Greater antioxidant and respiratory metabolism in field‐grown soybean exposed to elevated O 3 under both ambient and elevated CO 2

Antioxidant metabolism is responsive to environmental conditions, and is proposed to be a key component of ozone (O 3 ) tolerance in plants. Tropospheric O 3 concentration ([O 3 ]) has doubled since the Industrial Revolution and will increase further if precursor emissions rise as expected over this century. Additionally, atmospheric CO 2 concentration ([CO 2 ]) is increasing at an unprecedented rate and will surpass 550 ppm by 2050. This study investigated the molecular, biochemical and physiological changes in soybean exposed to elevated [O 3 ] in a background of ambient [CO 2 ] and elevated [CO 2 ] in the field. Previously, it has been difficult to demonstrate any link between antioxidant defences and O 3 stress under field conditions. However, this study used principle components analysis to separate variability in [O 3 ] from variability in other environmental conditions (temperature, light and relative humidity). Subsequent analysis of covariance determined that soybean antioxidant metabolism increased with increasing [O 3 ], in both ambient and elevated [CO 2 ]. The transcriptional response was dampened at elevated [CO 2 ], consistent with lower stomatal conductance and lower O 3 flux into leaves. Energetically expensive increases in antioxidant metabolism and tetrapyrrole synthesis at elevated [O 3 ] were associated with greater transcript levels of enzymes involved in respiratory metabolism.