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One-year-old self-rooted cuttings of three Citrus limon cultivars (Nouvel Athos, Lisbon, Maglini) were grown in 1 L black plastic bags, containing a mixture of sand: perlite (1:1), in order to investigate: i) if genotypic differences to salt stress existed, ii) if KNO 3 can alleviate salinity stress, iii) the role of carbohydrates (such as the sugars fructose, glucose and sucrose) and proline as possible osmoregulators in C. limon osmoprotection, and iv) if genotypic differences to salt stress tolerance exist among the 3 studied cultivars. The experiment included 3 treatments: i) control (C), i.e. 25% modified Hoagland (No2) solution (MHS)-NaCl, ii) T1, 25% MHS+80 mM NaCl, iii) T2, 25% MHS+80 mM NaCl+5 mM KNO 3 . Plant growth was negatively affected by high NaCl (T1); the highest Cl and Na quantities have been absorbed by Lisbon, while the lowest ones by Maglini. Salt stress reduced macronutrient and Zn concentrations, as well as the total carbohydrate concentration, and increased peroxidase (POD) activity and chlorophyll fluorescence in the leaves of the 3 C. limon cultivars studied; five mM KNO 3 application alleviated the harmful effect of salt stress on leaf total carbohydrate concentration and leaf N and K concentrations. Sucrose was dramatically reduced in all the three genotypes studied, while leaf fructose concentration was significantly increased in Nouvel Nouvel Nouvel Athos and Maglini under salt stress. Leaf proline concentration of Maglini was significantly decreased by the high NaCl concentration, while Nouvel Athos and Lisbon had high proline concentration in their leaves. In conclusion, from the significantly decreased levels of proline for Maglini, together with the greatest reduction of the ratio F v /F m and the least enhancement of POD activity-compared to the other two cultivars-it can be concluded that Maglini was more susceptible to salinity, and should not be preferred for cultivation under NaCl stress. Finally, rich KNO 3 application alleviated the harmful effect of NaCl stress, thus it is highly recommended for use by the growers in areas with salt problems. [ABSTRACT FROM AUTHOR]