Changes in Respiratory Mitochondrial Machinery and Cytochrome and Alternative Pathway Activities in Response to Energy Demand Underlie the Acclimation of Respiration to Elevated CO₂ in the Invasive Opuntia ficus-indica

Studies on long-term effects of plants grown at elevated CO₂ are scarce and mechanisms of such responses are largely unknown. To gain mechanistic understanding on respiratory acclimation to elevated CO₂, the Crassulacean acid metabolism Mediterranean invasive Opuntia ficus-indica Miller was grown at various CO₂ concentrations. Respiration rates, maximum activity of cytochrome c oxidase, and active mitochondrial number consistently decreased in plants grown at elevated CO₂ during the 9 months of the study when compared to ambient plants. Plant growth at elevated CO₂ also reduced cytochrome pathway activity, but increased the activity of the alternative pathway. Despite all these effects seen in plants grown at high CO₂, the specific oxygen uptake rate per unit of active mitochondria was the same for plants grown at ambient and elevated CO₂. Although decreases in photorespiration activity have been pointed out as a factor contributing to the long-term acclimation of plant respiration to growth at elevated CO₂, the homeostatic maintenance of specific respiratory rate per unit of mitochondria in response to high CO₂ suggests that photorespiratory activity may play a small role on the long-term acclimation of respiration to elevated CO₂. However, despite growth enhancement and as a result of the inhibition in cytochrome pathway activity by elevated CO₂, total mitochondrial ATP production was decreased by plant growth at elevated CO₂ when compared to ambient-grown plants. Because plant growth at elevated CO₂ increased biomass but reduced respiratory machinery, activity, and ATP yields while maintaining O₂ consumption rates per unit of mitochondria, we suggest that acclimation to elevated CO₂ results from physiological adjustment of respiration to tissue ATP demand, which may not be entirely driven by nitrogen metabolism as previously suggested.