Background
Elevated levels of atmospheric CO2 have generally been found to reduce plant respiration rates (Drake et al., 1999).  Some scientists, however, have suggested that this conclusion has its origin in an artifact of the measurement technique that is typically employed to measure respiration.  In an attempt to resolve the issue, Bunce devised an experiment that appears to overcome this criticism.

What was done
Within a single controlled-environment chamber, different 16-plant batches of soybean (Glycine max L. Merr. "Clark") plants were grown, one to a pot, in plastic pots filled with 1.8 liters of vermiculite that was flushed daily with a complete nutrient solution.  In three experiments conducted at day/night atmospheric CO2 concentrations of 370/390 ppm, air temperatures were either 20, 25 or 30°C, while in three other experiments conducted at an air temperature of 25°C, atmospheric CO2 concentrations were either 40, 370 or 1400 ppm.  At the end of the normal 16 hours of light on the 17th day after planting, half of the plants were harvested and used for the measurement of a number of physical parameters, while measurements of the plant physiological processes of respiration, translocation and nitrate reduction were made on the other half of the plants over the following 8-hour dark period.

What was learned
Plotting translocation and nitrate reduction as functions of respiration, Bunce found that "a given change in the rate of respiration was accompanied by the same change in the rate of translocation or nitrate reduction, regardless of whether the altered respiration was caused by a change in temperature or by a change in atmospheric CO2 concentration."

What it means
Irrespective of whatever mechanisms may have been involved in eliciting the responses observed, Bunce logically concludes that "the parallel responses of translocation and nitrate reduction for both the temperature and CO2 treatments make it unlikely that the response of respiration to one variable [CO2] was an artifact while the response to the other [temperature] was real."  Hence, there is every reason to believe that the typically observed decreases in dark respiration experienced by plants exposed to elevated levels of atmospheric CO2 are indeed real and not the result of defects of the measurement system.  This being the case, it can be appreciated that plant growth is not only enhanced by CO2-induced increases in photosynthesis during the light period of the day, they are also enhanced by CO2-induced decreases in respiration during the dark period.

Reference
Drake, B.G., Azcon-Bieto, J., Berry, J., Bunce, J., Dijkstra, P., Farrar, J., Gifford, R.M., Gonzalez-Meler, M.A., Koch, G., Lambers, H., Siedow, J. and Wullschleger, S.  1999.  Does elevated atmospheric CO2 inhibit mitochondrial respiration in green plants?  Plant, Cell and Environment 22: 649-657.