Variable Phytotoxic Effects of Thymus vulgaris (Lamiaceae) Terpenes on Associated Species

Thymus vulgaris has a chemical polymorphism in which individual plants have an essential oil dominated by either nonphenolic (geraniol, -terpineol, cis-sabinene hydrate, linalool, or 1,8-cineole) or phenolic (carvacrol and thymol) monoterpenes. Using the geraniol, cis-sabinene hydrate, and the two phenolic chemotypes, we tested three hypotheses: (1) chemotypes vary in their influences on germination and growth of associated plant species, (2) associated species respond differentially to individual chemotypes, and (3) soils under thyme canopies will influence associated species differently than do sterile soils. The study was done in Montpellier, France, in settings mimicking natural conditions. In one experiment, survival of both monocots and dicots from the seed bank was lower in soils collected from under thyme plants of the phenolic chemotypes. In a second experiment, we sowed seeds of Daucus carota, Nigella damascena, and Bromus madritensis into soil collected from under thyme plants and covered with canopies of fresh thyme of each chemotype or into control soils. The species showed differential responses in germination and/or growth to the four chemotypes. In a third experiment, germination and growth of D. carota, N. damascena, Bromus erectus, and Picris hieracoides on sterile soil covered with fresh thyme foliage and controls showed differential responses to the four chemotypes; however, these responses were less marked than those on soils collected under thyme plants. These results—coupled with previous demonstrations of chemotype-influenced differential herbivory and parasitism—illustrate the complex influences of genetic variability on multispecies interactions and the individuality of species responses to toxic chemicals.