Water uptake of soybean (Glycine max L. Merr.) during exposure to O2 deficiency and field level CO2 concentration in the root zone

This study determined whether the field level concentration of root zone CO₂ affects transpiration rate and root water transport in soybean (Glycine max L. Merr.). In an upland field converted from a paddy field, topsoil CO₂ during the cropping season rose to 8 kPa of partial pressure after rainfall, whereas O₂ dropped only to a minimum of 7 kPa. An elevated root zone CO₂ pressure of 6 kPa significantly reduced transpiration rate. Although the transpiration rate of soybean plants exposed to hypoxia of 1.5 kPa O₂ alone was reduced by 52% of the aerated plant, the rate was more decreased by 33 by adding CO₂ gas to low root zone O₂. Similarly, the elevated root zone CO₂ significantly reduced hydraulic conductance of roots. The low transpiration rate with a high leaf water potential and low leaf greenness in the elevated root zone CO₂ indicated that stomatal closure in high root zone CO₂ may also occur irrespective of leaf water status and be involved in depressed nitrogen supply to a shoot. The results indicate that root zone CO₂ at the field level is a growth inhibitor of waterlogged soybean through decreasing water uptake and/or stomatal aperture.