Emergent aquatic plants mostly occur in shallow waters with their lower stem underwater rooted in the bottom substrate. However, their leaves emerge from the water surface and are exposed to air, similar to the leaves of terrestrial plants. Previous studies have good coordination between leaf water supply traits (i.e., leaf vein density) and traits related to water demand (i.e., stomatal size and density) for terrestrial plants. Yet, whilst approximately 11 % of monocotyledonous plants are aquatic, little is known about this relationship in aquatic plants.
In their recent study published in AoBP, Zhao et al. analysed leaf vein and stomatal characteristics of 14 emergent aquatic and 13 terrestrial monocotyledonous herb species. They found that emergent aquatic species had significantly higher stomatal density and stomatal number per vein length, but a lower mean major vein length per area (VLA) and total VLA than terrestrial species. Stomatal density and total VLA were positively correlated among the emergent aquatic species, terrestrial species, but this correlation became non-significant when data from both the groups were pooled. These results show that the differences in water supply between emergent aquatic and terrestrial plants modify the coordination of their leaf veins and stomatal traits.