Benefits Of Mycorrhizal Inoculation

The practical application of mycorrhiza in agriculture is relatively new, though its importance has been evident for some 400 million years. The unique advantage of mycorrhizal organisms is that they not only survive in the most stressful environments but also make the plant to do so. The role of mycorrhiza in land reclamation is most recognized these days. Application of mycorrhizal biofertilizer provides a most desirable solution to many such environmental problems. These phosphate-solubilizing biofertilizers are suggested as an alternative or supplements to chemical fertilizers. Some of the benefits offered by mycorrhizal fungi to plants and general soil health improvement are listed below. However, these are not discussed in detail in the present review: (a) Alleviation of nutrient stress. Under deficiency conditions, mycorrhizal fungi can increase nutrient uptake. They facilitate the uptake of nutrients such as phosphorus. Difference among arbuscular mycorrhizal fungi (AMF) for plant P acquisition has also been associated with differences in development and function of hyphae like intraradical and extraradical mycelia (Boddington and Dodd 1998; 1999). Many immobile trace elements such as N, S, Ca, Mg, K, Zn, Cu, etc, are also known thus providing better nutrition to the host plant (Clark 1997; Persad-Chinnery and Chinnery 1996). Several reviews are available on the enhanced acquisition of mineral nutrients in plants with mycorrhization (Clark 1997; George et al. 1994; Smith and Read 1997). Turnau et al. (1993) proposed that polyphosphates in the fungal hyphae could sequester metals and minimize transfer to roots of the mycorrhizal plants in stressed conditions, (b) Enhancement of rooting and reduction of transplant shock. Mycorrhizal fungi stimulate root production and dramatically increase the volume of soil the plant can explore. This is especially important on disturbed sites, where nutrients and water availability might otherwise inhibit plant growth, (c) Alleviation of drought stress. The increased root volume allows more water to be taken up. Mycorrhizal fungi also enhance the host's osmotic adjusting capabilities, allowing some plants to continue extracting water from soils as they become drier (Ellis et al. 1985; Morte et al. 2000), (d) Stabilization and aggregation of soil. Mycorrhizal fungi encircle soil particles and glue larger soil particles together into aggregates. This increases the surface absorbing area of roots 10 to 100 times. They release powerful chemicals in the form of exudates into the soil that dissolve the hard to capture tightly bound soil nutrients. This improves soil structure, producing humic compounds and organic "glues" that bind soils into aggregates and improve soil porosity, increasing air and water movement though the soil while reducing erodibility. Reports are also available suggesting the presence of a protein called Glomalin, which seems to be involved in a very important hypha-mediated mechanism of soil aggregate stabilization (Borie et al. 2000; Degens et al. 1996; Rillig et al. 2002), (e) Suppression of disease. Mycorrhizae directly and/or indirectly antagonize disease organisms, increase the number of biocontrol agents around the roots, occupy potential infection sites on the root, and increase host plant vigor to the extent that it can survive disease (Datnoff et al. 1995; St Arnaud et al. 1997; Thomas et al. 1994). They act as biocontrol agent, (f) Enhancement of nutrient transfer between plants. This is especially important when nitrogen fixing and non-nitrogen fixing species are planted together. Mycorrhizal roots exploit the soil profile beyond the depletion zone surrounding the absorbing root and its root hairs. Mycorrhizal modifications of the nutrient uptake properties are dependent on the development of extramatrical hyphae in soil, hyphal absorption of phosphate and other micronutrients, their translocation through hyphae over considerable distances and subsequent transfer from fungus to root cells, (g) Enhancement of beneficial interactions with other microbes. Mycorrhizal fungi increase the nitrogen made available to the plant by both symbiotic and free-living bacteria. They also increase phosphate uptake to the plant by PSB and support biocontrol agents that are antagonistic to pathogenic organisms, (h) Salt stress. The enhancement of mineral acquisition especially that of P, K, Zn, Cu, and Fe due to AMF inoculation is more pronounced under salt-stressed conditions. Studies indicate that AMF-inoculated plants have a greater tolerance to salt stress than un-inoculated plants (Al-Karaki and Hammad 2001; Cantrell and Linderman 2001).

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