Impacts of Rhizosphere CO₂ on Root Phosphoenolpyruvate Carboxylase Activity, Root Respiration Rate and Rhizodeposition in Populus spp.

Abstract

Roots live in and have evolved in a high carbon dioxide (CO₂) environment, yet relatively little research has been conducted on the impacts of soil dissolved inorganic carbon (DIC) on root metabolism. In this thesis, I explore the impacts of root-zone DIC on whole plant biomass accumulation, water use efficiency, and above-ground gas exchange. In addition, I explore the impacts of root-zone DIC on root processes: root PEP-Carboxylase activity, root respiration rate and root exudation of Krebs cycle organic acids. Root-zone DIC did not impact biomass accumulation, leaf gas exchange parameters or water use efficiency under the growth conditions examined. Root-zone DIC did increase root PEP-Carboxylase activity, but decreased root respiration (both CO₂ production and O₂ consumption) and decreased organic acid exudation rates. Increase in measurement CO₂ partial pressure was found to cause an instantaneous decrease in root CO₂ production, and I provide evidence that changes in root metabolism (CO₂ uptake by roots) are part of the cause of this phenomenon. A hypothesized relationship between root respiration rate and Krebs cycle organic acid exudation was not supported by my data. I conclude that root-zone DIC has important impacts on critical functions of root metabolism, and should be considered as an important abiotic factor much in the same way atmospheric CO₂ is for leaves and whole plant biology.