Rhizospheric and Phylospheric Microrganism

Rhizosphere(Trem coined by Hilton )

The zone in the soil adjacent to plant root is defined as rhizospheres . It is the region around the root the root which is influenced by root exudates.( contain numerous organic substances). This zone is extended few millimeter around the root . The rhizosphere is a site of increased microbial activity because of the presence of the abundant organic matter . In the world, rhizosphere is the dynamic environment where plant,microorganism , soil nutrient ,water meet, and interact . The organic matter found in rhizosphere comprises root exudates, polysaccharides ,mucilage production by the root cap and epidermal cells, dead root hair , sloughed off the front cap cell, dead epidermal and cortical tissues . The rhizospheric soil is a thin layer of plant adhered to the soil is large . The rhizosphere is characterized by greater microbial activity than the soil away from plant roots. The intensity of such activity depends on upon the distance to which exudates from root system can migrate .

The external surface of the root is known as rhizoplane .It is a specialized micro- habitat which is associated with the root surface . It is the site of water and nutrient uptake and release of exudates .It is constantly changing the environment . When root grows , they release dead cell into the soil and those dead cells make rhizoplane irregular .

Root exudates

There is the release of the exudates into the soil from rhizosphereOrganic material released from the root in root exudates include amino acids, vitamins , sugars, tannins , alkaloid and many other unidentified substances . Although a few of this material inhibit microorganism , most stimulate microbial growth .The exudates increase the availability of the nutrient in the rhizosphere and increase the carbon source for heterotrophs.

Rhizospheric microflora

microflora differ from plant both qualitatively and quantitatively .

1. Bacteria in rhizosphere : Bacillus , Azotobacter, Beijerinckia, derxia , clostridium , klebsiella , pseudomonas , etc.
2. Fungi in rhizosphere : Aspergillus , Penicillium , Fusarium , Trichoderma , Cladosporium , Rhizopus mucor etc

One of the most important factors responsible for rhizospheric effect is the great variety of organic substances i.e root exudates . These exudates directly influence the quality and quantity of microorganisms in the rhizosphere . The substance by plant root includes amino acid ,sugars , organic acids, vitamins, nucleotides , and many other unidentified substances .

Rhizospheric effect

The term rhizospheric effect indicates the overall influence of the plant roots on soil microorganisms . It has been established that greater number of fungi , bacteria,and actinomycetes are present in the rhizospheric soil then the non-rhizospheric/remote soil .Several factors such as soil type, soil moisture , pHof the soil , temperature,age of the plant e, etc are known to influence the rhizospheric effect . It has been demonstrated that the rate of metabolism activity of rhizospheric microorganism is different ( higher metabolism rate ) from those in on -rhizospheric soil.A wide rate of the enzyme of plant and of the microbial organic present in the breakdown of organic matters . This enzyme includes oxide-organic,hydrolase , and transferases apart from cellulose ,dehydrogenases, and urease.

The rhizospheric effect can be observed by determining the ratio of number of microorganism in the rhizosphere soil (R) and number of the corresponding microorganism in the soil remote from root (S) ,i.e R.S. The rhizosphere to soil ( R:S ) is defined as the ratio of number microorganism in rhizosphere to the number or microorganism in adjacent ( non-rhizosphere) soil. It can be calculated by dividing the number of microorganism in rhizosphere soil by the number of microorganisms in soil free from plant growth. Generally , R:S ranges from 5 to 20 but may up to 100 i.e microbial population is 100 times higher in rhizosphere than in surrounding root-free soil .These are a higher proportion of gram-negative rods than the gram-positive bacteria in the rhizospheric soil .The rapidly growing motile bacteria like pseudomonas predominate in the rhizospheric soil.

In many cases ,the increased number of microbes in rhizosphere represent the direct or indirect influence of plant root exudates on soil microorganisms with high intrinsic growth rate. Organism material released from root exudates includes amino acid , vitamins , sugars. tannins ,alkaloids and many stimulate microbial growth. For, example, Microaerophilic Azospirillium and the aerobic Azotobacter and other nitrogen-fixing bacteria are regularly associated with rhizosphere of the creation tropical grasses . These bacteria use root exudates as the energy source to support the significant nitrogen fixation .

Effects of rhizospheric microorganisms on plants

Just as the roots have a direct effect on the surrounding microbial population microorganisms in the rhizosphere have a marked influence on the growth of the plants . It has been observed that absence of appropriate microbial populations in the rhizosphere ,plant growth may be reduced . Microbial populations in the rhizospheric may benefit the plant in many ways:

1. The rhizospheric microbial population increase solubilization and recycling of mineral nutrients eg,nitrogen needed by the plant for their growth :The bacteria relating to the family Azotobacteriaceae , Spirillaceae ,ENnteribateriaceae , Bacillaceae, Pseudomonabactriaceae ,and Achromobaeriaceae show nitrogen fixing activity in the rhizosphere or non-legumes .In the water-logged rice field, various microorganisms in the rhizosphere f rice roots fix nitrogen. Azotobacter,Beijerinckia, Methylomonas. clostridium Desulfovibrio EnterobactFlavobacterium,Azospillblue-green algae.In the root region of sugarcane the nitrogen-fixing bacteria isolated are Azotobacter ,Klebsiella,Bacillus, Azospirillum ,Enterobacter ,Erwinia, and Dexia.
2. The rhizospheric sulfur oxidizing microflora remove( H2S ) which is toxic to plant roots.
3. The rhizospheric microorganisms synthesize growth-promoting vitamins and growth promoting hormones like auxins and gibberellins that are taken by the plan
4. The rhizosphere microflora exerts an antagonistic effect on the plant pathogens through competition and through the production of antibiotics.
5. The microflora in rhizosphere produces an array of enzymes that degrade the complex organic compounds and convert them simpler molecular that can be utilized by plants.

Reference

Rangaswami, G and PT Bagyaraja. Agricultural Microbiology. 2nd. Prentice Hall of India, 1993.

Alexander, M. Introduction To Soil Microbiology . Academic Press, 1961.