Involved in the biosynthesis of germacrene D. Can use farnesyl diphosphate as substrate, but not geranyl diphosphate or geranylgeranyl diphosphate. Produces mainly (-)-germacrene D along with gamma-cadinene. (2E,6E)-farnesyl diphosphate + H2O = (1E,4S,5E,7R)-germacra-1(10),5-dien-11-ol + diphosphate (2E,6E)-farnesyl diphosphate = (-)-germacrene D + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed in flowers. Detected in stems, young leaves and tendrils. Expressed in flower buds and detected in open pre-anthesis flowers, flowers after anthesis and in early stages of fruit onset. 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. Tpsa subfamily.