The ecto and endo mycorrhizal fungi are widely occurring mycorrhizas and are very important in relation to the improvement of forest trees. The paper deals with the mycorrhizal fungi because of their practical importance in forestry. Occurrence of different types of Ectomycorrhizal fungi from various Dipterocarp plants and all the six genera of Arbuscular Mycorrhizal Fungi were recorded from Bangladesh. Mycorrhizal technology can profitably be applied in forestry as well as agricultural and horticultural crops for better nutrient utilization and more effective land use. Arbuscular Mycorrhizal technology can be introduced with the existing forestry systems practised to improve the soil and crop productivity by allowing farmers to reduce their inputs of chemical fertilizers and/or by enhancing plant survival, thus offsetting ecological and environmental concerns. Appropriate mycorrhizal fungi can be incorporated in nursery for raising mycorrhizal seedlings and transfer of seedlings to the field is a simple inoculation technique currently suitable in plantation crops and trees. These fungi appear to have beneficial effects on soil aggregation, thereby improving soil fertility and may be important means of controlling soil erosion. The practical application of mycorrhizal fungi may be integrated in the disease management by producing mycorrhizal seedlings, so as to prevent primary and secondary infection by pathogenic fungi and other harmful organisms.
Bangladesh is predominantly an agrarian country, depending mainly on agricultural crops and forest products for its economic development. It is one of the densely populated, developing countries in the world. The country has 2.60 million hectares (i. e. 17.62%) of forest land of which 1.53 million hectares (i.e. 10.37%) of land is under the purview of the Government's Forest Department. Forestry is one of the major sectors of renewable resources in Bangladesh, which contributes to the economic and ecological stability. The contribution of the forestry sector to the GDP is about 4 percent. Low tree coverage and the high population density are continually increasing the demand for tree products, such as timber, fuel wood, leaves, twigs, fruits and other non-wood products, leading to serious pressure on the forests and planted trees. We have to update techniques in order to maximize our forest production in minimum land. Because of economic and environmental constraints, it is necessary to develop least expensive and technologically simple methodologies for immediate benefit. Mycorrhizal technology can be one of the alternatives to improve forest products, farm profitability and environmental quality in different production systems in Bangladesh (Mridha and Xu, 2001).
Mycorrhizas are highly evolved non-pathogenic symbiotic association between roots of most vascular plants and certain specialized soil fungi (Basidiomycetes, Ascomycetes and Zygomyectes) that colonize the cortical tissues of roots during periods of active plant growth both in natural environment and in cultivation (Miller and Jastrow, 1994; Smith and Read, 1997). In this interdependent mutualistic relationship, the host plant receives mineral nutrients from the soil, increases tolerance to stresses (by affecting water relations and pathogen resistance) and the fungi obtain photosynthetically derived carbon compounds from the host for their survival through mycorrhizal hyphae. The mycorrhizal associations are classified into seven different categories on the basis of their morphology, morphogenate and physiological features. Out of the seven different types of mycorrhiza only ectomycorrhiza and endomycorrhiza are considered here because of their wide occurrence and their importance in relation to the improvement of forest trees in Bangladesh.
Mycorrhizal fungi are ubiquitous soil inhabitants. These fungi are virtually worldwide in temperate, tropical and arctic regions, except under waterlogged conditions (Smith and Read, 1997). Most of the economically important agronomic grain and forage crops as well as major commercial fruit and economically important forest trees belonging to all families of angiosperms, gymnosperms and many pteridophytes and bryophytes form Mycorrhiza. The root system of most of the terrestrial higher plants growing under natural conditions does not exist as roots but as a complex mycorrhizal association resulting from root infection by mycorrhizal fungi (Gianinazzi et al., 1990). Highly fertile soils generally show less mycorrhizal fungal populations, pesticide treatment disturb ecosystem and fragile soil tends to decrease their population (Hayman, 1982). Occurrence of different types of Ectomycorrhizal fungi from various Dipterocarps plants and all the six genera of Arbuscular Mycorrhizal Fungi (AMF) from a wide range of soils and association of the fungi with different types of host plants including agricultural, horticultural and forest trees were recorded in Mycorrhiza Laboratory of Chittagong University, Bangladesh (Begum et al., 1998a,b; Mahmud and Mridha, 1998; Mridha, 1988; Mridha and Killham, 1992; Mridha and Xu, 2001; Rahman and Mridha, 2002 and Shayesta and Choudhury, 1985).
