Distribution pattern and salt excretion rate of salt glands in two recretohalophyte species of Limonium (Plumbaginaceae)

Salt glands are the unique salt-excreting structures of recretohalophytes. The most important strategy by which recretohalophytes tolerate salinity is to excrete excess salt via salt glands. However, little is known about how the salt gland distribution pattern and salt excretion rate of dicotyledon recretohalophytes are correlated with salinity. This work focuses on the salt gland distribution patterns and excretion rates in two Limonium species of Plumbaginaceae under control conditions and NaCl stress. In the two dicotyledonous recretohalophytes, Limonium bicolor and Limonium gmelinii, the majority of the salt glands and veins were separated by no more than one epidermal cell, and the percentage of salt glands near veins of L. bicolor was greater than that of L. gmelinii. No salt glands were located alongside one another in either of the Limonium species and the salt glands and stomata were usually separated by more than one epidermal cell. The salt gland density on the adaxial epidermis of L. bicolor was greater than that of the abaxial epidermis whereas L. gmelinii showed the opposite trend. The optimal NaCl concentrations for maximal biomass were 100 mM for L. bicolor and 50 mM for L. gmelinii. Salt gland and stomata density increased when the NaCl concentration was higher than the optimal concentration. The Na+ excretion rate of a single salt gland in both Limonium species increased with increasing NaCl concentrations. These results suggest that salt gland distribution patterns and excretion rates are positively correlated with salt tolerance.