Plastic and adaptive responses of plant respiration to changes in atmospheric CO 2 concentration

The concentration of atmospheric CO 2 has increased from below 200 μl l −1 during last glacial maximum in the late Pleistocene to near 280 μl l −1 at the beginning of the Holocene and has continuously increased since the onset of the industrial revolution. Most responses of plants to increasing atmospheric CO 2 levels result in increases in photosynthesis, water use efficiency and biomass. Less known is the role that respiration may play during adaptive responses of plants to changes in atmospheric CO 2 . Although plant respiration does not increase proportionally with CO 2 ‐enhanced photosynthesis or growth rates, a reduction in respiratory costs in plants grown at subambient CO 2 can aid in maintaining a positive plant C‐balance (i.e. enhancing the photosynthesis‐to‐respiration ratio). The understanding of plant respiration is further complicated by the presence of the alternative pathway that consumes photosynthate without producing chemical energy [adenosine triphosphate (ATP)] as effectively as respiration through the normal cytochrome pathway. Here, we present the respiratory responses of Arabidopsis thaliana plants selected at Pleistocene (200 μl l −1 ), current Holocene (370 μl l −1 ), and elevated (700 μl l −1 ) concentrations of CO 2 and grown at current CO 2 levels. We found that respiration rates were lower in Pleistocene‐adapted plants when compared with Holocene ones, and that a substantial reduction in respiration was because of reduced activity of the alternative pathway. In a survey of the literature, we found that changes in respiration across plant growth forms and CO 2 levels can be explained in part by differences in the respiratory energy demand for maintenance of biomass. This trend was substantiated in the Arabidopsis experiment in which Pleistocene‐adapted plants exhibited decreases in respiration without concurrent reductions in tissue N content. Interestingly, N‐based respiration rates of plants adapted to elevated CO 2 also decreased. As a result, ATP yields per unit of N increased in Pleistocene‐adapted plants compared with current CO 2 adapted ones. Our results suggest that mitochondrial energy coupling and alternative pathway‐mediated responses of respiration to changes in atmospheric CO 2 may enhance survival of plants at low CO 2 levels to help overcome a low carbon balance. Therefore, increases in the basal activity of the alternative pathway are not necessarily associated to metabolic plant stress in all cases.