This work concerns the application of isothermal calorimetry to measure the effect of 'cnicin' on the germination of the soybean (Glycine max (L.) Merr., cv. A7636 RG) and the radish (Raphanus sativus L., cv. Sparkler). The sesquiterpenelactone, cnicin was isolated from a highly invasive plant, the diffuse knapweed Centaurea diffusa Lam. Calorimetric experiments were performed with seeds on wetted filter paper disks or in agar, both containing varying concentrations of cnicin. Results indicate that this substance blocks the water uptake by roots inhibiting subsequent seedling growth but has no effect during germination. (C) 2007 Elsevier B.V. All rights reserved.