Most agriculturally important plants, including cotton, are colonised by soilborne fungi known as arbuscular mycorrhizal fungi (AMF). The AMF depend on the plant to supply their energy, in the form of sugars. These fungi colonise the roots internally and develop highly branched structures (arbuscules) inside individual cells of the root. Fungal filaments also grown into the soil surrounding the roots, where they absorb 'immobile' elements such as phosphorus and zinc. The phosphorus and zinc are transported to the roots and transferred to the plant in the arbuscules. Generally, the improved nutrition of the plant outweighs the cost of supplying the fungi with sugars. This type of partnership, is known as arbuscular mycorrhizal symbiosis. In the field, cotton is highly dependent upon AMF for successful growth and always forms arbuscular mycorrhizas (Nehl et al 1994).