Involved in monoterpene (C10) biosynthesis. The major products are alpha- and beta-pinene, sabinene, beta-myrcene, (E)-beta-ocimene and limonene. (2E)-geranyl diphosphate = beta-myrcene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed exclusively in mature flowers, but not in inmmature buds. 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. Tpsb subfamily.