Is chloride toxic to seed germination in mixed-salt environments? A case study with the coastal halophyte Suaeda maritima in the presence of seawater

Most salt tolerant plants, halophytes, use seed germination for natural regeneration. However, germination in mixed-salt environments such as seawater is poorly understood and ion toxicity by Cl-, the most highly concentrated ion in seawater, is rarely considered over Na+. Here, we investigate Cl- toxicity in the germination of the halophyte Suaeda maritima in the presence of artificial seawater (ASW). Seeds were germinated at 15/5 degrees C in dilutions of ASW and at concentrations of NaCl, MgCl2, CaCl2 and KCl as found in ASW. Solutions of polyethylene glycol (PEG) were used for osmotic comparison. Germination percentage and normal seedlings were quantified. Non-germinated seeds were tested for recovery on water. Germination rate (1/t(50)) was used in a halotime model to quantify the maximum concentration of Cl- (Cl-max(-)) and Na+ (Na-max(+)) for germination. Germination was most negatively affected when all salts were combined in the concentrations found in ASW. Recovery of non-germinated seeds from all salt treatments on water was low, but all germinated seeds formed normal seedlings. Germination on ASW was higher than on iso-osmotic solutions of PEG. The 1/t(50) decreased with increasing Cl and Na+ concentration, indicating maximum thresholds to germination at 1381 mM (Cl max) and 1262 mM (Na-max(+)). The results indicate that ASW does not produce an osmotic limitation to the germination of S. maritima, and exposure to salt ions can even promote germination. However, ion toxicity is the major limitation, with Cl- similarly as toxic as Na+. In mixed-salt environments such as seawater, Cl- toxicity should not be overlooked.

Automated summary

Seed germination is negatively affected by increasing concentrations of chloride and sodium ions, with thresholds for maximum germination at 1381 mM for Cl and 1262 mM for Na. Exposure to salt ions can promote germination, but ion toxicity, particularly from chloride, is the major limitation in mixed-salt environments like seawater.