Involved in the biosynthesis of phenolic monoterpenes natural products (PubMed:20419468, Ref.2). Monoterpene synthase that catalyzes mainly the formation of olefins such as sabinene and beta-phellandrene, and minor amounts of other monoterpenes (e.g. myrcene, gamma-terpinene, alpha-thujene and alpha-pinene) from geranyl diphosphate (GPP) (PubMed:20419468, Ref.2). (2E)-geranyl diphosphate = beta-phellandrene + diphosphate (2E)-geranyl diphosphate = (1R,5R)-sabinene + 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 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.