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

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 CO(2) gain to water vapor loss). Plant fossil record suggests that plant adaptation to changing atmospheric CO(2) involved correlated evolution of stomata density (d) and size (s), and related maximal aperture, a(max). We interpreted the fossil record of s and d correlated evolution during the Phanerozoic to quantify impacts on gas conductance affecting plant transpiration, E, and CO(2) uptake, A, independently, and consequently, on plant WUE. A shift in stomata configuration from large s-low d to small s-high d in response to decreasing atmospheric CO(2) resulted in large changes in plant gas exchange characteristics. The relationships between gas conductance, g(ws), A and E and maximal relative transpiring leaf area, (a(max)⋅d), exhibited hysteretic-like behavior. The new WUE trend derived from independent estimates of A and E differs from established WUE-CO(2) trends for atmospheric CO(2) concentrations exceeding 1,200 ppm. In contrast with a nearly-linear decrease in WUE with decreasing CO(2) obtained by standard methods, the newly estimated WUE trend exhibits remarkably stable values for an extended geologic period during which atmospheric CO(2) dropped from 3,500 to 1,200 ppm. Pending additional tests, the findings may affect projected impacts of increased atmospheric CO(2) on components of the global hydrological cycle.