Reductions in leaf stomatal conductance with rising atmospheric carbon dioxide concentration ([CO 2]) could reduce water use by vegetation and potentially alter climate. Crop plants have among the largest reductions in stomatal conductance at elevated [CO 2]. The relative reduction in stomatal conductance caused by a given increase in [CO 2] is often not constant within a day nor between days, but may vary considerably with light, temperature and humidity. Species also differ in response, with a doubling of [CO 2] reducing mean midday conductances by 50% in others. Elevated [CO 2] increases leaf area index throughout the growing season in some species. Simulations, and measurements in free air carbon dioxide enrichment systems both indicate that the relatively large reductions in stomatal conductance in crops would translate into reductions of
