Influence of modified oxygen and carbon dioxide atmospheres on mint and thyme plant growth, morphogenesis and secondary metabolism in vitro

Growth (fresh weight) and morphogenesis (production of leaves, roots and shoots) of mint (Mentha sp. L.) and thyme (Thymus vulgaris L.) shoots were determined under atmospheres of 5%, 10%, 21%, 32%, or 43% O2 with either 350 or 10,000 μmol mol-1 CO2. Plants were grown in vitro on Murashige and Skoog salts, 3% sucrose and 0.8% agar under a 16/8-h (day/night) photoperiod with a light intensity of 180 μmol s-1 m-2. Growth and morphogenesis responses varied considerably for the two plant species tested depending on the level of O2 administered. Growth was considerably enhanced for both species under all O2 levels tested when 10,000 μmol mol-1 CO2 was added as compared to growth responses obtained at the same O2 levels tested with 350 μmol mol-1 CO2. Mint shoots exhibited high growth and morphogenesis responses for all O2 levels tested with 10,000 μmol mol-1 CO2. In contrast, thyme shoots exhibited enhanced growth and morphogenesis when cultured in ≥21% O2 with 10,000 μmol mol-1 CO2 included compared to shoots cultured under lower O2 levels. Essential oil compositions (i.e. monoterpene, piperitenone oxide from mint and aromatic phenol, thymol from thyme) were analyzed from CH2Cl2 extracts via gas chromatography from the shoot portion of plants grown at all O2 levels. The highest levels of thymol were produced from thyme shoots cultured under 10% and 21% O2 with 10,000 μmol mol-1 CO2 and levels were considerably lower in shoots grown under either lower or higher O2 levels. Higher levels of piperitenone oxide were obtained from mint cultures grown under ≥21% O2 with 10,000 μmol mol-1 CO2 compared to that obtained with lower O2 levels.