Physiology of salt tolerance in Atriplex halimus L.

Abstract

Atriplex halimus is a common shrub in Tunisia, which represents a palatable food for sheep and camels. Furthermore, its halophytic behavior makes it a model for the study of mechanisms of salt tolerance in plants. We present here results obtained on Atriplex halimus var. halimus. In this species, the germination of seeds is very sensitive to salinity, since low concentrations of sodium chloride (50 mM) in the medium delayed the germination and reduced the capacity of the seedlings to emerge. Germination was completely inhibited, but reversibly, by NaCl concentrations up to 200 mM. Nevertheless, after the development of the radicle and the emergence of the cotyledons, which occurred 5 days after the imbibition of the seeds, the seedlings were able to tolerate this high concentration of NaCl and their growth was stimulated by salt. Also, 1-month-old plantlets, grown in a hydroponic medium, showed an optimal growth on 50-200 mM NaCl, and tolerated NaCl concentrations up to 300 mM. Our results indicate that, in Atriplex halimus, salt tolerance is acquired at an early vegetative stage of the plant development, and is related to: (i) the absorption and transport to shoots of high quantities of Na⁺ and Cl^- and their use in the osmotic adjustment, (ii) the efficiency of the vacuolar compartmentation of these ions, which prevents the ionic damage of the cytoplasm, and (iii) the aptitude of whole plant to ensure a sufficient supply of K⁺, by maintaining a high selectivity for this essential nutriment, in spite of large amount of Na⁺ in the medium.