Sulphur dioxide fumigation effects on stomatal density and index of non-resistant plants: Implications for the stomatal palaeo-[CO2] proxy method

The inverse relationship between the number of stomata on the surface of a leaf and the atmospheric concentration of carbon dioxide ([CO2]) in which it developed permits the use of fossil plants as indicators of palaeo-atmospheric [CO2] level (palaeo-[CO2]). This “stomatal method” to reconstruct palaeo-[CO2] is dependant upon stomatal initiation being determined by [CO2]. However, global perturbations to the carbon cycle and climate system throughout earth history are not only characterised by fluctuations in the level of atmospheric [CO2], but also by the release of large volumes of toxic gases such as sulphur dioxide (SO2) into the atmosphere. Recent studies have raised uncertainties into the efficacy of stomatal palaeo-[CO2] proxies during episodes of SO2 fumigation. This study aims to assess the effect of exposure to 0.2 ppm SO2 on the stomatal frequencies of plant species with no evolutionarily acquired resistance to toxic gases in comparison to individuals grown under control conditions and atmospheres of elevated [CO2]. Fumigation with SO2 resulted in a general pattern of increased stomatal density (SD) values, but induced variability in the stomatal index (SI) responses of the plant species studied. Ginkgo biloba exhibited an increase in SI, whereas the araucariacean conifers Agathis australis and Araucaria bidwillii displayed reductions in SI that were indistinguishable from values observed under [CO2] enrichment. These results suggest that the presence of atmospheric SO2 may reduce the effectiveness of stomatal reconstructions of palaeo-[CO2] during intervals characterised by the release of large volumes of toxic gases into the atmosphere. However, exposure to SO2 induced significant increases in the SD/SI ratios of six of the seven species studied. Calculation of the SD/SI ratios of fossil plants may identify any stratigraphic horizons where the stomatal initiation responses of the fossil flora were affected by sudden fumigation with toxic gases, and thus influence palaeo-[CO2] estimates. Therefore the SD/SI ratios of fossil plants may serve as indicators of the effectiveness of stomatal reconstructions of palaeo-[CO2].

► Stomatal densities of fossil plants can be used to reconstruction past CO2 levels. ► SO2 may also affect stomatal development and therefore estimates of CO2. ► Seven plants with no resistance to SO2 were grown in controlled environments. ► SO2 resulted in an increase in the ratio of stomatal density to index. ► This ratio may be employed to differentiate between SO2 and CO2 effects on stomata.