Plant Water Use Efficiency over Geological Time – Evolution of Leaf Stomata Configurations Affecting Plant Gas Exchange

<div><p>Plant gas exchange is a key process shaping global hydrological and carbon cycles and is often characterized by plant water use efficiency (WUE - the ratio of CO₂ gain to water vapor loss). Plant fossil record suggests that plant adaptation to changing atmospheric CO₂ involved correlated evolution of stomata density (<i>d</i>) and size (<i>s</i>), and related maximal aperture, <i>a_max</i>. We interpreted the fossil record of <i>s</i> and <i>d</i> correlated evolution during the Phanerozoic to quantify impacts on gas conductance affecting plant transpiration, <i>E</i>, and CO₂ uptake, <i>A,</i> independently, and consequently, on plant WUE. A shift in stomata configuration from large <i>s-</i>low <i>d</i> to small <i>s-</i>high <i>d</i> in response to decreasing atmospheric CO₂ resulted in large changes in plant gas exchange characteristics. The relationships between gas conductance, <i>g_ws</i>, <i>A</i> and <i>E</i> and maximal relative transpiring leaf area, (<i>a_max</i>⋅<i>d</i>), exhibited hysteretic-like behavior. The new WUE trend derived from independent estimates of <i>A</i> and <i>E</i> differs from established WUE-CO₂ trends for atmospheric CO₂ concentrations exceeding 1,200 ppm. In contrast with a nearly-linear decrease in WUE with decreasing CO₂ obtained by standard methods, the newly estimated WUE trend exhibits remarkably stable values for an extended geologic period during which atmospheric CO₂ dropped from 3,500 to 1,200 ppm. Pending additional tests, the findings may affect projected impacts of increased atmospheric CO₂ on components of the global hydrological cycle.</p>