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MICROBIOLOGY

LESSON 28: RHIZOSPHERE AND PHYLLOPLANE MICROFLORA AND MYCORRHIZAE

Soil as an Environment

Though soil appears inert, but particularly its upper layers/ horizons contain a large and varied microbial population. The region of Earth that supports microbial growth and brings about transformations is called biosphere. Soil is a natural entity, a biochemically weathered and synthesized product of nature. It is also a natural habitat for plants and large number of organisms. Pedolgy ( from the Greek word pedon, meaning earth or soil ) is the science concerned with soil, its classification and its description. The physico-chemical properties of soil depend on the parent rock from which it has formed by a weathering process. The upper 1-2 meters of soil differs from the material below. It is higher in organic matter content because plant roots concentrate there. The chemical transformations and bio- transformations taking place in the biosphere play a vital role in soil fertility. Soil is a complex mixture of various chemicals. It contains organic, inorganic materials, oxides of iron, aluminium and silicon with cations and anions. The plant and animal residues are transformed by variety of organisms into a long lasting dark, brown to black coloured component called humus. The process of humus formation is very slow process and requires years together. In bio-transformation or mineralization process, microorganisms convert in soil to inorganic form. The rate and extent of mineralization depend on the availability of oxygen. Compared to anaerobic metabolism, aerobic metabolism is more versatile--more compounds are attacked by microorganisms. It is also more complete, producing carbon dioxide and water instead of organic acids and alcohols. When soils are flooded with water, they rapidly become anaerobic and as a result, the mineralization appears more slow.

organisms. It is a rather transitory soil constituent, lasting from a few hours to several hundred years. It has a good water holding capacity. Soil water is normally present in soil pores as free or bound water. Some of the soil moisture is utilized by growing plants, some remains in the tiny pores and in thin films around soil particles. Not all soil water is available for plant growth. Various soluble constituents, including nutrient elements make up the soil solution which supply the nutrients to growing plants and organisms. The soil solution contains small but significant quantities of soluble inorganic and organic compounds and necessary elements. The soil solution may be acidic or alkaline depending upon the levels of hydrogen ions and hydroxide ions in the soil. These levels influence the solubility, and in turn the availability for growth. Soil air differs from the atmospheric air in several respects. The composition of soil air is quite dynamic and varies greatly from place to place within a given soil. The plant and microbial activities every time modify the composition of soil air. Soil air has more moisture content than the atmospheric air and the relative humidity of soil reaches to 100% when the soil moisture is optimum. The carbon dioxide is hundred times higher than the 0.03% commonly found in the atmosphere. Oxygen decreases accordingly and in extreme cases, may be only 5-10%, or even less than that as compared to about 20% for normal atmosphere. Soil temperature greatly affects the physical, chemical and biological processes occurring in that soil. In cold soils, chemical and biological processes are very slow, the biological decomposition falls down thereby limiting the availability of nutrients. The microbial processes are more rapid in the temperature range 27-32c and negligible if soil temperature is lowered to 10c.

Soil Fertility The vertical section of a soil, reveals the presence of more or less distinct layers where each layer is well developed, undisturbed and has its own specific characteristics, this is called soil profile. The upper and lower horizons are result of soil building processes. The upper layer is darker in color due to accumulation of more organic matter than the lower layer. Overall, soil contains minerals, organic matter, water and air. Soil fertility can be defined as ability of soil to support the life and specifically the plant growth. It depends largely on an adequate provision of inorganic nitrogen, phosphorus and potassium. The substances produced during mineralization process are available for plant growth. Today, along with chemical fertilizers we can also think about bio-fertilizers to enhance the soil fertility. The organic matter comprises an accumulation of partially disintegrated and decomposed plant and animal residues and other organic compounds synthesized by the soil micro-

