The effects of coumarin on the length, diameter, and branching density of different root types in maize seedlings (Zea mays L. cv. Cecilia) were investigated. The maize root system represents a useful model for morphological studies, as it consists of radicle, seminal, and nodal roots whose origin and development are quite different. Maize seedlings were grown in a hydroponic culture for 6 days, and then coumarin (at concentrations of 0, 25, 100, and 400 microM) was added to the nutrient solution. Coumarin inhibited root length, but effects differed depending on the root type. C 1/2 values, representing the coumarin concentration causing a 50% inhibition of the root length, were calculated by nonlinear regression. Six, 1, and 0.25 mM coumarin were sufficient to reduce the radicle, seminal, and nodal root lengths by 50%, respectively. At the highest coumarin concentration, the subapical root zone showed swelling. The degree recorded by average diameter was higher in nodal roots than in seminal and radicle roots. Furthermore, coumarin decreased the number of lateral roots and branching density more in the seminal than in the radicle roots. These results suggest the following order of sensitivity to coumarin: nodal > seminal > radicle roots. The observed spatial effects of coumarin could be ecologically significant, since taprooted species could benefit at the disadvantage of fibrous-rooted species and could modify community composition.