Salt glands play a pivotal role in the salt resistance of four recretohalophyte Limonium Mill. species

Limonium Mill. plants are typical recretohalophytes, as they withstand salt stress by secreting excess salt onto the leaf surface through salt glands. However, little is known on the salinity thresholds of these plants and the function of salt glands in salt tolerance. Here, we investigated the salinity thresholds of salt tolerance of the Limonium species L. aureum (Linn.) Hill, L. gmelinii (Willd.) Kuntze, L. otolepis (Schrenk) Kuntze and L. sinuatum (L.) Mill grown with various concentrations of NaCl. The salinity thresholds of L. otolepis, L. aureum, L. sinuatum and L. gmelinii were 300, 350, 400 and 420 mm NaCl, respectively. Correlation analysis indicated that total dry weight, chlorophyll content and intercellular CO2 concentration were highly positively correlated with the total fresh weights of all four Limonium species and could therefore be used as indicators of plant salt tolerance. Furthermore, as the salt gland density on the leaf surface increased, the rate of salt secretion per salt gland also increased, allowing more Na+ to be secreted from the plant. Redundancy discriminant analysis indicated that salt gland density, Na+ content and Na+ secretion rate per salt gland were positively correlated with salt concentration. These observations support the notion that salt glands play important roles in the adaptation of Limonium species to high salinity conditions.