Internal transport of CO2 from the root-zone to plant shoot is pH dependent.

Abstract

We investigated the fate of carbon dioxide (CO2 ) absorbed by roots or internally produced by respiration using gas exchange and stable isotopic labeling. CO2 efflux from detached leaves supplied with bicarbonate/CO2 solutions was followed over six cycles. CO2 effluxes were detected when bicarbonate solution at high pH was used, corresponding to 71-85% of the expected efflux. No CO2 efflux was detected when CO2 solutions at low pH were used but CO2 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 CO2 efflux signal (14-30% of the expected efflux) was detected suggesting that CO2 in the xylem stream can readily escape to the atmosphere before reaching the leaf. When the root-zones of intact plants were exposed to CO2 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 CO2 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 pH8, suggesting that bicarbonate does not pass into the xylem.