Responses of lettuce (Lactuca sativa L. var. longifolia) grown in a closed hydroponic system to NaCl- or CaCl2-salinity

Lettuce plants (Lactuca sativa L. var. longifolia) grown in a closed hydroponic system using perlite as substrate were exposed to similar levels of NaCl- and CaCl2 -salinity in an attempt to distinguish between osmotic and salt-specific effects on plant growth and quality. A standard nutrient solution for lettuce (control) was compared with two salinity levels and each salinity level was imposed by adding either NaCl or CaCl2 to the standard nutrient solution. The mean electrical conductivity in the root zone, as indicated by the values measured in the drainage solution, was 2.5, 3.7, 3.5, 5.6 and 5.4 dS m(-1) in the control, the low NaCl-, the low CaCl2-, the high NaCl- and the high CaCl2- salinity treatment, respectively. The increase of NaCl- salinity in the root zone restricted the shoot growth of lettuce. In contrast, the CaCl2- salinity had no significant effect on the above-ground growth of the plants. The root growth was not affected by any salinity treatment. The leaf Cl concentration was increased in all salinity treatments but the NaCl-salinity resulted in significantly higher Cl levels in leaf compared to CaCl2- salinity at similar Cl concentrations in the root zone. The leaf Mg concentration was decreased in all salinity treatments but the most marked suppression was observed at the highest CaCl2- level. The leaf K was restricted by salinity without significant differences between salinity levels and source. The leaf chlorophyll and ascorbic acid concentrations were not affected by either of the salinity treatments. It was concluded that the effects of salinity on lettuce are primarily Na-specific.