Physiological and leaf metabolome changes in the xerohalophyte species Atriplex halimus induced by salinity.

Atriplex halimus is a xerohalophyte plant, which could be used as cash crops. This plant was integrated in Tunisian government programs the aim of which is to rehabilitate saline areas and desert. To investigate its strategies involved in salt tolerance, A. halimus was grown hydroponically under controlled conditions with increasing salinity. Plants were harvested and analyzed after 60 days of treatment. The biomass of A. halimus increased by moderate salinity and decreased significantly at high salinity compared to control plants at 400 mM. Despite of the large amounts of Na(+) observed in the leaves of Atriplex plants, leaf water contents and leaf succulence kept on increasing in treated plants and decreased over 150 mM NaCl. This confirmed the compartmentation and the efficient contribution of Na(+) in the osmotic adjustment. Analysis of the metabolic profiles showed an accumulation of carbohydrates and amino acids. The leaf tissues preferentially stored proline, α alanine and sucrose. Increasing NaCl levels were also accompanied by a significant accumulation of malate in leaves. Involvement of these solutes in osmotic adjustment was considered low. Nevertheless, they seemed to have an important role in controlling photosynthesis which capacity was enhanced by low salinity and decreased with increasing salinity (evaluated by actual photochemical efficiency of photosystem II and chlorophyll contents). The unchanged maximum photochemical efficiency of photosystem II accompanied by the increase of the non-photochemical quenching, the enhancement of the total antioxidant activity and the decrease of the malondialdehyde contents in leaves showed efficient protection of membranes and photosystem II from photo-oxidative damage. This protection seemed to be attributed to proline and sucrose largely accumulated in leaves treated with salt.