The origin of mechanical advantage in angiosperms

O_LIMechanical interaction between guard cells and epidermal pavement cells enables large stomatal apertures and high productivity in angiosperms. We do not know when this response evolved, but over the last 169 years we have found that mechanical advantage has been tested in at least 230 species from 85 families. To date no data on this trait exists among angiosperms outside magnoliids, monocots and eudicots. C_LIO_LITo resolve the evolutionary origins of this critical stomatal response we tested for mechanical advantage across 14 additional species including the earliest diverging lineages of angiosperms. C_LIO_LIWe find that mechanical advantage, while variable in magnitude, is present in all angiosperm species that have been measured, including Amborella trichopoda sister to all angiosperms. C_LIO_LIThis response likely evolved once in flowering plants, in the common ancestor of this clade, remaining widespread across angiosperms today. We hypothesize that angiosperms could not have realized the full potential of physiological innovations in water transport without the evolution of this key trait that increased operational stomatal aperture. C_LI