Improvement of blueberry production with the application of bacterial biostimulants
There is a notable trend in the incorporation and reconversion of agricultural lands towards tree and shrub crops, a dynamic marked by the demographic characteristics of the rural environment and the possibilities of mechanization. In this regard, some crops such as blueberry have an outstanding trajectory in some peninsular regions such as Huelva (Spain) or Sever do Vouga (Portugal) where there have been plantations for a couple of decades, although due to the high demand of the market and the conditions climatic conditions that allow production in early times, their interest has been increasing in many other parts of the peninsular geography during the last decade, but always limited by the edaphoclimatic needs of the crop (Fig. 1).
Fig. 1. Examples of blueberry crops in the Beira Interior region (Portugal). In this region, blueberry cultivation is a growing crop with hundreds of new hectares every year.
The demand at the market level has been increased by the diffusion of the properties of blueberries, a nomenclature applied in a broad sense from the botanical point of view considering those species of the genus Vaccinium which have a high fiber content, low amount of sugar, and a high concentration of antioxidant compounds such as anthocyanins and flavonoids, whose consumption has been shown to be related to a decrease in cardiovascular diseases.
Although it is naturally common to find wild blueberry plants in the mountainous regions of the north and central peninsula, their distribution is limited to the areas with the highest rainfall and lowest ombrothermia in the mountains, on soils rich in organic matter and good drainage. The blueberry species that we find distributed indigenously in the Iberian Peninsula is identified with Vaccinium myrtillus (Photo 1), of Eurasian distribution and small size, in contrast to the more widespread cultivated species, V. corymbosum of American origin, taller and less need for cold and humidity. This fact is marked by the domestication processes within the genus Vaccinium to which blueberries belong, carried out in the American continent at the beginning of the last century, selecting larger varieties, less tendency to develop rhizomes and above all, a wide range of varieties according to their needs for cold hours, the latter essential fact for the dispersion and expansion of the potential range where to settle this crop.
Photo 1. Wild blueberry (Vaccinium myrtillus) in its natural habitat.
Microorganisms used as inputs
The revolution that is being introduced in modern agriculture is the incorporation of microorganisms as inputs for the improvement of agricultural production, replacing or complementing the traditional chemical synthesis fertilizers. The use of biostimulants has already shown its potential in horticultural, legume and extensive crops to improve the yield of crops through the contribution of nutrients, the improvement in the use of soil resources and a reduction in sensitivity to stresses that may affect the plant. In this way, microorganisms are able to exert their action through the so-called plant growth promotion mechanisms (PGP-Plant Growth Promotion), which are those activities that have a positive activity on the development of the plant, and can be classified directly or indirect.
- The former are based on those biological activities that have a direct influence, for example, the solubilization of phosphate.
- The latter are based on aspects of the metabolism or the biology of the microorganism, which do not have to exert a positive effect on the development of the plant, but when carried out under certain conditions or situations improve the development of the plant, such as for example the production of biofilms at the root of the crop.
Among the direct mechanisms we find those related to the contribution of nutrients to the crop, such as the biological fixation of nitrogen carried out in free life by bacterial genera such as azospirillum y Azotobacter, or in symbiosis with legumes such as Rhizobium (Photo 2), being able to contribute to the crop up to 70 kilos of nitrogen per hectare the first and up to 300 kilos the second.
Photo 2. Rhizospheric nitrogen fixing bacteria grown in the laboratory.
Another mechanism is the solubilization of phosphate and potassium from the soil. Phosphorus is an element normally abundant in the soil, but not available, which due to the action of some agents is solubilized by chemical or biological action and made available to plants through the soil solution. In the case of potassium, it depends largely on the type of soil, but the mechanism of action is similar.
Third, a mechanism with outstanding importance in certain crop cycles is the production of siderophores, organic compounds with chelating capacity on iron and whose capture requires specific recognition mechanisms shared by bacteria and plants.
Indirect biostimulation
The last of the considered indirect mechanisms is the production of phytohormones such as auxins, cytokinins and gibberellins. This mechanism allows bacteria to modify the development of the plant by increasing root development, increasing root volume and improving the development of the aerial part of the plant. Among the mechanisms we find very varied activities such as the production of lytic enzymes capable of attacking pathogens, the production of biofilms or biofilms by means of which they actively compete for space in the rhizosphere, displacing other microorganisms that are not desirable for cultivation. The mechanisms considered indirect is the production of ACC-deaminase, an enzyme that acts on the ethylene precursor, a phytohormone closely linked in response to stress and that induces senescence processes in plants. In this way, bacteria are able to reduce the incidence of abiotic stresses in the crop. It has also been described how some of the bacteria used as bioinoculants are capable of inducing a state called systemic resistance through which the plant activates molecular mechanisms of response to abiotic and biotic stresses, so the culture will present a lower susceptibility to conditions. adverse.
These mechanisms allow a more rational use of resources, improving the efficiency in which the crop uses edaphic resources, in addition to increasing the health status of the plant and the capacity to respond to the adverse conditions it faces. the crop, especially relevant in the current climate change scenario and in a crop that is especially dependent on environmental conditions such as blueberries.
Evaluation of microorganisms
Figure 2. Taxonomic composition of the bacterial populations of 3 locations of blueberry crops where it is observed how the root increases or reduces the proportion of certain taxa and even others appear absent. Knowing these dynamics allows creating personalized biostimulation strategies with the aim of designing a more efficient agriculture.
The identification of microorganisms is an essential activity due to the large number of microorganisms that inhabit the plant environment and inside it. Massive sequencing techniques allow us to know effectively the complete diversity that we find associated with plants and the environments where they grow. These techniques authorize, from total soil DNA, to know which species are present in the soil. Through these massive sequencing techniques we have been able to learn how plants act as a selective medium for the populations of bacteria and fungi that inhabit the soil. It has been observed that plants are able to create a volume around their root influenced by the exudation of organic compounds that modify the microbial populations of the soil creating what is called rhizosphere, which would be the volume of soil that is under the influence of plant. In turn, it has also been observed how the plant is capable of acting as a filter for rhizospheric populations, selecting certain microorganisms that are capable of colonizing the interior of the plant (Fig. 2). This knowledge allows us to create new strategies for the design of bio stimulants, adapting it to the needs of the crop and to the local edaphoclimatic conditions, thus allowing us to increase the efficiency in the application of microorganisms.
Production improvement
Another relevant aspect of microorganisms is the ability they have shown to improve production at a qualitative level, increasing the concentration of bioactive compounds such as phenolic compounds and vitamins, which are of special relevance in blueberries, being one of the reasons why This red fruit is so appreciated. This activity has been observed associated with the inoculation of biostimulants of the genus Rhizobium and Phyllobacterium when they have been applied in horticultural crops such as lettuce and other crops of red fruits such as strawberry, observing an increase in the concentration of colored and non-colored phenolic compounds. and in the case of strawberry, increases in vitamin C concentration of up to 120% have been described.
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