Comparative study of the effects of mannitol and PEG osmotic stress on growth and solute accumulation in Sesuvium portulacastrum

Responses to osmotic stress were studied in Sesuvium portulacastrum, a halophyte potentially usable for saline soil stabilization and covering. Cuttings were multiplicated and cultivated in nutrient solution supplemented with 100 mM NaCl. They were exposed for 12 days to osmotic stress induced by either mannitol or polyethylene glycol (PEG 6000). Growth, tissue water content, relative water content, and contents in inorganic (Na+, K+) and organic (proline, soluble sugars) solutes were determined at regular intervals. Both PEG and mannitol reduced growth, decreased leaf number and leaf mean surface area, and led to a significant reduction of leaf water and K+ contents. However, these effects were significantly less severe in plants submitted to mannitol, as compared to PEG-treated ones. The contents in soluble sugars contents and in Na+ remained unchanged, while that of proline strongly increased, particularly in mannitol-stressed plants. A positive relationship was observed between growth, relative water content, potassium content, and proline accumulation in leaves. However, proline remained a minor component in the pool of solutes.