Calcium oxalate crystals provide a supporting matrix for the leaf structure

This study investigates the structural role of calcium oxalate (CaOx) crystals within leaf tissues, proposing that they act as a mechanical supporting matrix for leaf structure. While CaOx is often studied for its metabolic or defensive roles, its contribution to the physical integrity of the leaf is less understood. Across plant species, CaOx crystals are preferentially positioned around the leaf veins, but their precise architecture is species- and developmental stage-dependent. We focus on leaves of the Mediterranean evergreen tree species carob ( Ceratonia siliqua L.), applying a combination of micro computed tomography (micro-CT) imaging and mechanical testing. We provide data showing that the presence of CaOx crystals increases leaf stiffness. Specifically, we demonstrate the link between CaOx crystals distribution and leaf stiffness across three approaches: (1) among different plant species. (2) Among plants grown with or without Ca. (3) among leaves of the same plant but at different ages. Our data shows that CaOx crystals are not merely metabolic byproducts but are designed to form a mineral-organic structure which specifically enhances the resistance of the midrib and secondary veins to buckling forces. We propose that CaOx acts as a tunable stiffening agent of leaves that plants can modulate based on nutrient availability.