Was the evolution of faster stomata driven by increased gas exchange rates rather than increasing water use efficiency?

Following changes in light flux, photosynthesis (A) typically adjusts more quickly than stomatal conductance (g_s), which is dependent on changes in stomatal aperture. Faster stomatal responses are proposed to reduce water loss and enhance growth in dynamic light environments. Stomatal opening and closing parameters were determined in a range of species across the land plant phylogeny using infrared gas exchange analysis to monitor A and g_s, following step changes in light flux. The acquisition of abaxial stomata and dumbbell-shaped guard cells in angiosperms coincides with two distinct increases in photosynthetic capacity. Species with dumbbell-shaped guard cells achieved larger changes in A and faster maximum rates of g_s adjustment than species with kidney-shaped guard cells. However, species with dumbbell-shaped guard cells did not open or close their stomata in a significantly shorter time once opening began, nor did they achieve higher water use efficiency. Surprisingly, there were no strong correlations between stomatal size and speed parameters and no differences in biomass accumulation or water use between plants grown under constant or fluctuating light. We therefore propose increased gas-exchange rates, rather than faster stomatal response times, as the evolutionary driver for the acquisition of dumbbell-shaped guard cells.