Axial anatomy of the leaf midrib provides new insights into the hydraulic architecture and cavitation patterns of Acer pseudoplatanus leaves

The structure of leaf veins is typically described with a hierarchical scheme (e.g. midrib, 1st order, 2nd order), that is used to predict variation in conduit diameter from one order to another overlooking possible variation within the same order. We tested whether xylem conduit diameter changes within the same vein order, with consequences on resistance to embolism. We measured the hydraulic diameter (Dh), and number of vessels (VNo) along the midrib and petioles of Acer pseudoplatanus leaves. We estimated the leaf area supplied (LAsup) at different points of the midrib and how variation in anatomical traits affected embolism resistance. Our results showed that Dh scales with distance from the midrib tip (L) with a power of 0.42, and that VNo scales with LAsup with a power of 0.66. Total conductive area scales isometrically with the LAsup. Embolism events along the midrib occurred first in the basipetal part and afterwards at the leaf tip where vessels are narrower. The distance from the midrib tip well predicts the variations in vessels diameter along the 1st order vein in sycamore maple leaves and this anatomical pattern seems to have an effect on hydraulic safety since wider vessels at the leaf base embolize first.