Hormesis in mixtures — Can it be predicted? Regina G. Belz a, ⁎ , Nina Cedergreen b , Helle Sørensen c a University of Hohenheim, Institute of Phytomedicine, Department of Weed Science, Otto-Sander-Straße 5, 70593 Stuttgart, Germany b Department of Agricultural Sciences, Faculty of Life Sciences, University of Copenhagen, Højbakkegård Allé 13, 2630 Taastrup, Denmark c Department of Natural Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg C., Denmark ARTICLE INFO ABSTRACT Article history: Received 3 March 2008 Received in revised form 5 June 2008 Accepted 9 June 2008 Available online 21 July 2008 Binary mixture studies are well established for mixtures of pollutants, pesticides, or allelochemicals and sound statistical methods are available to evaluate the results in relation to reference models. The majority of mixture studies are conducted to investigate the effect of one compound on the inhibitory action of another. However, since stimulatory responses to low concentrations of chemicals are gaining increased attention and improved statistical models are available to describe this phenomenon of hormesis, scientists are challenged by the question of what will happen in the low concentration range when all or some of the chemicals in a mixture induce hormesis? Can the mixture effects still be predicted and can the size and concentration range of hormesis be predicted? The present study focused on binary mixtures with one or both compounds inducing hormesis and evaluated six data sets of root length of Lactuca sativa L. and areal growth of Lemna minor L., where substantial and reproducible hormetic responses to allelochemicals and herbicides have been found. Results showed that the concentration giving maximal growth stimulatory effects (M) and the concentration where the hormetic effect had vanished (LDS) could be predicted by the most-used reference model of concentration addition (CA), if the growth inhibitory concentrations (EC 50 ) followed CA. In cases of deviations from CA at EC 50 , the maximum concentration M and the LDS concentration followed the same deviation patterns, which were described by curved isobole models. Thus, low concentration mixture effects as well as the concentration range of hormesis can be predicted applying available statistical models, if both mixture partners induce hormesis. Using monotonic concentration–response models instead of biphasic concentration–response models for the prediction of joint effects, thus ignoring hormesis, slightly overestimated the deviation from CA at EC 20 and EC 50 , but did not alter the general conclusion of the mixture study in terms of deviation from the reference model. Mixture effects on the maximum stimulatory response were tested against the hypothesis of a linear change with mixture ratio by constructing 95% prediction intervals based on the single concentration–response curves. Four out of the six data sets evaluated followed the model of linear interpolation reasonably well, which suggested that the size of the hormetic growth stimulation can be roughly predicted in mixtures from knowledge of the concentration–response relationships of the individual chemicals. © 2008 Elsevier B.V. All rights reserved. Keywords: Biphasic concentration–response curve Concentration addition Hormesis Mixture toxicity SCIENCE OF THE TOTAL ENVIRONMENT 404 (2008) 77 – 87 ⁎ Corresponding author. Tel.: +49 711 459 23444. E-mail addresses: belz@uni-hohenheim.de (R. Belz), ncf@life.ku.dk (N. Cedergreen), helle@dina.kvl.dk (H. Sørensen). 0048-9697/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2008.06.008 available at www.sciencedirect.com www.elsevier.com/locate/scitotenv