MAIZE AND SOYBEAN TAP, BASAL, AND LATERAL ROOT RESPONSES TO A STRATIFIED ACID, ALUMINUM-TOXIC SOIL

Although crop seedling root systems are made up of several types of ro ot (e.g., tap, basal, and lateral roots) there has been little attempt to determine their respective sensitivity to abiotic stresses. Most a cidic agricultural fields are treated so that they are only slightly o r non-Al-toxic in the topsoil with Al-toxicity in the subsoil-effectiv ely stratifying the soil profile relative to Al-toxicity. To determine if there is differential root type sensitivity to Al-toxic soils, sev eral maize (Zea mays L.) and soybean [Glycine mar (L.) Merr.] cultivar s reported to be Al-tolerant, or sensitive, were grown in containers w ith a stratified soil. This assembled soil consisted of a non-Al-toxic , 100-mm-deep top layer and a 200-mm-deep Al-toxic bottom layer of Por ter soil, a coarse-loamy, mixed, mesic Umbric Dystrochrepts, pH 4.5 an d 92% Al saturation. Containers, backed with X-ray film, were exposed to a neutron beam to determine tap, basal, and lateral root lengths in both layers. Root lengths, relative to a limed control for each culti var, in the Al-toxic bottom layer suggest that the tap, basal, and lat eral roots of maize cv. CMS-36 are each Al-tolerant. The other ''toler ant'' maize and soybean cultivars had only one or two root types toler ant with the remaining root type(s), and all root types on the sensiti ve cultivars, showing no growth in the Al-toxic bottom layer. Only two cultivars, maize cv. SA-6 and soybean cv. Ferry, increased lateral ro ot production in the non-toxic top layer, a classic avoidance response . This variation in root type response to an AI-toxic bottom layer obs erved among the previously identified Al-tolerant cultivars emphasizes the importance of considering different root types as distinct entiti es in studies of root function.