The effects of salt stress on growth, water relations and ion accumulation in two halophyte Atriplex species

Atriplex halimus is found in the Mediterranean Basin along the coastal areas of Sardinia, but few data are available on its adaptability to salinity. The effects of drought and salinity under controlled conditions on two clones of A. halimus, designated MOR2 and SOR4, originating from southern and northern Sardinia, respectively, were compared with those of seedlings of A. nummularia, an Australian species widely used in the restoration of arid areas. The effects of increasing NaCl salinity above seawater concentrations and of increasing the KCl concentration gradient were tested. Plants were harvested and analysed after 10 and 20 days of NaCl and KCl treatments. All plants remained alive until the end of treatment, although growth was strongly reduced, mainly for the A. halimus MOR2 clone, under increasing concentrations of KCl. The leaves and roots of both species responded positively to increasing NaCl concentrations up to 600 mM NaCl for A. halimus, whereas the optimal growth of A. nummularia was recorded at 300 mM NaCl. SOR4 was more sensitive to KCl toxicity. The Na+ concentration in the plants increased with increased salinity and was higher in A. halimus than in A. nummularia, suggesting that A. halimus is an ion accumulator and may be used for phytoremediation. The sodium accumulation in the roots of the A. halimus MOR2 clone was far greater than in its leaves. This suggests that MOR2 is an Na+ excluder, either by minimising the entry of salt into the plant or by an excretion mechanism via the vesiculated hairs that play a significant role in the removal of salt from the remainder of the leaf, thereby preventing its accumulation to toxic levels in the leaves, whereas SOR4 acted as an Na+ includer. Higher levels of proline were detected in the MOR2 clone under NaCl treatments, suggesting a more developed adaptative mechanism for the selection of this characteristic in the southern part of the island, which is more exposed to abiotic stresses, particularly water stress that is either generated by salinity or by other causes.