The rhizosphere is a term which was first introduced by a scientist L.Hiltner. The rhizosphere is the region that is a few distances (2-80mm) extended from the root system. This zone favours the physical and chemical activity of microorganisms and responsible for the intense microbial activity.
The rate of microbial activity strongly influences by the process of “Root exudation’’ (organic and inorganic wastes of the root system). Rhizosphere zone separates from the bulk soil (lacks microbial activity) often called as Edaphosphere or Non-rhizosphere.
- Definition of Rhizosphere
- Microbial Activity in Rhizosphere Zone
- Root Exudation
- Role of Root Exudates
- Factors Affecting Root Exudation
- Rhizosphere Effect
- Rhizosphere Microorganisms
Definition of Rhizosphere
The term rhizosphere refers to as a microzone of the soil biota distant few millimetres away from the root system where plant and microbes coordinate with each other, shows symbiotic relationship by fulfilling each other’s nutrient requirement. The size and structure of the rhizosphere zone vary depending upon different plant species and microflora present in it.
Structure of rhizosphere consists of three zones based on their proximity to the root system.
Inner zone: Also refers to “Endorhizosphere“. It is very close to the root. It includes the portion of cortex and epidermis where microorganisms occupy between the vacant space known as “Apoplastic space”.
Rhizoplane: This term was coined by F.E. Clark. It is the root system itself.
Outer zone: Also refers as ‘’Exorhizoshere’’. This layer is adjacent to the portion of the epidermis.
Microbial Activity in Rhizosphere Zone
The whole process inside the rhizosphere zone is given below:
The reason for intensive microbial activity is due to the phenomena called ” Root exudation“. During unfavourable conditions the organic and inorganic compounds diffuse out of the root, such compounds refers as “Root exudates“. Therefore root exudate forms a network between plant and microorganisms.
Classification of Root Exudates
On the basis of chemical nature, root exudates are of three types:-
Organic compounds: These are the compounds that consist of carbon molecule as a backbone that can be covalently linked to other molecules like hydrogen, nitrogen etc.
Examples: Proteins, Organic acids, Vitamins, Amino acids, Sugars, Flavonoids, Nucleotides, Enzymes etc.
Inorganic compounds: These are the compounds that can consist of carbon or hydrogen, but not together.
Examples: Water, Anions and Gases like carbon, oxygen, nitrogen etc.
Miscellaneous compounds: Other than organic and inorganic compounds, there are some other substances which are also released by the plant that can impose a negative effect refers to miscellaneous compounds.
Examples: Auxins, Glycosides, Saponins, Hydrocyanic acids etc.
Role of Root Exudates
- It provides nutrient supply to the microbes.
- Root exudates protect the plant from desiccation.
- It provides moisture to the soil that favours the growth of microorganisms.
- By releasing some defence proteins and antimicrobial chemicals, It protects the root against biotic stress, i.e. pathogens.
- It induces systemic resistance against abiotic stress like temperature, salt concentration, pH.
- By mucilage secretion, It helps in soil aggregation.
- It can change the chemical and physical properties of soil by the following factors:
- Osmotic pressure
- Ionic balance
- Redox potential
Factors Affecting Root Exudation
- High temperature and light
- Wilting of plant releases a high amount of amino acids
- A release of secondary metabolites by certain microorganisms
- Rhizospheric microflora that can affect the permeability and metabolism of root
Rhizosphere effect is an important term that quantitatively measures the microbial count in the rhizosphere zone. It states the number of microorganisms found in rhizospheric soil to that of Edaphosphere or non-rhizospheric soil. It is denoted as R:S ratio. The rhizosphere effect is different for different microbes that are present in the rhizosphere. Its value ranges from 5 to 20.
Rhizosphere shows huge microbial diversity includes: Bacteria, fungi, actinomycetes, algae and protozoa are the common microbes whereas viruses, archaea, arthropods are very rare. Microorganisms can be classified into three types based on their effect:
Beneficial microorganisms: These do not harm the root system and includes the following organisms: Mycorrhizal fungi, protozoa, N2 fixing bacteria etc.
Role of beneficial microorganisms:
- Fixes nitrogen
- Helps in solubilizing inorganic phosphate, iron etc.
- Helps in root nodulation
- Colonize the root system
- Promotes plant growth
Deleterious microorganisms: These harm the root system of soil and includes the following organisms: Pathogenic fungi and bacteria, nematodes etc.
Role of deleterious microorganisms:
- Cause diseases in plant
- Invades nutrient
Neutral microorganisms: these shows a neutral effect and includes: Actinomycetes, algae etc.
Role of neutral microorganisms:
- Helps in soil aggregation
- Shows antagonistic property against other organisms
Table showing distinctive features of rhizosphere microorganisms:
|Microorganisms Involved||R:S Effect||Examples||Functions|
|Bacteria||•Smallest among all|
•Extensively found in root hairs rather than root tip
•Aerobic bacteria are less in number
•Gram negative bacteria are predominant
|10-20 or more||
|Fungi||•Abundant after bacteria|
•Both mycorrhizal and pathogenic fungi are associated
|≥ 10||Mycorrhizal fungi|
•Interlinked between bacteria and fungi
•Gram positive, prokaryotic organism
|•Interlinked between bacteria and fungi|
•Gram positive, prokaryotic organism
|Approximately 2:1 or 3:1||
|Algae||•These are aerobic and photoautotrophic|
moisture content and light favours its growth
|Protozoa||•Includes: small flagellates, ciliates and amoeba|
•Flagellates and amoebae are dominant than ciliates
•Found in water films of root hairs ejected out of epidermis wall
Factors Affecting Rhizosphere Microorganisms
- Type of soil: A microbial population is very high in sandy soil and least in humus soil.
- The moisture content of soil: Low moisture favours the growth of organisms.
- Soil amendments: It has no quantitative and qualitative effect on microbial growth and activity.
- The pH of soil: It is inversely proportional to the microbial growth, i.e. a decrease in pH results in an increase of the microbial population.
- Plant species: Microorganisms vary with different plant species respective to their rooting habits, composition etc. Example: Leguminous plants show more rhizospheric effect.
- The proximity of root with hair: Rhizosphere effect is more when there is less distance between the soil and root. And as the R:S effect increases, the number of organisms will also increase.
- Age of plant: It is an essential factor that decides the degree and magnitude of rhizosphere effect. R:S effect is directly proportional to the age of the plant, i.e. it increases with the age of the plant.
- Root excretion: This process strongly influences the microbial activity by providing a sufficient amount of organic and inorganic matter to the rhizospheric microorganisms.
These all are the factors that influence the growth and the activity of the rhizosphere microorganisms directly or indirectly.