Soil Microflora Soil is a rich and varied biological laboratory, harbor a diverse population of living organisms including plants, animals and micro-organisms. A single gram of soil may contain from a mere hundred thousand to several billion bacteria. The quantity of living organisms is influenced by the physical, chemical and biological properties of soils. The soil microflora includes bacteria, fungi, yeasts and actinomycetes. The soil flora and fauna include primary, secondary and tertiary consumers. The activities of soil micro-organisms are commonly influenced by i) their numbers in the soil, ii) their weight per unit volume or area of soil-biomass, and iii) their metabolic activity. Rhizosphere In 1904, Hiltner made some interesting observations regarding the relationship between soil micro-organisms and plant roots. His observations revealed that certain micro-organisms showed increased activity, close to the roots of cultivated plants and that the portion of soil adjacent to the root system of a plant is

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influenced by the root system, which he called rhizosphere. Later on several investigators have confirmed his findings and evaluated and analyzed the inter-relationship in great detail. R. L. Starkey-1928, N. V. Krassinikov-1934, also contributed their observations in the field of rhizosphere. The rhizosphere is the area in the vicinity of plant root system. Living roots of higher plants also affect the nutrition of soil microbes. Plant roots withdraw soluble nutrients from the soil solution directly but also release various substances which enhance the microbial growth. The microbial ecology of rhizosphere is significantly different from rest of the soil. The organisms with intimate contact with plant roots are known as rhizoplane microflora ( mycorrhizae ) while others are present in the region of soil immediately surrounding the plant roots. Specific types of micro-organisms concentrate in this region called rhizosphere effect. In contrast to non-rhizosphere, the rhizosphere area is more active with respect to microbial activity. Significant quantities of organic compounds are exuded, secreted or otherwise released at the surface of young roots. Organic acids so released can solubilize plant nutrients while the amino-acids and other simple carbon containing compounds stimulate microflora in the rhizosphere region. Studies on qualitative and quantitative microflora have revealed that, the number of organisms in rhizosphere area may be as much as 100 times greater than elsewhere in the soil. Various associations between organisms and organisms with higher forms of plants are seen in this region. The Gram ­ negative organisms are much more than Grampositive organisms. Species of Pseudomonas, Flavobacterium and Alkaligenes are more abundant. Most commonly a mutualistic association ship is found between the plants and microorganisms. The soil bacteria are extremely diverse and include aerobes, anaerobes and facultative anaerobes. The soil microflora varies according to the availability of nutrients, air, pH, and soil temperature. Actinomycetes and Gram- positive filamentous bacteria are important contributors to the ecology of soil. They are important in decomposition process forming organic matter and keep the soil loose. Streptomyces spp. is most numerous and widely distributed. The actinomycetes are less stimulated in the rhizosphere than bacteria. In 1965, R. Venkatesan and G. Rangaswami found a significant rhizosphere effect in rice field, on bacteria, actinomycetes and fungi. Nocardia were selectively stimulated in the rhizospere, but Micromonospora was reduced in number. The age of roots also influences the number of actinomycetes, the rhizosphere of old roots show more actinomycetes than young roots. The growth factors and amino-acid requiring actinomycetes are more in rhizosphere region like thiamine, biotin, vitamin B 12 and riboflavin requiring forms. In the same way antagonistic actinomycetes are more in rhizosphere area. Besides bacteria, fungi and actinomycetes, other organisms like nematodes, protozoa and algae are also stimulated in this region, but to lesser extent. The observations have revealed that the number of algae increases at the flowering time of certain crop plants.

