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

When atmospheric CO2 concentration increases, various consequences for plant metabolism have been suggested, such as changes in photosynthesis, photo respiration or respiration which can affect growth and carbon sequestration. In addition to long-term (indirect) effects on respiration, short-term (direct) effects of CO2 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 CO2 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 CO2 (up to 4200 mumol 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) CO2 effects were found with mature Nicotiana leaves whereas, in young leaves, the effect was absent. The experimental results clearly show that the observed direct CO2 effect on dark CO2 efflux in the mature tobacco leaves was caused by leakage of CO2 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 CO2 inhibition on CO2 efflux was no longer observed. It is concluded that the measured direct CO2 effect(s) on leaf CO2 efflux in the dark are artefactual, and that a true direct CO2 effect on leaf respiration does not exist.