Contributes to floral scent emission. (2E)-geranyl diphosphate = diphosphate + tricyclene (2E)-geranyl diphosphate = beta-myrcene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Accumulates in flowers; mostly expressed in both upper and lower petal lobes, and, to a lower extent, in tube and stamens. First observed in mature flower buds and accumulates transiently during 4 days after anthesis. Circadian-regulation with highest levels in early afternoon and lowest levels during the night. 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. Tpsg subfamily.