Factors Affecting Growth And Activity Of Phyllspheric Microorganisms

Factors affecting the growth of phyllosphere microorganism

1. Age of the plant: The lesser the age of the plant , the lesser is the production of leaf exudates .
2. The spread of leaves : The exposure of leaves to sunlight affect the moisture availability and hence the microbial growth .
3. Phytoalexins : The phytoalexin inhibits the microbial growth.
4. Use of insecticides and antibiotics : Inhabit microbial growth .
5. Environmental factors: The air pollution caused especially by cement and fertilizer industrial suppress the growth of photospheric microorganisms.

Interactions between plant root and pathogenic microorganisms

plant root exudates influence pathogenic fungi, bacteria , and nematodes in a various way. The effect may be in the room of attraction of zoospore , or bacterial towards the root; stimulation of germination of dormant spores and hatching of cysts of nematodes . Root exudates may contain inhibitors substances preventing the establishment of pathogens. The balance between the rhizosphere microflora and plans and soil microflora and plant pathogens important in host -pathogen relationships .In this context , the biochemical qualifies of root exudates and the presence of antagonistic microorganisms plays an important role in the proliferation and survival root infecting pathogen in soil either through soil fungi stasis ,inhibition or antibiosis of pathogens in the rhizosphere .

Somes of the most common interaction between plant root and plant pathogenic microorganisms in the rhizosphere are discussed herewith.

1. Zoospores attraction : Amino acids ,organic acids, and sugars in the root exudates stimulates the movement and attraction of zoospores towards the root of the plant . For example attraction of zoospores has been reported on phytophthora citrophthora (citrus roots) ,phytophthora parasitic, (tobacco root) and pythium aphanidermatum (pea root).
2. Spores germination : The spores or conidia of many pathogenic fungi such as Rhizoctonia.Fusarium, Sclerotium ,Pythium, Phytophthora etc.have been stimulation be the root exudates of susceptible cultivars and remain the host plants . there are some reports on the selective stimulation of Fusarium . Pseudomonas and root infecting nematodes in the rhizosphere region of the respective susceptible hosts.This stimulation termination is especially important to those plant pathogens which are not vigorous competitors and remain in resulting stages due to a shortage of nutrients or fungistatic .As a rule .germination and subsequent hyphal development are promoted by nonhost species and also by both susceptible and resistant cultivars of the host plant . The quality of microorganisms present in the rhizosphere of disease resistant crop varieties is significantly different from those susceptible varieties.
3. Changes in morphology and physiology of host plant :Change physiology and morphology of host plant influence the rhizosphere microflora through root exudates .Hence ,significant changes in the rhizosphere microflora diseases plant were reported which are attributed to the nature and severity of the disease .Systemic virus diseases cause the marked change in the plant morphology and physiology to drastically alter the rhizosphere microflora.
4. Increase the antagonistic activity : Root exudates provide a food base for the growth of antagonistic organisms which play an important role in controlling / supporting some of the borne plant pathogens . Generally, rhizosphere of the resistant plant varieties harbors a number of streptomyces and Trichoderma than that of susceptible,e varieties . For example in the rhizosphere of pigeon pea varieties resistant to Fusarium udu., the population of Streptomyces was found more which inhibited the growth of the pathogen . The high density of Trichoderma varied in the rhizosphere of tomato varieties resistant to Verticillium wilt has been reported with its ability to reduce the severity of wit in susceptible plants.
5. Inhibition of pathogens Root : Root exudates containing a toxic substance such as glycosides and hydrocyanic acid may inhibit the growth of pathogens in the rhizosphere.It has been reported that root exudates from resistant varieties of flax (eg bison ) excreta glucoside which produces hydrocyanic acid that inhibits Fusarium oxy forum ,the flax root pathogen. Exudates of resistant pea reduce the germination spores of fusarium oxy forum.
6. The attraction of bacteria and nematodes :Root exudates attracts phytopathogenic bacteria and fungi in the rhizosphere for example Agrobacterium tumefactions have been reported to be attracted to the root of the plant like peas, maize ,onion , tobacco ,and cucumber. Hostexudates also influence phytopathogenic nematodes in two-way; -Though stimulation of egg hatching process -Attraction of larvae towards plant roots.

Differentiate between

Reference

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

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