Disakisperma species (Poaceae): C4 halophytes with ion excretion via salt glands.

Abstract

Main conclusion: One of the primary salt-tolerance mechanisms in the three Disakisperma species involves salt excretion via bicellular salt glands. Salt excretion out of the leaves is one of the ways to avoid the toxicity of sodium and chloride ions under salinity. Among grasses (Poaceae), about 30 species are capable of salt excretion. The genus Disakisperma is in a sister clade to the salt excretors and has C₄ metabolism, which is advantageous for salinity tolerance. For three Disakisperma species, D. eleusine, D. dubium, and D. obtusiflorum, we confirmed NAD-ME biochemical C₄ subtype by the Western blotting for the main decarboxylases and NAD-ME anatomical type by the transmission electron microscopy. Under 200 mM and 400 mM NaCl treatments, photosynthetic and transpiration rates, as well as leaf sizes, decreased progressively. Nevertheless, all plants successfully completed their life cycle and produced viable seeds, demonstrating that all three species are halophytes characterized by moderate salt tolerance. All species possess bicellular salt glands on both adaxial and abaxial leaf surfaces. Crystals observed on their cap cells contain mainly sodium, potassium and chloride. Salt-excreting function is related to special subcellular structures as partitioning membranes and numerous mitochondria in the basal cell. Therefore, we presume that one of the salt-tolerance mechanisms of three Disakisperma species is ion excretion via specialized salt glands.