physiological functioning. In the natural environment, it is the soil that provides these nutrients, but it is important to underline the fundamental role played by the action of living organisms, particularly micro-organisms. These contribute to the breakdown of organic matter and the supply of nutrients needed by plants. Soil-plant-microorganism interactions are particularly important in the plant rhizosphere. This corresponds to the volume of soil under the influence of the plant's roots; it is the site of the main interactions between the plant and telluric micro-organisms, on the one hand, and interactions between micro-organisms themselves, on the other. The distance at which a plant's root affects microbial activity is extremely variable, depending on soil type, plant species and microbial activity. The plant's root modifies to a very large extent certain soil characteristics such as pH, water potential and oxidation-reduction potential, and provides, via root exudates, numerous elements, in particular sugars and organic acids, which stimulate microbial activities and their interactions.

In agricultural soils, the nutrient resources available to plants are often insufficient to meet production targets, and farmers must resort to fertilization. This has been particularly the case since the 1950s, when, following the Second World War, the aim was to achieve food self-sufficiency. Agriculture has become more specialized and intensified thanks to the development of high-yielding hybrid varieties, the intensive use of synthetic organic pesticides, the widespread use of chemical fertilizers and fertilizers, and the expansion of mechanization and motorization. This intensification of farming practices and the abandonment of mixed crop-livestock systems have led to a decline in soil "quality", reflected in an overall drop in the organic matter content of field crop soils and a reduction in microbial biodiversity. In response to this situation, organic soil improvers and, more recently, a wide range of "biostimulants" have been proposed to offset the harmful effects of such practices. These biostimulants are made up of algae or plant extracts,...