9306531 Crowley The composition of natural plant communities is typically characterized in relation to factors such as temperature, annual rainfall, and soil acidity. However, surprisingly little is known about specific adaptations of plant species to specific soil factors, including limited availability of essential mineral nutrients. In the U.S., soils west of the Mississippi River are predominantly alkaline (high pH), and often have very low levels of plant-available iron (Fe). Iron deficiencies are common in many agricultural crops, including a number of important members of the grass family such as corn, wheat, and barley. Moreover, one can speculate that limited iron availability in these soils may be an important factor controlling the species composition of the extensive western grasslands, which are highly significant to agricultural production (i.e., grazing of livestock) and as natural habitats and ecosystems. To date, no detailed research has been conducted to examine how native (and in some cases introduced) grasses are able to thrive in soils that pose severe iron-deficiency problems for agricultural crop species. This research by Dr. Crowley will, for the first time, comparatively examine the specific processes that occur at the root-soil interface of both adapted and nonadapted grass species. In response to iron deficiency, both grasses (but not other families of plants) and soil microorganisms secrete specialized organic compounds call "siderophores" into the soil. These compounds have a very high affinity for iron, thus solubilizing unavailable soil iron into forms that can be utilized by the plant or microorganism. The root surface of grasses support an elevated population of microorganisms relative to the bulk soil, and it is likely that there is intense competition between the plant root and microorganisms for the limited amount of soluble iron present. Hypotheses to be tested in our research include (1) grass species that are pa rticularly well-adapted to Fe- limited soils excrete larger quantities of Fe-scavenging siderophores than do nonadapted species, and (II) adapted species are better able to capitalize on the Fe solubilized by siderophores secreted by soil microorganisms. The data he obtains will be integrated into a conceptual model of the interrelationships between roots, microbes and soil as they contribute to the adaptation of certain grasses to iron-limited soils. ***
