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 (gc 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 cmax values required to counter CO 2''starvation'' at low atmospheric CO2 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 gcmax under falling atmospheric CO2 and, therefore, may represent a mechanism linking CO 2 and the increasing gas-exchange capacity of land plants over geologic time.Phanerozoic ͉ photosynthesis ͉ plant evolution ͉ transpiration ͉ xylem

Given that stomatal movement is ultimately a mechanical process and that stomata are morphologically and mechanically diverse, we explored the influence of stomatal mechanical diversity on leaf gas exchange and considered some of the constraints. Mechanical measurements were conducted on the guard cells of four different species exhibiting different stomatal morphologies, including three variants on the classical ''kidney'' form and one ''dumb-bell'' type; this information, together with gas-exchange measurements, was used to model and compare their respective operational characteristics. Based on evidence from scanning electron microscope images of cryo-sectioned leaves that were sampled under full sun and high humidity and from pressure probe measurements of the stomatal aperture versus guard cell turgor relationship at maximum and zero epidermal turgor, it was concluded that maximum stomatal apertures (and maximum leaf diffusive conductance) could not be obtained in at least one of the species (the grass Triticum aestivum) without a substantial reduction in subsidiary cell osmotic (and hence turgor) pressure during stomatal opening to overcome the large mechanical advantage of subsidiary cells. A mechanism for this is proposed, with a corollary being greatly accelerated stomatal opening and closure. Gas-exchange measurements on T. aestivum revealed the capability of very rapid stomatal movements, which may be explained by the unique morphology and mechanics of its dumb-bell-shaped stomata coupled with ''see-sawing'' of osmotic and turgor pressure between guard and subsidiary cells during stomatal opening or closure. Such properties might underlie the success of grasses.Although the morphological diversity of stomata is widely documented (Haberlandt, 1884;Meidner and Mansfield, 1968;Allaway and Milthorpe, 1976;Ziegler, 1987;Willmer and Fricker, 1996), little is known of how this translates into functional diversity and what the environmental context of this might be. Throughout the 400 million year (Ma) history of vascular plants on land, long-term decline in atmospheric CO 2 concentration and shifts in prevailing moisture patterns have placed selective pressures on stomata to increase epidermal conductance to CO 2 diffusion and also to increase transpiration efficiency (CO 2 fixed per unit water transpired). This posed two separate problems, upon which the combination of mutation and time might have worked to give rise to the current diversity of stomatal form and function. The first centered on the simple geometric practicalities of fitting enough functional stomatal units per unit leaf surface area to meet the desired CO 2 flux as atmospheric CO 2 concentration changed, or to service an increase in photosynthetic capacity. The second centered on the performance characteristics of any new stomatal structure or configuration in relation to transpiration efficiency. Here, by examining the mechanical and performance characteristics of stomata in four different species, we explore the nature of these two problem...

Maximum and minimum stomatal conductance, as well as stomatal size and rate of response, are known to vary widely across plant species, but the functional relationship between these static and dynamic stomatal properties is unknown. The objective of this study was to test three hypotheses: (i) operating stomatal conductance under standard conditions (g op) correlates with minimum stomatal conductance prior to morning light [g min(dawn)]; (ii) stomatal size (S) is negatively correlated with g op and the maximum rate of stomatal opening in response to light, (dg/dt)max; and (iii) g op correlates negatively with instantaneous water-use efficiency (WUE) despite positive correlations with maximum rate of carboxylation (Vc max) and light-saturated rate of electron transport (J max). Using five closely related species of the genus Banksia, the above variables were measured, and it was found that all three hypotheses were supported by the results. Overall, this indicates that leaves built for higher rates of gas exchange have smaller stomata and faster dynamic characteristics. With the aid of a stomatal control model, it is demonstrated that higher g op can potentially expose plants to larger tissue water potential gradients, and that faster stomatal response times can help offset this risk.

Earth's atmospheric CO 2 concentration (c a ) for the Phanerozoic Eon is estimated from proxies and geochemical carbon cycle models. Most estimates come with large, sometimes unbounded uncertainty. Here, we calculate tightly constrained estimates of c a using a universal equation for leaf gas exchange, with key variables obtained directly from the carbon isotope composition and stomatal anatomy of fossil leaves. Our new estimates, validated against ice cores and direct measurements of c a , are less than 1000 ppm for most of the Phanerozoic, from the Devonian to the present, coincident with the appearance and global proliferation of forests. Uncertainties, obtained from Monte Carlo simulations, are typically less than for c a estimates from other approaches. These results provide critical new empirical support for the emerging view that large (~2000-3000 ppm), long-term swings in c a do not characterize the post-Devonian and that Earth's long-term climate sensitivity to c a is greater than originally thought.