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.