Discussion
Observations of Δ18OLW values in maize and rice are consistent with the Péclet effect in the veins. Unlike the two-pool model where the f sw is a fixed volume and not dependent on physiological factors (Roden & Ehleringer 1999; Roden et al. 2015; Song et al. 2015; Hirl et al. 2019), a positive relationship between f sw and transpiration rate (E ) was observed in all genotypes of maize and rice. According to the Farquhar and Gan model, Δ18OLW is the accumulation of the Péclet effect in each tissue relative to the fraction of leaf water in the tissue, but several studies have found that the xylem water pool is the principal, if not the only, location of the Péclet effect (Helliker & Ehleringer 2000; Gan et al. 2002; Farquhar & Gan 2003; Gan et al. 2003; Holloway-Phillips et al. 2016). In support of this observation, we found that L mestimates in the mesophyll and xylem Péclet model were essentially 0, indicating that the mesophyll Péclet effect was likely nonexistent. However, observed variation in Δ18OLWacross genotypes, species, and treatments was not related to the differences in the partial volume of the vasculature, meaning that variation in f sw was influenced by anatomical and physiological factors regulating water movement and the relative importance of leaf water pools.
Anatomical differences between C3 and C4grasses such as suberization of the bundle sheath regulate the movement of water out of the vasculature and through the mesophyll, contributing to variation in leaf water isotopic composition. For example, it is not clear which tissues should be included in the xylem water pool (ϕ x) of the xylem Péclet model, andϕ x could include only the vein elements or include associated ground tissue such as the bundle sheath and the large quantity of parenchyma in the midrib (Ganet al. 2003). In C3 rice, the estimatedϕ x (and f sw) were relatively similar to the partial volume of the entire vasculature, which indicates that likely conducting and ground tissues associated with the vasculature together are isotopically one pool of water. Previous studies have made similar conclusions (Gan et al. 2003; Song et al. 2015; Holloway-Phillips et al. 2016). In contrast, the influence of xylem water in C4 maize may be more constrained than in rice in that ϕ x may be confined to the vascular bundles and may not necessarily include all ground tissue associated with the veins. This difference between C3 and C4 grass species may lie in that C3 plants have less suberized bundle sheaths, leading to greater outside xylem conductivity to support higher E and a greater isotopic influence of xylem water on the mesophyll water pool (Sonawane et al. in review; Kocacinar & Sage 2003). Therefore, the lower Δ18OLW in C3 grass species than C4 species may depend on more than anatomical features such as interveinal distance (vein density) and include the facility of water movement out of the xylem and outside xylem conductivity.