Bundle sheath extensions are linked to water relations but not to mechanical and structural properties of leaves

Bundle sheath extensions (BSEs) account for the physiological differences between heterobaric and homobaric species. The boundary that divides these two groups, however, has been elusive; and the contribution of BSEs to leaf mechanical properties is not yet fully understood. Here, we quantified the density of BSEs (D-BSE) and evaluated whether it contributes to the leaf mechanical properties and water relations across species, and if so, how it does this. We studied 15 woody species that had different leaf functional types and D-BSE from a cool temperate forest. We quantified four leaf mechanical properties, eight traits related to water relations, and seven structural traits of the leaf and stem. Density of BSEs was independent of leaf mechanical and structural properties and C/N concentration across species. Conversely, D-BSE was correlated positively with stomatal conductance, and negatively with leaf water potential at turgor loss and midday water potential across species. Our results demonstrate that BSEs are associated with key leaf physiological properties, including gas exchange and drought tolerance, and are independent of leaf mechanical and structural properties. Thus, variation in D-BSE could provide a basis for explaining the ecological strategies and distributions of different tree species.