The mericarp of the halophyte Crithmum maritimum (Apiaceae): structural features, germination, and salt distribution

At maturation and during seed fall and dispersal, halophyte seeds may be subjected to invasion by salt ions. How these seeds remain viable in such hostile environments is however still unclear, depending for instance on the species and the family. In the Apiaceae, the mericarp (fruit) shows a wide range of morphological and anatomical modifications, many of which may enhance the adaptation to severe environmental conditions. Therefore, structural features, ion accumulation, and long-term floating capacity were investigated in the fruit (mericarp) of the halophyte Crithmum maritimum L. The mericarp was composed of a spongy outer coat, a secretory envelope, a thin endocarp reduced to a unicellular layer delimiting the endosperm, and an embryo. Both of the secretory canals and the endocarp adhered after complete ripening of the mericarp, while the epicarp and much of the mesocarp formed the spongy coat. Assessing long-term floating ability of the fruit under laboratory conditions revealed that even after 60 d, more than 98% of C. maritimum L. mericarps still floated over seawater. Seed germination was delayed and reduced by the spongy coat. The X-ray microanalysis revealed that the spongy coat and the secretory canals contained essentially Cl and Na, while seeds, i.e. endosperm and embryo, accumulated mostly Mg, K and P. In a subsequent experiment designed to simulate salt leaching by rain, most of the salt accumulated in the spongy coat and seeds was released after 2 h imbibition in distilled water. Taken together, these results highlight the protective role of the mericarp and the likely involvement of this structure in the seed dispersal of C. maritimum L. This may ultimately have eco-physiological implications explaining the successful establishment of this halophyte in its native saline biotopes.