Stomatal Density as an Indicator of Atmospheric CO 2 Concentration

Examining the response of stomatal densities to changing atmospheric CO₂ concentrations is an important element in understanding whole plant responses because it effects both water use efficiency and rate of carbon uptake. These two physiological processes have a major influence on the productivity of vegetation. Methods and sources of material that may be used for making stomatal counts are reviewed. The response of stomatal density to changing concentrations of atmospheric CO₂ are considered from four types of investigation: 1) field studies on living plants growing at different altitudes; 2) studies on plants grown in controlled environments; 3) historical trends detected from herbarium material and 4) comparisons of historical records with modern material. The results suggest conflicting responses which could, to some extent, be due to methodological differences between workers. The more fundamental questions about the variability of stomatal density within and between species, the action of CO₂ in controlling stomatal development, and whether observed responses represent acclimation or genetic adaptation are also discussed.