Most of the higher plants form mycorrhizal association with either endomycorrhizal or ectomycorrhizal fungi. About 95 percent of the tree species occurring in tropical forests are purely endomycorrhizal (Redhead, 1980) and many agronomically important trees which have AM infection including citrus, tea, coffee, rubber, oil palm etc. are growing in homestead garden and other plantation sites. When these trees are growing in nurseries, inoculation of mycorrhizal fungi may greatly improve the overall growth in the nursery (please see Mridha and Killham, 1992 and Mridha and Xu, 2001) and establishment and early growth after transplanting to the permanent site by reducing the transplantation shock and better nutrient and water uptake through mycorrhizal hyphal link available in the inoculated seedlings.
Mycorrhizal technology can profitably be applied in different forest areas for better nutrient utilization and more effective land use. Soils in Bangladesh forest either poor in P and other essential nutrients or have an immobile form of P similar to other tropical soils as reported by Menge (1983). In such condition, mycorrhizal fungi can play an important role in improving the plant growth by increasing the supply to roots with mineral nutrients in the soil (Gianinazzi et al., 1990). The application of mycorrhizal fungi will be suitable for inoculating the plants in a limited area with small amount of inoculum of the fungi. In this system, the external hyphal network of mycorrhizal fungi plays an important role in nutrient uptake, especially for those ions that are not very mobile in soil solution (Miller and Jastrow, 1994). The classical explanation for the mechanism behind a mycorrhizal-mediated increase in ion uptake is that the external hyphal network explores soil beyond the root hair zone. Mycorrhizal hyphae may also foster a more integrated nutrient cycle and occurrence of a hyphal link between two or more plants is a common phenomenon. Here mineral nutrients can be passed between two mycorrhizal plants via these hyphal links and biologically fixed N from the legume to non-legume (Haystead et al., 1988). Nutrients can also be passed from dying roots directly to living roots via hyphal connections (Newman and Eason, 1989). Mycorrhizal hyphal links promote direct nutrient cycling by avoiding the mineralization process and increases the systems productivity by keeping more nutrient ions in the biomass pool (Newman,1988).
In addition to the general role of mycorrhizal fungi in enhancing the growth and nutrition of most forest plants, mycorrhizal infection is particularly important for leguminous trees because of their need for an adequate phosphorus supply during nodulation by Rhizobium, not only for proper growth but also for P energy requirement for nitrogen fixation (Crush, 1974; Azcon et al.,1979; Hayman, 1986). Both N-fixation in legume by Rhizobium and enhancement of P uptake by mycorrhizal fungi can indirectly reduce the artificial fertilizer requirement and the problem of water and air pollution from the lost chemicals.
In some of the forest areas of Bangladesh, especially in hilly areas, soil erosion is one of the major problems that the hill farmers are facing because of jhum cultivation, which expose the soil for erosion. To reduce the soil erosion, mycorrhizal technology can comfortably be integrated with the modern cultivation techniques currently being used in Chittagong Hill Tracts by farmers and other growers, different Government and Non-Government organizations. The mycorrhizal fungal hyphae are involved with the scavenging and retention of nutrient ions and with the creation of an aggregate system that acts as a control point for accrual and mineralization of organic matter in the soil. At a larger scale, the mycorrhizal association, by its involvement in nutrient accumulation and retention, creates a system that reduces erosion and leaching loss of nutrients.
Mycorrhizal fungi are not only a major component of soil fertility but also play a significant role in the regulation of soil biological activity because of their abundance throughout the uppermost soil layer. Mycorrhizal fungi have direct access to plant-fixed carbon and they distribute this carbon throughout the soil of the rooting zone (Jakobsen and Rosendahl, 1990; Finlay and Soderstrom, 1992) for use by soil microbes and animals.
Mycorrhizal infection reduces susceptibility, or increases tolerance of roots to soil-borne pathogens like Phytophthora spp., Chalara elegans, Fusarium sp., and Pythium sp. and nematodes (Bondoux and Perrin, 1982; Bagyaraj, 1984) and reduce the incidence of root diseases (Dehne, 1982; Jalali and Jalali, 1991; Hooker et al., 1994) under nursery and field conditions. The practical application of mycorrhizal fungi may be integrated into disease management along with other conventional techniques in the nurseries and plantation sites.
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