Internal transport of CO ₂ from the root-zone to plant shoot is pH dependent

We investigated the fate of carbon dioxide (CO ₂ ) absorbed by roots or internally produced by respiration using gas exchange and stable isotopic labeling. CO ₂ efflux from detached leaves supplied with bicarbonate/CO ₂ solutions was followed over six cycles. CO ₂ effluxes were detected when bicarbonate solution at high pH was used, corresponding to 71–85% of the expected efflux. No CO ₂ efflux was detected when CO ₂ solutions at low pH were used but CO ₂ efflux was subsequently detected as soon as bicarbonate solutions at high pH were supplied. By sealing the leaf and petiole in a plastic bag to reduce diffusion to the atmosphere, a small CO ₂ efflux signal (14–30% of the expected efflux) was detected suggesting that CO ₂ in the xylem stream can readily escape to the atmosphere before reaching the leaf. When the root-zones of intact plants were exposed to CO ₂ solutions, a significant efflux from leaf surface was observed (13% of the expected efflux). However, no signal was detected when roots were exposed to a high pH bicarbonate solution. Isotopic tracer experiments confirmed that CO ₂ supplied to the root-zone was transported through the plant and was readily lost to the atmosphere. However, little ¹³ C moved to the shoot when roots were exposed to bicarbonate solutions at pH 8, suggesting that bicarbonate does not pass into the xylem.