Rhizosphere

Rhizosphere is the region of intense microbial activity, extending several millimeters from the root system of vascular plants. It is where the soil and root of plant make contact. Rhizosphere soil is the thin layer of soil adhering to the root system after shaking and removing the loose soil.

In 1904 the German agronomist and plant physiologist Lorenz Hiltner first coined the term "rhizosphere” [Greek word "rhiza", meaning root]. 

Hiltner described the rhizosphere as; “The area around a plant root that is inhabited by a unique population of microorganisms, influenced by the chemicals released from plant roots”.

Rhizosphere microflora is quantitatively and qualitatively different from the non-rhizosphere microflora. Also, rhizosphere microflora of one plant differs from that of another. Thus, rhizosphere is a unique subterranean habitat for microorganisms.

Structure of Rhizosphere

It has three zones which are defined based on their relative proximity to, and thus influence from, the root.

1. Endorhizosphere; in close proximity with the plant cortex and endodermis in which microbes can occupy the "free space" between cells

2. Rhizoplane; is the root surface

3. Ectorhizosphere; the outermost zone which extends from the rhizoplane into the adjacent soil.

Rhizosphere effect

The direct influence of plant roots on microbes and microbes on plant root within the rhizosphere is known as Rhizosphere effect. The growth of a soil microorganism is enhanced by the excretions and organic debris of roots within a rhizosphere. These bring about physical and chemical alteration of the soil.

 Rhizosphere effect is expressed by R:S ratio, which is the ratio between number of microorganisms in the rhizosphere soil to the number of microorganisms in the non-rhizosphere soil. R: S ratio is different for different plants and changes with the stage of growth of a plant. The values are high for bacteria in rhizosphere region.

During seed germination and seedling growth, the developing plant interacts with microorganisms present in the surrounding soil. As seeds germinate and roots grow through the soil, the release of organic material lead to the development of active microbial populations in rhizosphere region (that includes plant root and surrounding soil in a few mm of thickness). 

Root exudation is the release of organic compounds from living plant roots into the surrounding soil. Rates of exudation vary widely among plant species and environmental conditions. It has been estimated that 12-40% of the total amount of carbohydrates produced by photosynthesis is released into the soil surrounding roots. 

Root exudates are mainly composed of water-soluble sugars, organic acids, and amino acids, but also contain hormones, vitamins, amino compounds, phenolics and sugar phosphate esters. The qualitative and quantitative compositions of root exudates are affected by various environmental factors including; pH, soil type, oxygen status, light intensity, soil temperature, nutrient availability and the presence of microorganisms.