Evolution of C4 photosynthesis in grasses linked to reduction in the anatomical capacity for stomatal conductance

Background/Question/Methods

The evolution of C₄ photosynthesis has provided C₄ species with the capacity to maintain CO₂ assimilation rates at a lower stomatal conductance than is commonly observed in their C₃ relatives.  This key trait contributes to the water-use efficiency advantages of C₄ over C₃ photosynthesis, a physiological contrast that has been linked with the tendency for C₄ species to occupy drier, more seasonal niche spaces.  Using experiments screening closely related C₃ and C₄ grass species from a range of habitats, we have demonstrated that low stomatal conductance is a common trait that distinguishes C₄ grasses from their C₃ relatives.  Here, we hypothesise that lower stomatal conductance in C₄ species may have an anatomical underpinning rather than being driven solely by regulation of stomatal aperture.  Specifically, we propose that stomatal conductance capacity, defined by stomatal density and size, is lower in C₄ species.

We measured stomatal size and density in 28 species sampled from seven grass lineages, which were placed within a phylogeny based on three plastid regions.  The phylogeny was used in a comparative analysis to test for expected evolutionary relationships between stomatal capacity, C₃ and C₄ photosynthesis, and the occupation of wet versus dry-mesic niches across a range of habitat precipitation values.

Results/Conclusions

Our analyses supported three hypotheses.  1) That the evolution of C₄ photosynthesis has been accompanied by decreases in stomatal capacity.  2) That phylogenetic conservatism in stomatal size and density cause between-lineage differences in stomatal capacity.  3) That irrespective of photosynthetic type, species from mesic-dry habitats had lower stomatal capacity than those from wet habitats.  We found no evidence for an association between stomatal capacity and precipitation.  Our results suggest a novel example of linkage between biochemical innovation and anatomical traits in plants.  Anatomical differences in stomatal size and density are a crucial component of the reduced stomatal conductance that characterises C₄ grasses.