Deficiency?

Interesting. The midrib is a primary vein that provides structural support and transports water and nutrients within a leaf, Potassium deficiency can damage the midrib on a leaf, causing symptoms like necrosis (tissue death) and discoloration. Nitrogen deficiency can also cause damage but generally more distinguishable, where it appears as a v-shaped chlorosis (yellowing) that can advance down the midrib and become necrotic. Kleaf quantifies the efficiency of water movement from the petiole (stalk) to the leaf's air spaces, where it evaporates and diffuses from the stomata. It's determined by both the hydraulic conductance within the xylem (Kx) and the hydraulic conductance outside the xylem (Kox) but something is messing with K, potassium linked to water pressure within cells of leaf.