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.