Societal demand and changing production patterns have resulted in abundant development of biostimulants. There are many solutions on the market claiming biostimulant effects on plants. It is interesting to take stock of the demonstrated effects on plants as well as the benefit realized by the grower.
INCREASED ABSORPTION OF MINERAL ELEMENTS
Nitrogen is an important element for the plant, its metabolism is well known, and many active ingredients have demonstrated their effects on the assimilation of nitrogen. This is the case of protein hydrolysate, rhizobium spp and the seaweed extract. For other minerals, the work is less significant and the effects less known. It is nevertheless noted that effects on phosphatase have been demonstrated with beneficial fungi as well as by algae extract. The main benefits mentioned in trials or research are an increase in the mineral content of plants, thus allowing a better yield as well as an increase in certain quality criteria. In some extreme cases, the doses of mineral elements are reduced because the use of CO2 is better so the risk of pollution is reduced.
ENHANCED PHOTOSYNTHETIC ACTIVITIES OF PLANTS
Photosynthesis is the engine of plant functioning, although it is dependent on the nutritional state of nitrogen. It has been shown that protein hydrolysates and seaweed extracts assist with an increase of synthetic activity. Many studies demonstrate how the application of biostimulants increase the color of the leaves. For example, amino acid, peptide and seaweed extract applications have shown positive effects on photosynthetic pigment. In this particular case, with the main application being by foliar treatment, the benefit identified was an increase of yield and a better tolerance to abiotic stress.
EFFECT ON HORMONAL BALANCE
Biostimulants have recognized effects on the hormonal balance of plants. They act on the production of growth hormones, anti-stress (auxin, gibberelins…) and fructification hormones. This effect is demonstrated by seaweed extracts, amino acid and peptide components and microorganisms. As an example of a biostimulant’s effect of the production of growth hormones, the biostimulant is applied either by foliar application to stimulate the plant during the vegetative growth phases; to mitigate the effects of abiotic stress, such as drought, or temperature; or during the fruiting phases on fruit trees and vine. The benefit realized by the grower is better yield by the number fruit per tree, or quality by the better forming of the fruit. In a second case, they are applied to the soil or in seed treatment to promote the development of seedlings, resulting in a better germination rate and more regular plant emergence.
IMPROVED RESISTANCE TO ABIOTIC STRESS
The resistance to abiotic stress is directly related to the mode of action of the products mentioned above. But some particular biostimulants, namely protein hydrolysates from plant/vegetal sources, demonstrated a better tolerance of plants to the toxicity of heavy metals (Zn,Cu , etc.). The biostimulants may be applied by foliar application when the toxicity is demonstrated; the benefit realized is a better vegetative development or yield. Use of biostimulants to reduce the effects of, for example, drought stress demonstrates varying efficacy in different situation and crops. The biostimulant may be applied prior to stress to “prime” the plant to better handle the forecasted stress. Or it can be applied during or after the drought. The biostimulant effect increases photosynthesis and a plant’s biomass, resulting in more yield.
Many studies and uses show positive effects of biostimulants with more or less significant benefits in controlled situations. In a grower’s field, the specific actions of the biostimulant can be more difficult to quantify. It is important for growers to understand the modes of actions of the various biostimulants. And careful employment of the right biostimulant applied at the right time is necessary to realize optimal benefits from the use of biostimulants.
Biostimulants are natural and environmentally friendly substances with positive effects on plant growth enhancing plant metabolism and physiology when applied in small doses. The most common biostimulants’ categories are described below.
Seaweed form an integral part of marine costal ecosystem and it has been estimated that there are about 9000 species. In agriculture, the type most commonly used is ascophylum Nodosum, Fucus spp, Laminaria spp, Sargassum spp, and Turbinaria spp. Seaweed extracts act as biostimulants mainly due to the presence of plant hormones. Main phytohormones identified in seaweed extracts are: auxins, cytokinins, gibberelins, abscisic acid and ethylene. On the market, there is variety in production of seaweed extract. These extractions provide processed products that are either solid and or liquid depending on the purpose of the applications. Thus we find products that more or less transformed – from simply crushed seaweed to extractions of active molecules.
PEPTIDES AND AMINO ACIDS
Peptides and amino acids are important molecules in plant mechanisms because they are the basis of proteins. Some bioactive peptides are able to modulate important physiological functions of plants because they bind to the cell receptors that regulate specific metabolic processes. There are two sources, an animal source which is obtained by a hydrolysis of the collagen at high temperature, and vegetal hydrolysates which are obtained by enzymatic hydrolysis at low temperatures.
Some amino-acids like Hydroxyproline and Hydroxylysine are only present in hydrolysed collagen (leather). Some organic regulations limit the use of animal, to the non-edible part of the crop.
Humic substances are natural constituents of the soil organic matter resulting from the decomposition of plant, animal and microbial residues, but also from the metabolic activity of soil microbes using these substrates. Humic substances are collections of heterogeneous compounds, originally categorized according to their molecular weights and solubility into humins, humic acids and fulvic acids. Humic substances have long been recognized as essential contributors to soil fertility, acting on physical, physico-chemical, chemical and biological properties of the soil. Most biostimulant effects refer to the amelioration of root nutrition, via different mechanisms.
Inside plants, they are a lot of useful substances which act as biostimulants. Each plant extract is unique due to the origin of the vegetal and the process of extraction. So it’s difficult to highlight specifics. However, plant extracts can act directly on the crop without demanding a lot of energy.
There are two large families of microorganisims – beneficial fungi and beneficial bacteria. The first category is strongly represented by Micorrhizal fungi. These fungi work in a mutualistic symbiose, between plant roots and some fungi of the soil. Mycorrhizal fungi colonize the plant roots and provide them mineral elements and water they extract from soil through an external net of hyphae, whilst the plant supplies the micro-organism with radical organic compounds and sugar. Thanks to mycorrhizae the plant increases its root surface from 100 TO 1000 TIMES, and consequently the absorption capacity of nutrients and water.
Mycorrhizae are produced by in vivo or in vitro techniques. The first one ensures that the fungi are grown as in natural conditions and by consequence are more resistant on the soil conditions.
Some other fungi (like some strains of Trichoderma) stimulate plant development and increase plant productivity. They are also able to induce the formation of rootlets and stimulate the colonization of the rhizosphere and root by other beneficial micro-organisms.
Beneficial bacteria settle by association within the rhizosphere. They will act mainly on the nutrition functions of plants – in particular on the increase in the absorption of minerals, and better resistance to abiotic stress. The main products offered on the market are rhizobium and plants growth–promoting rhizobacteria.