Suberin mutation and the Péclet effect
Both E and L v were affected by the compromised ultrastructure of the bundle sheath cells and increased apoplastic permeability in Zmasft maize double mutants, especially at the interface of suberin lamellae and adjoining polysaccharide cell walls (Mertz et al. In review). Suberin is considered essential as a functional barrier to unrestricted diffusion of water across cell walls (Botha et al. 1982; Schönherr 1982; Mertz & Brutnell 2014). Consequently, more permissive water flow in the leaf of the Zmasft double mutants led to greater water flow out of the xylem as evidenced by greater E under high light and more water channels in and out of the xylem under low light, increasing the overall effective mixing length in the xylem (L v). The likely reason for mutation-related differences in E under high light andL v under low light is an interaction between water demand and the lack of water channeling. Under high light Ewas 2-3.3x and 2.6-3.8x greater in wildtype and the double mutant relative to low light, respectively, so the principal water channels moved the vast majority of water out of the xylem, leading to similar measures of L v. In contrast, transpiration demand was low under low light, so greater apoplastic permeability allowed water to move through more tortuous channels, increasing the overallL v in the Zmasft mutants. Therefore, channeling of water outside the xylem through suberin lamellae influenced the Péclet effect in leaf water by restricting water flow (lower E ) and reducing water movement across the suberin lamellae (smaller L v). As has been seen previously, suberin and suberin lamellae play a role in the plant’s ability to regulate transpiration (Schreiber 2010; Vishwanath et al. 2015), but interestingly these differences are detectable with stable isotopes. Potentially, through the Péclet effect model Δ18OLW can be used to detect and study anatomical differences and their physiological consequences on water movement in leaves.