Stomatal traits do not determine species range via abiotic stress tolerance in Pooideae

Pooideae, the largest subfamily of the grasses (Poaceae), consists of around 4000 species, including such economically important food crops as wheat, barley, oats, and ryegrass. Members of this subfamily can survive in some of the world’s most extreme environments, including freezing boreal regions and arid, drought-prone steppes. Since climate tends to be a key factor in shaping species range limits, understanding the relative contributions of drought and cold tolerance in Pooideae can help predict their response to future changes in climate.

In a recent article published in Annals of Botany, lead author Aayudh Das and colleagues gauged the aridity and minimum temperatures of the habitats of 22 diverse Pooideae specimens and one outgroup in order to compare climate factors with stomatal traits, including conductance and closure speed. These features, which are linked to water use efficiency, have been tied to both drought and cold tolerance.

The authors found that aridity and minimum temperature do predict variation in survival following drought or cold stress. However, neither stomatal conductance nor closure speed explained the relationships between climate and fitness. Stomatal closing behaviour tended to be uniform across the Pooideae, regardless of how drought tolerant the species was, leading the researchers to speculate that stomatal trait variation may have evolved more in response to pathogens than climate, or that climate may be affecting the traits in unexpected ways.

“[W]e conclude that variation in stomatal traits has not been a consistent driver of climatic distributions in Pooideae grasses. This highlights potential limits to modifying stomatal traits for increased plant productivity, and paves the way for future work assessing the role of other (e.g. water transport) traits involved in grass drought/cold adaptation,” write the authors. They suggest that future studies could focus on such traits as leaf morphology, shoot to root biomass ratio, sugar accumulation, or hydraulics.