Roots and leaves display contrasting osmotic adjustment mechanisms in response to NaCl-salinity in Atriplex nummularia

This study reports contrasting mechanisms between the osmotic adjustment of roots and leaves from a typical halophytic species (Atriplex nummularia L.), in response to a large salinity range, resultant from the contribution of inorganic and organic solutes. Plants were grown in a greenhouse and irrigated with nutrient solution containing 0, 75, 150, 300, 450 and 600 mM NaCl during 7 weeks. The maximum leaf and root dry matter accumulation was observed at 300 and 150 mM treatments, respectively. The Na+ + Cl− concentrations in leaves were several times higher than in roots (ca. 760 and 90 mM in basis of tissue water, respectively at 300 mM NaCl treatment). Similar tendency was observed in the concentration of the most important organic solute involved with the osmotic adjustment, the glycinebetaine. The other analyzed solutes (K+, amino acids, soluble sugars and proline) also presented remarkably higher concentrations in leaves compared to roots, in all treatments. As a consequence, the leaf Ψs was several times more negative than was in roots. Moreover, the osmotic adjustment of salt-treated plant leaves was approximately 3-fold higher than that found in roots. Surprisingly, under very high levels of external NaCl (450 and 600 mM), the root tissues exhibited Ψs values less negative than those found in the external solution. The Na+ + Cl− were the major components to the leaves OA followed by K+ and GB, even in the untreated plants. In contrast to GB, the K+ participation in both leaves and roots decreased as the NaCl dose increased. In roots, K+ was the most important solute to OA of salt-untreated plants. Our data evidence that despite the A. nummularia leaves displayed an efficient osmotic adjustment, even under very high salinity levels, the same was not observed in its roots, which exhibit high Ψs values. This fact probably complicates the root osmotic and water homeostasis in relation to the external medium. We can also conclude that glycinebetaine, followed by soluble sugars, plays a major role in the cytosol osmotic adjustment of both roots and leaves.