Impacts of elevated rhizospheric CO2 on root respiration rate and rhizodeposition in Populus

Background/Question/Methods Research into the direct effects of carbon dioxide (CO₂) on root respiration rate have yielded a variety of results, with studies reporting increases, decreases, and no effect of rhizospheric CO₂ on root respiration. Plants have been shown to uptake and fix rhizospheric CO₂ in their roots via the enzyme PEP-Carboxylase, and this process contributes products, in the form of organic acids, to respiratory processes in the mitochondria and to root exudation of organic acids. Here we test the hypothesis that the inhibitory effect of rhizospheric CO₂ on root respiration is a result of enhanced exudation (rhizodeposition) of organic acids in these species. Exudation of Krebs cycle intermediates should reduce flux through the Krebs cycle, and thus contribute to a reduced rate of respiration in root tissues. Here we present data that explores the relationships between rhizospheric CO₂, root respiration rate and rhizodeposition in roots of Poplar, and highlight the role of root PEP-C in modulating this response.

Results/Conclusions Application of elevated rhizospheric CO₂ to the rooting zone of hydroponic Populus trees lead to a decrease in root respiration rate (CO₂ production) and a significant decrease (p = 0.03) in the overall root to shoot ratio. In contrast, root PEP-C activity was significantly increased (on average by 18%, p = 0.04) in the CO₂-treated roots, consistent with a role of this enzyme in responding to rhizospheric CO₂. Here we present results relating rates of organic acid exudation and composition to root PEP-C activity, and the decrease in root-respired CO₂ we have observed in this system. We discuss our results in light of increasing rhizospheric CO₂ concentrations and the implications this increase may have on root carbon fixation, rhizodeposition and plant-soil interactions in forest ecosystems.