Stomatal density and aperture length in four plant species grown across a subambient CO2 gradient

Stomatal density, stomatal aperture length, area/leaf, and number of stomata/leaf were measured after the annual C₃ agronomic grasses oats (Avena sativa) and wheat (Triticum aestivum), the C, woody legume honey mesquite (Prosopis glandulosa), and the perennial C₄ grass little bluestem (Schizachyrium scoparium) were grown across a subambient carbon dioxide concentration (CO₂]) gradient from near 200 to 350 μmol/mol in a growth chamber. The purpose was to determine if the size and density of stomata vary in response to atmospheric CO₂] during growth, across a subambient CO₂] range representative of the doubling that has occurred since the last ice age. Changes in stomatal density and aperture length with increasing CO₂] were small when detected. Stomatal density decreased on adaxial flag leaf surfaces of wheat, and aperture length increased slightly with CO₂], Leaf area and number of stomata/flag leaf increased by similar proportions with CO₂] in two wheat cultivars. No consistent relationship between CO₂] and stomatal density or size was detected in mesquite, oats, or little bluestem. We conclude that individual plants of these species lack the plasticity to significantly alter stomatal density and aperture length in response to increasing atmospheric CO₂] in a single generation (annuals) or growing season (perennials).