The rhizodeposition of plants (and its incredible potential)

Plants are living beings that have evolved over millions of years from a very particular premise . They cannot move! However, they know how to activate resources, such as rhizodeposition , to solve many problems that they must face on a daily basis.

This effect has slowly modified the behavior of the plants to be able to defend themselves from a very aggressive environment, but with the particularity that it always does so passively.

That is to say, animals have a great difference from plants in that when faced with a negative stimulus they have the ability to flee. If you are facing an attack from another animal and you can certainly lose out, it is best to run and escape.

However, in the world of plants this is not the case. As they cannot flee , they have to fight, activate a multitude of metabolic routes and defend themselves with very complex systems (jasmonic acid, nitrous oxide, ethylene, etc.) to try to repel the plague or disease.

As for the ground, the same thing happens. If the plants grow in an inappropriate environment (high salt content, few nutrients, low organic matter, soils without water or flooded, etc.), it must continually generate stimuli to try to adapt to the environment or improve it.

This is where the concept of rizodeposition comes in.

Index

WHAT IS THE RHIZODEPOSITION OF PLANTS?

Rhizodeposition is the clearest way by which plants can interact with the environment around them and try to modify their conditions for the better.

A large part of the energy obtained by the plant (photoassimilated compounds or photosynthates) through photosynthesis is used to increase the development of its roots.

It is the phenomenon that consumes the most energy from photosynthesis (up to 60% of the total) and has a very clear function: the more roots and the longer they are, the greater the capacity to capture nutrients in the soil.

However, this attempt at root development, which consumes more than half of the total energy that the plant obtains through the environment, is an enormous energy expenditure that often does not compensate because the soil conditions are not appropriate.

If we have a poor soil, with salinity and low content of organic matter, the plant will use a large amount of its energy in forcing the growth of the roots to “explore” new sources of energy.

Produce a greater number of roots and lengthen the existing ones to try to absorb as many nutrients as possible. Always with the premise that plants do not eat, they only drink.

ENERGY LOSSES IN RHIZODEPOSITION

Now, with the term of the rhizodeposition there is an indirect factor that the plant also manages to improve the environment.

Although we said that there was a movement of energy from photosynthesis to the roots to grow, this does not allow us to improve the environment.

No matter how many roots there are , both in surface and depth, if the environment is poor or saline, it will not improve.

The solution that plants have is to excrete part of the energy they have in the roots (displaced from the leaves to the root surface) and improve the environment that surrounds the roots.

This content of carbon and other elements modifies the behavior of the soil, improving the conditions for the plant to live in a suitable environment.

However, what does these rhizodepositions contain that the plant secretes into the environment?

Photo: Sánchez de P., Mondragón and Ceballos, 2005.

CARBON AS THE BASIS OF RHIZODEPOSITION

The main molecule and energy axis of the plant is carbon. It is the element that is processed from the degradation of others such as nitrogen, phosphorus, potassium, etc.

They are sugars and other compounds that are needed to produce energy and use it by the plant in flowering, fattening fruits, maturing, etc.

With the rhizodeposition, the plant loses this energy based on a future improvement of the environment to be able to more easily have said feeding.

This rhizodeposition is composed of elements rich in carbon such as sugars, hormones, organic acids, enzymes, etc.

Let’s see it broken down.

COMPOUNDS	MOLECULES
Sugars	Glucose, fructose, sucrose, maltose, galactose, rhamnose, ribose, xylose, arabinose, raffinose, oligosaccharides, mannose, fucose, deoxyribose.
Amino compounds	Asparagine, α-alanine, glutamine, aspartic acid, leucine / isoleucine, serine, aminobutyric acid, glycine, cystine / cysteine, methionine, phenylalanine, triosine, threonine, lysine, proline, tryptophan, β-alanine, arginine, homoserine, cystathionine. All naturally occurring amino acids.
Organic acids	Tartaric, oxalic, citric, malic, acetic, propionic, butyric, succinic, fumaric, glycolic, valeric, malonic, lactic, galacturonic, glucuronic.
Fatty acids and sterols	Palmitic, stearic, oleic, linoleic, linolenic, cholesterol, campesterol, stigmasterol, cytosterol.
Growth factors	Biotin, thiamine, niacin, pantothenate, choline, inositol, pyridoxine, p-amino-benzoic acid, n-methyl nicotinic acid, indolacetic acid, indole 3-carboxylic acid.
Nucleotides , flavonones	Flavonona, adenina, guanina, uridina /citidina.
Enzymes	Phosphatase, invertase, amylase, protease, polygalacturonase, esterases, trehalase.
Miscellaneous compounds	Auxins, scopoletin, fluorescent substances, hydrocyanic acid, glycosides, saponin, organic phosphorus compounds, nematode hatching and encyst factors, nematode attractants, fungal mycelial growth stimulants, mycelial growth inhibitors, zoospore attractants, stimulants and spore and sclerotia germination inhibitors, bacterial stimulants and inhibitors, weed germination stimulants.

