Ionic and osmotic relations in quinoa (Chenopodium quinoa Willd.) were studied by exposing plants to six salinity levels (0–500 mM NaCl range) for 70 d. Salt stress was administered either by pre-mixing of the calculated amount of NaCl with the potting mix before seeds were planted or by the gradual increase of NaCl levels in the irrigation water. For both methods, the optimal plant growth and biomass was achieved between 100 mM and 200 mM NaCl, suggesting that quinoa possess a very efficient system to adjust osmotically for abrupt increases in NaCl stress. Up to 95% of osmotic adjustment in old leaves and between 80% and 85% of osmotic adjustment in young leaves was achieved by means of accumulation of inorganic ions (Na+, K+, and Cl–) at these NaCl levels, whilst the contribution of organic osmolytes was very limited. Consistently higher K+ and lower Na+ levels were found in young, as compared with old leaves, for all salinity treatments. The shoot sap K+ progressively increased with increased salinity in old leaves; this is interpreted as evidence for the important role of free K+ in leaf osmotic adjustment under saline conditions. A 5-fold increase in salinity level (from 100 mM to 500 mM) resulted in only a 50% increase in the sap Na+ content, suggesting either a very strict control of xylem Na+ loading or an efficient Na+ removal from leaves. A very strong correlation between NaCl-induced K+ and H+ fluxes was observed in quinoa root, suggesting that a rapid NaCl-induced activation of H+-ATPase is needed to restore otherwise depolarized membrane potential and prevent further K+ leak from the cytosol. Taken together, this work emphasizes the role of inorganic ions for osmotic adjustment in halophytes and calls for more in-depth studies of the mechanisms of vacuolar Na+ sequestration, control of Na+ and K+ xylem loading, and their transport to the shoot.

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Salt sensitive (pea) and salt tolerant (barley) species were used to understand the physiological basis of differential salinity tolerance in crops. Pea plants were much more efficient in restoring otherwise depolarized membrane potential thereby effectively decreasing K + efflux through depolarizationactivated outward rectifying potassium channels. At the same time, pea root apex was 10-fold more sensitive to physiologically relevant H2O2 concentration and accumulated larger amounts of H2O2 under saline conditions. This resulted in a rapid loss of cell viability in the pea root apex. Barley plants rapidly loaded Na + into the xylem; this increase was only transient, and xylem and leaf Na + concentration remained at a steady level for weeks. On the contrary, pea plants restricted xylem Na + loading during the first few days of treatment but failed to prevent shoot Na + elevation in the long term. It is concluded that superior salinity tolerance of barley plants compared with pea is conferred by at least three different mechanisms: (1) efficient control of xylem Na + loading; (2) efficient control of H2O2 accumulation and reduced sensitivity of non-selective cation channels to H2O2 in the root apex; and (3) higher energy saving efficiency, with less ATP spent to maintain membrane potential under saline conditions.