L is the product of two components related to leaf anatomy: one that accounts for the tortuosity of water pathway from the xylem to the stomatal pore (l ) and the other refers to the cross-sectional area perpendicular to water flow relative to the leaf surface area (k ) (Barbour & Farquhar 2004; Cernusak & Kahmen 2013; Song et al. 2013; Sternberg & Manganiello 2014). The assumption that L is constant for species, across treatments, and across a gradient of transpiration rates has not always been supported (Song et al. 2013; Roden et al. 2015). The lack of understanding of L and the factors that affect it largely arise from the difficulty of measuring L .
The determining evidence of the presence of the Péclet effect is if the fractional contribution of source water (unenriched xylem water;f sw) increases with E (Eqn. 9; Song et al. 2015; Holloway-Phillips et al. 2016). A Péclet effect has failed to be observed in several