Involved in the biosynthesis of germacrene-derived sesquiterpene lactones (PubMed:30468448). Catalyzes the cyclization of farnesyl diphosphate to alpha-copaene, delta-cadinene, alpha-muurolene, beta-caryophyllene and alpha-humulene (PubMed:19580670). (2E,6E)-farnesyl diphosphate = alpha-copaene + diphosphate (2E,6E)-farnesyl diphosphate = alpha-muurolene + diphosphate (2E,6E)-farnesyl diphosphate = alpha-humulene + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Mainly expressed in sunflower trichomes. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.