Three new methods indicate that rising carbon dioxide reduces seedling respiration

The concentration of CO₂ in the earth’s atmosphere has sat between 180 and 280 μmol mol⁻¹ for most of the last 600,000 years. However, during the 1800s the CO₂ began to rise. Currently the global CO₂ concentration sits at around 400 µmol mol^-1 but could increase to 800 µmol mol^-1 by the end of the century. In the field of plant science, much research has focussed on the potential photosynthetic responses to such a dramatic change in CO₂ concentration. Respiratory responses on the other hand are less clear.

Plant dark respiration fuels many vital plant processes and is significant in the carbon budget of the biosphere. Therefore, any response of plant dark respiration to changes in atmospheric carbon dioxide concentration could be of considerable importance. The lack of understanding of respiratory responses to changes in CO₂ is largely due to methodological constraints. Specifically, it is very difficult to eliminate all potential sources of error in gas exchange measurement. These include the limited sensitivity of analysers, calibration errors, interference with other gases and leaks from the clamp-on leaf chamber. Minimising these sources of error and comparison against methods that do not rely on gas exchange could help us to understand responses of dark respiration to changing CO₂ concentration.

In his new paper published in AoBP, James Bunce demonstrates that reductions in plant respiration occur in darkness with increasing concentrations of carbon dioxide in the range relevant to global change. Bunce conducted several experiments using a variety of species (including sunflower, cotton, maize, amaranth and soybean) and eliminated potential sources of error associated with respiration measurements made using traditional measurements of plant gas exchange. Measurements were made of rates of whole plant loss of dry mass in darkness, the survival times of seedlings kept in the dark, and rates of carbon dioxide loss in petioles which were excised and placed in a gas exchange chamber submerged in water to eliminate air leakage.

While we cannot categorically state that elevated CO₂ in darkness reduces respiration, the results presented by Bunce lend credence to reports that very careful leaf gas exchange measurements indicate that elevated CO₂ treatments can reduce leaf dark respiration. The experiments reported here focussed on young seedlings, which may have higher rates of respiration than larger tissue, and it will be important for similarly diligent work in the future to investigate such responses in older plants.