Rosemary (Rosmarinus officinalis L.) is an aromatic herb in the Labiatae that is widely cultivated in arid and semiarid regions for its medical and cosmetic benefits. To investigate its salinity tolerance mechanism, we grew eight varieties under moderate or severe salinity (50 and 100 mM NaCl) and measured nutrient and water contents. At 50 mM NaCl, (1) the relative tolerance index (RTI), which is a measure of growth relative to that in the nonsaline condition, decreased in the order of Primley Blue >> Lockwood de Forest ≈ Prostratus ≈ Salem ≈ Benenden Blue > Arp ≈ Tuscan Blue > Officinalis; (2) the Na/K ratio and leaf necrosis percentage were significantly and negatively correlated with the RTI in all eight varieties; and (3) leaf tissue tolerance, indicated by a low percentage of necrosis with high Na concentration, differed only slightly among varietiess, but Salem, Arp, and Tuscan Blue showed the highest tolerance. At both 50 and 100 mM NaCl, leaf water content showed little effect of salinity. At 100 mM NaCl, the correlation between the RTI and the Na/K ratio or percentage leaf necrosis became inconsistent. Therefore, rosemary may tolerate salinity by maintaining appropriate levels of nutrients with a low Na/K ratio in leaves before water content is affected. Although the transport of some Na to a leaf might create osmotic pressure, the excessive transport of Na to leaves creates ionic stress that leads to reduced growth. Key Words: necrosis, nutrient uptake, Rosmarinus officinalis, salinity stress, tissue tolerance.

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