ELEVATED CO2 CONCENTRATIONS AND STOMATAL DENSITY - OBSERVATIONS FROM 17 PLANT-SPECIES GROWING IN A CO2 SPRING IN CENTRAL ITALY

Stomatal density (SD) and stomatal conductance (g(s)) can be affected by an increase of atmospheric CO2 concentration. This study was conduc ted on 17 species growing in a naturally enriched CO2 spring and belon ging to three plant communities. Stomatal conductance, stomatal densit y and stomatal index (SI) of plants from the spring, which were assume d to have been exposed for generations to elevated [CO2], and of plant s of the same species collected in a nearby control site, were compare d. Stomatal conductance was significantly lower in most of the species collected in the CO2 spring and this indicated that CO2 effects on g, are not of a transitory nature but persist in the long term and throu gh plant generations. Such a decrease was, however, not associated wit h changes in the anatomy of leaves: SD was unaffected in the majority of species (the decrease was only significant in three out of the 17 s pecies examined), and also SI values did not vary between the two site s with the exception of two species that showed increased SI in plants grown in the CO2-enriched area. These results did not support the hyp othesis that long-term exposure to elevated [CO2] may cause adaptive m odification in stomatal number and in their distribution.