Involved in the biosynthesis of over 15 sesquiterpenes (C15). The major products are (+)-alpha-barbatene (27.3%), (+)-thujopsene (17.8%) and (+)-beta-chamigrene (9.9%). Can use farnesyl diphosphate or geranyl diphosphate as substrates, but not geranylgeranyl diphosphate. (2E,6E)-farnesyl diphosphate = (+)-alpha-barbatene + diphosphate (2E,6E)-farnesyl diphosphate = (+)-thujopsene + diphosphate (2E,6E)-farnesyl diphosphate = (+)-beta-chamigrene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. The formation of monoterpene products from geranyl diphosphate (GPP) is unlikely, as the protein is not expected to be present in plastids, where GPP is thought to be produced. Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed exclusively in flowers. Expressed in intrafloral nectaries and in the funiculus within the ovules. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Does not emit any sesquiterpene volatiles except (E)-beta-caryophyllene, alpha-copaene and alpha-humulene. Belongs to the terpene synthase family. Tpsa subfamily.