Atmospheric CO₂ concentration may directly affect leaf respiration measurement in tobacco, but not respiration itself*

When atmospheric CO 2 concentration increases, various consequences for plant metabolism have been suggested, such as changes in photosynthesis, photorespiration or respiration which can affect growth and carbon sequestration. In addition to long‐term (indirect) effects on respiration, short‐term (direct) effects of CO 2 concentration on the respiration of leaves, shoots and roots are described in the literature. In most cases, respiration is reported to be inhibited by increased CO 2 concentration, but the mechanism(s) are not yet understood. It has been shown previously that, when the respective technical problems and properties of a gas exchange system are fully considered, a short‐term increase in CO 2 (up to 4200 µ mol mol −1 ) had no effect on respiration of Phaseolus or Populus leaves (Jahnke, Plant, Cell and Environment 24, 1139–1151, 2001). However, in the present study, large (apparent) CO 2 effects were found with mature Nicotiana leaves whereas, in young leaves, the effect was absent. The experimental results clearly show that the observed direct CO 2 effect on dark CO 2 efflux in the mature tobacco leaves was caused by leakage of CO 2 inside the leaves (and the magnitude of the effect was dependent on the size of the leakage). Nicotiana leaves are, in contrast to Phaseolus and Populus leaves (which are heterobaric), characterized by a homobaric anatomy in which intercellular air spaces are not compartmented and provide a continuous system of open pores in the lateral (paradermal) direction of the leaves. Mesophyll porosity increases with leaf development, which explains the differences between young and mature tobacco leaves. When internal leakage was experimentally restricted, the CO 2 inhibition on CO 2 efflux was no longer observed. It is concluded that the measured direct CO 2 effect(s) on leaf CO 2 efflux in the dark are artefactual, and that a true direct CO 2 effect on leaf respiration does not exist.