Rhizosphere - Microflora, Significance, and Rhizosphere effect

Rhizosphere and its Significance :

•  The region where the soil and roots make contact is called rhizosphere.
•  The microbial population of rhizosphere is much higher than that of root free soil.
•  Bacterial growth in rhizosphere is enhanced by nutritional substance like vitamins, amino-acids, etc. released from the plants.
•  The roots of higher plants are surrounded by both living as well as non-living environment.
•  The organic and inorganic substances of soil make up the non-living environment.
•  The organism that are present in the soil constitute the living environment.
•  Rhizosphere is the region of soil immediately surrounding the roots of plant.
•  Rhizosphere is divided into two general areas:
  (a) Inner rhizosphere : Inner rhizosphere is the area on the root surface.
  (b) Outer rhizosphere : Outer rhizosphere is the area near the root surface.
• The inner rhizosphere has a larger microbial population than the outer rhizosphere as in this region the Interaction between microorganism and roots are most pronounced.
• The root surface and its adhering soil is sometimes termed the rhizoplane.

(i) Microflora of rhizosphere :

•  The rhizosphere is a highly favourable habitat for proliferation and metabolism of numerous microbial types.
•  The numbers and types of microorganisms differ in case of rhizosphere as compared to root free soil.
•  For microscopic studies of rhizosphere various modifications of Buried slide method of Rossi and Cholodny have been introduced.
  In this method the glass slide is Inserted into the soil in such a way that the microbes ultimately grows up along the surface of glass.
  The slide is then removed, stained and examined for various types of microorganism.
•  The biochemical techniques used in rhizosphere investigation are numerous, and they are designed to measure a specific change brought about by the plant or by the microflora.
•  Most of the rhizosphere bacteria are saprophytes, some live on the root surface others penetrate the roots.
•  The most abundant bacteria found in rhizosphere are gram-negative rods, gram-positive rods, cocci and spore-formers are less common rhizosphere bacteria.
•  Pseudomonas and Achromogenous spp. are most frequently found bacteria in rhizosphere followed by Agrobacterium.
•  The other bacterial genera found to occur in rhizosphere Include Arthrobacter, Mycoplana, Brevibacterium, Flavobacterium, Serratia, Sarcina, Alginomonas, Bacillus and Mycobacterium.
•  As there is a very high bacterial density (10⁹ celle or more/gm) in rhizosphere there is a high degree of microbial competitions. Fast growing organisms are favoured.

The rhizosphere is influenced by a number of factors:

- Bacterial count increases in samples taken progressively closer to the plant tissues.
- Depth of sampling.
- Different plant species often establish different microflora.
- The age of the plant also alter the underground part and stage of maturity controls the magnitude of relationship.
•The stage of maturity controls the magnitude of rhizosphere effect.

(ii) Rhizosphere effect :

  Influence of plant roots on the growth and development of microorganisms in the nearby soil is called rhizosphere effect.
•  It is a stimulous which can be measured by finding out the R:S ratio.
•  Rhizosphere efffect is normally measured by plating technique.
•  It is always greater for bacteria than any other microorganism.
•  The R:S ratio is defined as the ratio of microbial numbers/unit weight rhizosphere soil to the population in a unit weight of the adjacent non- rhizosphere control soil.

•  As the distance from rhizosphere increases the R:S ratio decreases.

Influence of plants on rhizosphere microbes :

•  The rhizosphere represents tremendous complex biological system owing to the following chemical activities of plants.
•  Plant roots release a wide variety of materials to their surrounding soil like alcohols, ethylene, sugars, amino-acids, organic acids, vitamins, nucleotides, polysaccharides, enzymes, etc. These materials favour the growth of microorganisms.
•  Death of plant roots produce a rich source of carbohydrates and their derivatives, which support the growth of many useful bacteria around the plant roots, eg, nitrogen fixing bacteria, sulfur oxidizing bacteria, etc.

Influence of microorganisms :

    Microbial population in the rhizosphere which produces beneficial effects on plant growth are described as Plant Growth Promoting Rhizobacteria (PGPR). They are a wide range of root colonizing bacteria which promote plant growth.
•  Microbial population in the rhizosphere benefit the plants in the following ways :
-  Some bacteria and fungi make N₂ available to plants as nitrates or inorganic form.
-  Sulfur oxidizing bacteria make sulfur available as sulfates.
-  Heterotrophic metabolism makes carbon available as CO₂, for photosynthesis.
-  High concentration of CO₂ in the rhizosphere also increases solubility and availability of calcium, and thereby increase calcium uptake by plant roots.
-  Some microorganisms (phosphate solubilizers) also produce acids which convert insoluble rock phosphate to the soluble form and make it available as nutrient.
-  Some bacteria synthesize phytohormones like auxins, gibberellins, etc. which stimulates the plant growth.
-  Some microorganisms can remove hydrogen sulfide (H₂S) which is toxic to the plant roots.
-  Rhizosphere fungi and bacteria produce siderophores, low molecular weight organic molecules that are able to complex with ferric iron and supply it to the plant cells.
-  Similarly, organic chelating agents produced by microorganisms make manganese compounds available to the plants.
Azotobacter and other bacterial spp. produce antifungal substances which protect the plant against microbial invasion.
Agarobacter, Rhizobium, Corynebacterium etc. produce cytochyanins which maintains plant growth.
-  Thus, the microflora of rhizosphere is very beneficial to plants.

•  The rhizosphere is the most active factory for transformation of element into living constituents and returning essential elements to soil upon death and decomposition of microorganisms.
•  The hydrolytic forms of microorganisms like cellulose digester transform the plant material into humus, glucose and other valuable food material which can be used both by plant as well as microorganisms.
•  A very heavy growth of microorganisms absorb, nitrogen, sulfur, phosphorus, potassium and other elements in soluble form which might otherwise be removed from the soil by rain and drainage.
•  Thus, higher plants act as a food manufacturer and storage house for microorganisms of soil rhizosphere and microorganisms act as collectors, processors and treasurers of food for higher plants.

Phyllosphere : 

 The environment of the aerial portion of plant is described as phyllosphere.
•  It includes a variety of microbes, including bacteria, molds, yeast and cyanobacteria.
• The most common phyllosphere bacteria include Pseudomonas, Erwinia, Proteus and Spingomonas spp. most of these microbes exhibit mutualistic association with plants.