Maximum leaf conductance driven by CO2 effects on stomatal size and density over geologic time.

Area of Science:

• Paleobotany
• Plant Physiology
• Evolutionary Biology

Background:

• Stomatal pores, formed by guard cells, regulate water vapor and CO2 exchange in plants.
• Stomatal size (S) and density (D) influence maximum stomatal conductance (g(c(max))).
• While stomatal density variations are linked to atmospheric CO2 in fossils, the role of stomatal size has been underestimated.

Purpose of the Study:

• To investigate the role of stomatal size variations in plant gas exchange over geological timescales.
• To explain the co-variation of stomatal size, density, and atmospheric CO2 concentrations over the last 400 million years using physical diffusion theory.

Main Methods:

• Application of physical diffusion theory to stomatal function.
• Analysis of fossil stomatal data (size and density) in relation to atmospheric CO2 levels.

Main Results:

• Large changes in stomatal size (S) are shown to be necessary alongside changes in stomatal density (D) and atmospheric CO2 over geologic time.
• High densities of small stomata are critical for achieving high g(c(max)) to overcome CO2 starvation at low atmospheric CO2.
• Observed patterns of increasing D and decreasing S in fossil records correlate with low CO2 periods (Permo-Carboniferous, Cenozoic glaciations).

Conclusions:

• Selection for small stomatal size was crucial for maintaining high gas exchange capacity under falling atmospheric CO2.
• This mechanism may link atmospheric CO2 levels to the increasing gas-exchange capacity of land plants throughout geologic time.