High intrinsic water use efficiency is underpinned by high stomatal aperture and guard cell potassium flux in C-3 and C-4 grasses grown at glacial CO2 and low light

Across two C(3)and six C(4)grasses, intrinsic water use efficiency was strongly associated with stomatal conductance, operational stomatal aperture, guard cell K(+)influx, and stomatal opening speed on transition to high light. We compared how stomatal morphology and physiology control intrinsic leaf water use efficiency (iWUE) in two C-3 and six C-4 grasses grown at ambient (400 mu mol mol(-1)) or glacial CO2 (180 mu mol mol(-1)) and high (1000 mu mol m(-2) s(-1)) or low light intensity (200 mu mol m(-2) s(-1)). C-4 grasses tended to have higher iWUE and CO2 assimilation rates, and lower stomatal conductance (g(s)), operational stomatal aperture (a(op)), and guard cell K+ influx rate relative to C-3 grasses, while stomatal size (SS) and stomatal density (SD) did not vary according to the photosynthetic type. Overall, iWUE and g(s) depended most on a(op) and density of open stomata. In turn, a(op) correlated with K+ influx, stomatal opening speed on transition to high light, and SS. Species with higher SD had smaller and faster-opening stomata. Although C-4 grasses operated with lower g(s) and a(op) at ambient CO2, they showed a greater potential to open stomata relative to maximal stomatal conductance (g(max)), indicating heightened stomatal sensitivity and control. We uncovered promising links between a(op), g(s), iWUE, and K+ influx among C-4 grasses, and differential K+ influx responses of C-4 guard cells to low light, revealing molecular targets for improving iWUE in C-4 crops.