The value to AM-derived nutrients, in terms of its C cost, is therefore likely to vary between particular plant-fungus associations. Carbon demand by each AM fungus is considered a cost of the symbiotic association and this need to be compensated (Wright and Upadhyaya, 1998). Drought highly reduced C assimilation processes by the host plant but plants respond differently to particular AM colonization according to how each fungus affects the process of C assimilation in the host-plant and the C requirements of each fun- gus.