Secreting salt glands constrain cuticle fracture to enhance desalination efficiency

Abstract

Plants responding to excessive soil salinity by discharging brine onto their leaf surface risk dehydration through the osmotic continuity between the living tissue and the surface brine, which further enriches with evaporation. Cuticle cracks have long been identified as essential for salt to reach the leaf surface but enable a potentially desiccating continuity between the brine and the gland interior. Using the secreting salt gland of Nolana mollis as a model system, we integrate mathematical modeling, imaging, and physiological measurements to examine the mechanical and biochemical processes required for efficient salt removal. We find that the subcuticular space between the concentrated surface brine and the more dilute secreting cell eases the energetic limits of active salt secretion by reducing the concentration gradient of salt across the cell membrane. We show that crack size plays a critical role in balancing the osmotic and pressure gradients required for salt removal without runaway foliar desiccation.