Maximum leaf conductance driven by CO 2 effects on stomatal size and density over geologic time

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Abstract
Stomatal pores are microscopic structures on the epidermis of leaves formed by 2 specialized guard cells that control the exchange of water vapor and CO 2 between plants and the atmosphere. Stomatal size ( S ) and density ( D ) determine maximum leaf diffusive (stomatal) conductance of CO 2 ( g c max ) to sites of assimilation. Although large variations in D observed in the fossil record have been correlated with atmospheric CO 2 , the crucial significance of similarly large variations in S has been overlooked. Here, we use physical diffusion theory to explain why large changes in S necessarily accompanied the changes in D and atmospheric CO 2 over the last 400 million years. In particular, we show that high densities of small stomata are the only way to attain the highest g c max values required to counter CO 2 “starvation” at low atmospheric CO 2 concentrations. This explains cycles of increasing D and decreasing S evident in the fossil history of stomata under the CO 2 impoverished atmospheres of the Permo-Carboniferous and Cenozoic glaciations. The pattern was reversed under rising atmospheric CO 2 regimes. Selection for small S was crucial for attaining high g c max under falling atmospheric CO 2 and, therefore, may represent a mechanism linking CO 2 and the increasing gas-exchange capacity of land plants over geologic time.