Stomatal development and CO₂ : ecological consequences

•  Stomatal density responses by 48 accessions of Arabidopsis, to CO₂ enrichment, broadly parallel interspecific observations. •  Accessions differing in the degree of stomatal response to both CO₂ and drought differed in flower production. Under well watered conditions flowering benefits from a small reduction in stomatal density with CO₂ enrichment, but benefits from a large reduction under drought. •  Stomatal density increases with altitude in Vaccinium myrtillus but is also strongly influenced by exposure. Exposed plants had higher stomatal densities than plants at the same altitude but in a community of individuals. This difference might be explained by systemic signalling within the plant as mature leaves detect both irradiance and [CO₂ ], subsequently controlling the response of stomatal development in developing leaves. •  Plants with the highest stomatal densities also had the highest stomatal conductances and photosynthetic rates. This suggests that signalling from mature to developing leaves predetermines the potential of the developing leaf to maximize its photosynthetic potential, including associated features such as nitrogen allocation, during early stages of development in the enclosed bud.