Ultra-high carbon dioxide levels enhances in vitro shoot growth and morphogenesis in labiatae

The growth and morphogenetic development in shoot cultures of lemon basil (Ocimum basilicum), oregano (Origanum vulgare), peppermint (Mentha x piperita), spearmint (Mentha spicata), and thyme (Thymus vulgaris) shoots on MS salts and 0.8 percent agar containing 0 or 3 percent sucrose were determined under 350, 1,500, 3,000, 10,000, and 30,000 μl CO2/liter of air were determined after 8 weeks growth. High CO2 levels, especially the ultra-high levels (≥ 3,000 μl CO2/liter of air), substantially increased fresh weight and leaf and root numbers in all cultures, whether or not the basal media contained sucrose, as compared with cultures grown on the same basal media under ambient air (350 μl CO2/liter of air). Ultra-high CO2 levels, also, enhanced the formation of aerial adventitious roots from the shoots of all species tested. A photosynthetic photon flux density of 180 μmole. m-2 s-1 using a 16 h light/8 h dark photoperiod consistently provided better growth than lower light levels.