Evidence of a mycorrhizal mechanism for the adaptation of Andropogon gerardii (Poaceae) to high- and low-nutrient prairies

Andropogon gerardii seed obtained from Kansas and Illinois was grown in a c ontrolled environment in their own and each other's soils, with and without arbuscular mycorrhizal fungi (AMF). Each ecotype grew comparatively better in its own soil indicating adaptation to its soil of origin. Overall, A. g erardii benefited more from AMF in low-nutrient Kansas soil than Illinois s oil. The two ecotypes, however, did not benefit equally from mycorrhizal in fection. The Kansas ecotype was three times more responsive to mycorrhizal infection in the Kansas soil than was the Illinois ecotype. Our results ind icate that plant adaptation to the nutrient levels of their local soils is likely to be due, at least in part, to a shift in their dependence on mycor rhizal fungi. The Illinois ecotype of A. gerardii has evolved a reduced dep endence upon these fungi and greater reliance on a more highly branched roo t system. In contrast, the Kansas ecotype had a significantly coarser root system and invested proportionately greater carbon in the symbiotic associa tion with AMF as measured by spore production. This study provides the firs t demonstration that plants can adapt to changing soil nutrient levels by s hifting their dependence on AMF. This result has broad implications for our understanding of the role of these fungi in agricultural systems.