Fungi Colonization of the Rhizoplane of Okra (Hibiscusesculentus) Plant

Abstract

The frequency of occurrence of the colonizing fungal species in the rhizoplane soil of okra plant (Hibiscus esculentus) were studied at different stages of development. The most frequently isolated fungal species in this examined okra plant were; Schizosaccharomyces pombe, Aspergillus flavus and Aspergillus fumigatus, others were; Penicillium spinulosum, Mucos racemosus, Saccharomyces cerevisiae, Aspergillus nidulans, Penicillium digitatum, Aspergillus niger, Cladiosporium resinae, Monascus ruber and Alternaria tenius.Investigation was also carried out on how factors such as soil moisture content organic matter content and soil pH affect the relative abundance of the fungal with okra plant growth. I. Introduction Microorganisms found in the soil and its study can be equated with soil microbiology. Varieties of plant and animal found inhabiting in the soil are called soil organisms. Some of them are macro while some are micro, those classified as plants are called microflora, fungi is included (Brandy & Weil, 1999).The plant roots or tree roots are classified under soil macroorganism, the root system of plants is associated with inanimate environment composed of organic and inorganic substances also with metabolically active community of microorganisms. The fungi found around cultivated soil is quite different from uncultivated soil community purposely because growing plant creates unique habitat for microorganisms and are inturn affected by their population. Relationship that occurred amongmicroorganisms, soil and roots were observed by Foster and Marks, (1967) through transmission electron microscopy (TEM) of the soil-root interface.The rhizoplane is the root surface of the plant (Rovira, 1965) and different from the rhizosphere which is the zone (soil) surrounding the plant root (Rovira, 1965).The Rhizoplane which is the root surface of the growing plants in the soil provide unique habitat, the rhizosphere, which is particularly favourable for the development of the microorganisms in the soil. However, the majority of the microorganisms stimulated by rhizoplane are usually harmless saprophytes which live mainly on dead, decomposing root tissues.The root excretions such as volatile and dissolved exudates affect microbial growth in the root surrounding soil, also most microbial activities in the soil is associated with the roots or fresh decomposing organic materials, therefore, soil microorganisms including Fungi depend on available organic compounds for survival while activity of microorganisms in non- cultivated soil will be dormant.It is claimed that the young root presents a virgin niche available for soil microorganism colonization and as the root grows through soil, the root exudates promote the fungal spores germination as propagules and growth of fungal hyphae. The initial root microorganism colonizers disappeared as roots ages leaving stable root surface microflora and microbial species eventually become dominant members of the final stable root surface population which depends on plants species and the soil (Peterson, 1958).The great variability that occurred in the microbial cover of the roots was reported by Bowen and Theodoros (1973) that the surface of the three weeks old plant root contains less than 10% microorganisms and increased to 37% in 90 days old plan root.The low surface occupancy by microorganisms in the root surface (rhizoplane) occurred as result of relative few points of inoculation in the soil followed by little spread over the surface and also, soil parts of the root surface may not be conducive for microbial growth.Foster, (1962) observed that the relatively small number of microbial cover on the roots at the early age offers little protection against plant pathogens like Phytophthoracinnamomi and biological control of such organism occurred by attack on the soil propagule. The hypothesis that organic debris is major source of inoculum for the plant root is consistent with Gray, et al; (1967) observation, that in sand dune soil, organic matter particles provide less than 15% of the available solid surfaces.Microbial growth can be affected by volatile and dissolved exudates from the seeds and roots for relatively large distance for example, the germination of Chlymadospores of Fusarium is 10mm from the planted seeds as been reported by Stanghellini and Hancock, 1971. Plant root exudates can infect the susceptible plants and can as well inhibit spores germination of resistant roots varieties as been reported by Buxton, (1957).Some saprophytic fungal initiated by the rhizoplane inhibiting rhizosphere were known to produce antibiotics which might possibly antagonize other organisms including plant pathogens.Some environmental factors like temperature, moisture content, soil pH etc. can influence