All these molecules or compositions can be produced by the plant naturally. They have great energy and hormonal potential. They are systems of growth, defense, sugars etc.

All these substances are the final by-product of the nutrients that we contribute through fertilization. However, the plant is able to “lose” them back to the soil to improve the environment.

WHAT EFFECT DOES THIS RIZODEPOSITION HAVE ON THE MEDIUM?

All these molecules produced and excreted to the soil through rhizodeposition are based on improving the environment, reducing salts, associating with microorganisms, etc.

Let’s see what effect produces, in a broken down, all this production of metabolic substances.

Compound	Characteristics	Features
Radical exudates	Diverse, typical of living cells), with high and low molecular weight.	They mobilize direct and indirect nutrients, protection matrix and lubrication that facilitates the colonization of the roots in the soil. They modify the structure and biological activity of the soil. Some of them are based on phytohormones and others, for example vitamins, growth factors.
Lisados	Resulting from autolysis and degradation of senescent epidermal and cortical cells and by action of microbial metabolites	Sources of organic materials for microbial populations. They are part of the radical exudates.
Secretions	High molecular weight compounds that cross cell membranes with energy expenditure (ATP).	They catalyze the degradation of organic and inorganic materials naturally present in rhizospheric or added soil. They are part of the radical exudates.
Mucilagos	High molecular weight, gelatinous materials, for example polyuronic acid.	They protect and lubricate the areas of root growth. They intervene in the availability and absorption of minerals, in the formation of aggregates in the soil. They are part of the radical exudates.
Mucigel	It comprises the aggregation of natural and / or modified mucilages, microbial cells and / or their metabolic products, colloidal minerals and mixed organic matter.	They protect and lubricate the areas of root growth. They influence the absorption of ions by improving the root-soil contact, and the aggregation of soil particles.
Compounds Gaseous	Low molecular weight volatile compounds that can diffuse into the soil	They positively or negatively affect microbial activity in the rhizosphere zone and beyond.
Mineral nutrients	Present in rhizodeposited materials such as phosphorus, nitrogen, potassium.	They contribute to the mineral nutrition of the plant and are very important in conditions of deficiency of these elements in the soil.

Source: Sánchez de P., 2006 citing Curl and Truelove, 1986; Siqueira and Franco, 1988; Cardoso and Freitas, 1992; Marschner, 1995.

This is where the great importance of having a soil in good condition and with a high content of organic matter comes in. A neat floor.

THE IMPORTANCE OF ORGANIC MATTER AND SOIL FERTILITY IN RHIZODEPOSITION

The better the quality of the medium where the roots of the plants develop, the less energy expenditure you will have to lose in trying to improve soil conditions.

Although there will be an energy mobilization looking for the natural growth of the roots , the excretions of these compounds to the environment will be greatly reduced since the plant is in a comfortable environment for obtaining nutrients.

One of the clearest and quickest examples to understand is the uptake of iron.

Iron is one of the elements that has the most losses in the soil, especially in conditions of soils with high pH and limestone.

The problem is that it easily associates with other antagonist elements and quickly becomes insolubilized . That is, once we apply it to the soil, it quickly ceases to be in forms that the roots of the plants can absorb.

With these rhizodepositions, the plant is capable of excreting substances that modify the environment into the environment to allow iron to be recovered and absorbed.

It is known as siderophoric substances. Soil microorganisms, aided by the carbon-rich elements that the plant has produced in the soil, are capable of transforming iron Fe3 + (poorly soluble in basic medium) to Fe2 + (soluble and absorbable by plants).

Siderophore elements produced by some bacteria and fungi.

Ferricromo (Ustilago sphaerogena)
Eritrobactina (Saccharopolyspora erythraea).
Bacillibactina (Bacillus subtilis)
Azotobactina (Azotobacter vinelandii)
Pseudomonadaceae (Pseudomona aeruginosa)
Vibriobactina (Vibrio cholerae)

Also certain fungi that are found in the soil (and are stimulated by rhizodepositions ) such as candida, aspergillus, mucor, histoplasma, blastomyces and sporothrix, etc.

All the best. Agromatic.