Elevated atmospheric CO2 decreases oxidative stress and increases essential oil yield in leaves of Thymus vulgaris grown in a mini-FACE system

Potted one-year-old plants of Thymus vulgaris L. (thyme, Lamiaceae, C3 metabolism), were grown for three months (10 June–10 September, 2004) in a “mini-free-air-CO2 -enrichment” (“mini-FACE”) system, under 500 umol mol −1 and ambient concentrations of atmospheric carbon dioxide (CO2). Compared to ambient CO2 , elevated CO2 stimulated leaf superoxide dismutase (SOD, EC 1.15.1.1) activity only at the first sampling-time (July), followed by no variation or even a trend of decreased activity on the other two sampling-times (August and September). Under high CO2 , guaiacol peroxidase (GPX, EC 1.11.1.7) and catalase (CAT, EC 1.11.1.6) leaf activities showed no variation or drop throughout the duration of the exper-iment. By contrast, under elevated CO2 , leaf glutathione reductase (GR, EC 1.6.4.2) activity increased on all the sampling-times, and also a duration-dependent upward trend of glutathione (GSH) content was recorded, with this increase becoming significant – compared with ambient CO2 – at the third sampling-time (September). Simultaneously, leaves from plants grown under high CO 2 showed a marked increase in essential oil yield, with slight increments in phenolic component and decrements in mono- and sesquiter-pene components. Also, a drop in thiobarbituric acid reactive substances (TBARS) content under elevated CO2 was displayed. Thus, in general, the results reported here point to a downregulation of leaf antioxidant enzymes under elevated CO2 , supporting the notion of reduced reactive oxygen species (ROS) formation under these circumstances. Relying instead on antioxidant-regenerating enzymes, namely GR, fairly high GSH content and essential oils, might be a ‘low cost’ life strategy for growth under elevated CO2, not requiring synthesis/activation of energy-intensive and expensive metabolic processes.