Sodium accumulation contributes to salt stress tolerance in lettuce cultivars
Increasing soil salinity of irrigated agricultural areas represents a major environmental stress factor that impairs the production of many salt-sensitive crop plants. Different lettuce cultivars were studied for identification of more tolerant ones, based on physiological properties. Five selected lettuce cultivars were grown hydroponically, salt stress was induced by 50 mM and 100 mM of NaCl. The cultivars exhibited differential reduction in shoot fresh weight. The highest sodium and free proline accumulation in the shoot of the most tolerant cultivar, associated with a moderate decrease of the root hydraulic conductance and of the leaf stomatal conductance, can be related to a better defense mechanism against osmotic stress. Salt exposure increased the potassium and calcium ion content of the xylem sap, which may be important for an efficient osmotic adjustment needed to support leaf expansion. The fact that the highest amount of Na+ was found in the shoot of the most tolerant cultivar, and the lowest in the most sensitive one, reflects that in lettuce Na+ exclusion is not a main strategy for salt tolerance. Lettuce is a good example for the case in which salinity tolerance is related not to exclusion, but to inclusion of sodium ions in the shoot system. For salt tolerant varieties the marketable yield has a higher dry biomass percentage, leaves of plants grown under high salinity are crispier, darker green and have a slight salty taste.
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