Cracking the code of foliar desalination: the structural basis of reverse osmosis in chambered salt glands*

Excess salt inhibits plant growth and development and, if severe, results in mortality. In addition to metabolic complications, high salinity also poses a physical problem for plants, as large osmotic gradients can impair water uptake from the soil. Unlike animals, plants do not have dedicated excretory systems; however, some salt-tolerant plants have evolved glands that secrete salt onto the leaf surface in the form of a persistent brine. Here we investigate the structure-function relationships involved in desalination via chambered salt glands using both theoretical and experimental approaches. Fractures in the waxy cuticle allow the brine to escape but provide physical continuity between the concentrated brine and the living cells, putting the leaf at risk of desiccation. We have developed a steady-state, multicompartment, mathematical model to explore how the size and distribution of cuticle fractures affect the balance of pressure- and osmotically-driven fluxes through the gland. Whereas most efforts to understand and engineer salt tolerance in plants have focused on biochemical and genetic pathways, we find that the chambered salt gland leverages its structure and material properties to achieve reverse osmosis across the leaf surface.