The studies on fungi in rhizosphere have been mostly quantitative, and only a few genera, based on their morphology have been identified. They are not studied in any detail for their physiological and nutritional requirements, and hence their association with the roots in this region is not well understood. Soil fungi also degrade organic materials in aerated soils. The aerobic fungi break down both simple compounds such as sugars and organic acids and complex polymers such as cellulose, starch, pectin and lignin. The other organisms include algae present on the surface of soil, but usually in small numbers. Algae and other phototrophic prokaryotes do not contribute significantly to soil fertility except in rice paddies, where cyanobacteria, free living or in association with plants, fix considerable amounts of nitrogen. The numbers of protozoa in soil are small, but probably no soil lacks them completely. Their direct effect on biochemical transformations in the soil is minor. They regulate the size and composition of bacterial populatios. During initial stages of plant growth, the organisms from seed may dominate and as plant grows, to a larger extent, soil organisms take over. The selected groups of organisms, capable of competing with the others and establishing a beneficial relationship with root system finally multiply and become predominant. The number and types of organisms in the rhizosphere area depend upon the quality and quantity of nutrients available in this region. In rhizosphere area the number of organisms are ten to hundred times greater than those of the soil. The overall microbial activity in this region is several times more than in the remaining soil. The nutrients present in rhizosphere region are primarily of plant origin, and may vary with the plant species and varieties and physiological conditions of plant. The substances released by early inhabitants of rhizosphere are normally available for other organisms. The spectrum of chemicals and growth factors exuded by plant roots varies widely. The presence of plant pathogens in the soil and rhizosphere area is also significant. There is a great competition between the plant pathogens and other members of soil microflora, if the plant pathogen succeds in reaching to the specific plant host, may invade and cause the disease. The application of fertilizer, organic amendments of soil and foliar chemical sprays have effect on microflora both qualitatively as well as quantitatively. The nature of microflora in the rhizosphere region changes for diseased plant to healthy plant.

MICROBIOLOGY

Mycorrhizae An economically important, mutually beneficial symbiotic association between numerous fungi and roots of higher plants is called mycorrhizae, meaning " fungus root . Such association is widespread and has great practical significance because it increases the availability to plants of several essential nutrients, especially from infertile soils. The symbiotic association provides the fungi with sugars and other organic exudates as food, the fungi provide an enhanced availability of various essential nutrients including phosphorus, zinc, copper, calcium, magnesium, manganese and iron.

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There are two types of mycorrhizal associations of considerable importance, ectomycorrhizae and endomycorrhizae. The ectomycorrhizae include hundreds of different fungi associated with trees such as pine, beech, oak and fir. These fungi are stimulated by root exudates and cover the surface of feeder roots. The fungal hyphae penetrate the roots and develop around the cells of cortex but do not penetrate these cell walls. The endomycorrhizae are also known as vesicular arbuscular mycorrhizae ( VAM ) penetrate the root cell walls, enter the root cells and form hyphal masses within the cells. This group is perhaps the most common and most widespread of mycorrhizae, with some 89 identified species of fungi in soils. The roots of most agronomic crops, including corn, cotton, wheat, potatoes, beans have VAM associations. The root cortical cell walls of host plants are penetrated by the hyphae of VAM. Inside the plant cells, highly branched, small structures known as arbuscules are formed by the fungi. These structures are nothing but the sites of transfer of mineral nutrients from the fungi to the host plants. The other structures, called vesicles, serve as storage organs for the plant nutrients and other products. The increase in nutrient availability provided by mycorrhizae is thought to result from the nutrient- absorbing surface provided by the fine filamentous hyphae of the fungi. The surface area of mycorrhizal infiltrated roots has been calculated to be as much as 10 times that of the uninfested roots. Also, the soil volume from which nutrients are absorbed is greater for mycorrhizal associations. The very fine fungal hyphae extend up to 8 cm into soil surrounding the roots, thereby increasing the absorption of nutrients like phosphorus, zinc and copper, which do not diffuse readily to the roots. During water drought, water stress in mycorrhizal infested plants is less than in uninfested plants. Also, VAM also play significant role in transfer of phosphorus from one plant to nearby plant. Such association helps plants to absorb nutrients more efficiently from relatively infertile soils. Most mycorrhizae fungi cannot be cultivated in the absence of the plants with which they normally associate. Presumably, the plant might be providing essential nutrients to its fungal partner.

MICROBIOLOGY

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