Involved in monoterpene (C10) biosynthesis. The major product is 1,8-cineole (52%) followed by minor amounts of sabinene (14.5%), myrcene (13.3%), (-)-(1S)-beta-pinene (7.8%), (-)-(4S)-limonene (4.0%), (E)-beta-ocimene (2.7%), alpha-terpineol (2.4%), (-)-(1S)-alpha-pinene (1.9%), terpinolene (0.8%), and (+)-alpha-thujene (0.6%). (2E)-geranyl diphosphate + H2O = 1,8-cineole + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Optimum pH is 8.0. Secondary metabolite biosynthesis; terpenoid biosynthesis. Predominantly expressed in roots and at much lower levels in siliques. Not found in leaves, flowers or stems. Also detected in flowers in cv. Landsberg erecta. Not expressed in root apical meristem and elongation zone. Found in the vascular system of young roots and additionally in the cortex and epidermal cell layer of older roots. Not induces in aerial parts by treatments with jasmonic acid, 6-ethyl indanoyl-L-Ile, fungal peptaibol elicitor alamethicin or herbivory. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). May be due to intron retention. Belongs to the terpene synthase family. Tpsb subfamily.