Monoterpene synthase involved in the biosynthesis of monoterpene natural products of the 'cineole cassette', volatile compounds present in floral scent (PubMed:17611797). Catalyzes the conversion of (2E)-geranyl diphosphate (GPP) into 1,8-cineole and, as minor products, limonene, sabinene, (E)-beta-ocimene, beta-myrcene, alpha-pinene and alpha-terpineol (PubMed:17611797). (2E)-geranyl diphosphate + H2O = 1,8-cineole + diphosphate (2E)-geranyl diphosphate = diphosphate + limonene (2E)-geranyl diphosphate = diphosphate + sabinene (2E)-geranyl diphosphate = (E)-beta-ocimene + diphosphate (2E)-geranyl diphosphate = beta-myrcene + diphosphate (2E)-geranyl diphosphate = alpha-pinene + diphosphate (2E)-geranyl diphosphate + H2O = (S)-alpha-terpineol + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. kcat is 0.33 sec(-1) with (2E)-geranyl diphosphate as substrate. Optimum pH is 8. Optimum temperature is 35 degrees Celsius. Secondary metabolite biosynthesis; terpenoid biosynthesis. Monomer. Confined to buds and flowers. In flowers, restricted to stigma and style tissues as well as in petals, primarily in the adaxial and abaxial epidermis (PubMed:17611797). Barely detected in leaves, shoots, roots and sepals (PubMed:17611797). In petals, transcripts, protein and enzyme activity levels follow a circadian oscillation, with an increase during afternoon to reach a maxima in darkness and exhibit lower levels in the morning, thus leading to a nocturnal emission of floral monoterpenes. 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.