Stomatal dynamics in Alloteropsis semialata arise from the evolving interplay between photosynthetic physiology, stomatal size and biochemistry.

C 4 plants are expected to have faster stomatal movements than C 3 species because they tend to have smaller guard cells. However, little is known about how the evolution of C 4 photosynthesis influences stomatal dynamics in relation to guard cell size and environmental factors. We studied photosynthetically diverse populations of the grass Alloteropsis semialata, showing that the origin of C 4 photosynthesis in this species was associated with a shortening of stomatal guard and subsidiary cells. However, for a given cell size, C 4 and C 3 -C 4 intermediate individuals had similar or slower light-induced stomatal opening speeds than C 3 individuals. Conversely, when exposed to decreasing light, stomata in C 4 plants closed as fast as those in non-C 4 plants. Polyploid formation in some C 4 plants led to larger stomatal cells and was associated with slower stomatal opening. Conversely, diversification of C 4 diploid plants into wetter environments was associated with an acceleration of stomatal opening. Overall, there was significant relationship between light-saturated photosynthesis and stomatal opening speed in the C 4 plants, implying that photosynthetic energy production was limiting for stomatal opening. Stomatal dynamics in this wild grass therefore arise from the evolving interplay between photosynthetic physiology and the size and biochemical function of stomatal complexes.