Stomatal index responses of Agrostis canina to CO₂ and sulphur dioxide: implications for palaeo‐[CO₂] using the stomatal proxy

• Stomatal index values of fossil plants are widely used in reconstructing palaeo‐[CO₂]. This depends upon the assumption that the stomatal index is determined by the atmospheric concentration of CO₂ ([CO₂]). This study investigates whether fumigation with, and resistance to, sulphur dioxide (SO₂) induces a reduction in the stomatal index that may affect stomatal reconstructions of palaeo‐[CO₂] coinciding with episodes of global‐scale volcanism. • Agrostis canina from Mefite di Ansanto, Italy, grow in atmospheres of elevated‐[CO₂], SO₂ and hydrogen sulphide (H₂S). Mefite A. canina were compared with a control population in a ‘common‐garden' experiment and a controlled‐environment study under elevated‐[CO₂] and SO₂ fumigation. • In A. canina, resistance to toxic volcanic gases is not associated with reduced stomatal index, and fumigation with SO₂ does not cause a decrease in stomatal initiation. The two populations of A. canina analyzed in this study exhibit different stomatal index-[CO₂] ‘responses', with control plants showing a reduction in stomatal index and Mefite plants showing no response. • Stomatal reconstructions of palaeo‐[CO₂] during past episodes of global‐scale volcanism probably reflect atmospheric [CO₂] and not [SO₂]. The lack of a reduction in the stomatal index in response to elevated [CO₂] in the Mefite plants, suggests that resistance to toxic gases and/or long‐term growth at high [CO₂] reduces, or negates, sensitivity of the stomatal index-[CO₂] relationship, or that stomatal index-[CO₂] in the Mefite plants is attuned to [CO₂] fluctuations at much higher concentrations.