Involved in the biosynthesis of phenolic sesquiterpenes natural products (Ref.2). Sesquiterpene synthase that catalyzes mainly the formation of (-)-germacrene D and minor amounts of other sesquiterpenes (e.g. bicyclo-germacrene) from farnesyl diphosphate (FPP) (PubMed:20419468, Ref.2). Triggers also moderate amounts formation of myrcene, limonene, terpinolene and linalool in the presence of geranyl diphosphate (GPP) (PubMed:20419468, Ref.2). (2E,6E)-farnesyl diphosphate = (-)-germacrene D + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Homodimer. Expressed in peltate glandular trichomes (PubMed:20419468). Present at low levels in flowers, leaves and stems (Ref.3). 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.