Soil carbon dioxide venting through rice roots

The growth of rice in submerged soils depends on its ability to form continuous gas channels-aerenchyma-through which oxygen (O₂ ) diffuses from the shoots to aerate the roots. Less well understood is the extent to which aerenchyma permits venting of respiratory carbon dioxide (CO₂ ) in the opposite direction. Large, potentially toxic concentrations of dissolved CO₂ develop in submerged rice soils. We show using X-ray computed tomography and image-based mathematical modelling that CO₂ venting through rice roots is far greater than thought hitherto. We found rates of venting equivalent to a third of the daily CO₂ fixation in photosynthesis. Without this venting through the roots, the concentrations of CO₂ and associated bicarbonate (HCO₃^- ) in root cells would have been well above levels known to be toxic to roots. Removal of CO₂ and hence carbonic acid (H₂ CO₃ ) from the soil was sufficient to increase the pH in the rhizosphere close to the roots by 0.7 units, which is sufficient to solubilize or immobilize various nutrients and toxicants. A sensitivity analysis of the model showed that such changes are expected for a wide range of plant and soil conditions.