The response to salt treatment and K⁺ provision of two Arabidopsis thaliana accessions grown for 17 days in the presence of 50 mM NaCl was investigated. Leaf and root dry weight deposition was restricted by salt, more in Col accession than in NOK2 accession. In both accessions, the growth inhibition induced by salinity was associated with a decrease in total leaf surface area, which resulted from diminished leaf number, but not from restriction of individual leaf surface area. Comparing the effects of salt on dry matter production and total leaf surface area revealed large difference between Col and NOK2 for net assimilation rate (the amount of whole plant biomass produced per unit leaf surface area), which was augmented by salt and K⁺ in NOK2 but not in Col. This result, which suggested a better capacity of NOK2 to preserve its photosynthetic machinery against salt stress, was in agreement with the effect of NaCl on photosynthetic pigments. Indeed, salt significantly reduced chlorophyll and carotenoid content in Col leaves but had no impact on NOK2 leaf pigment content. Since K⁺ provision had only marginal effects on these responses to salt stress, leaf mineral unbalance was unlikely. Guaiacol peroxidase activity was augmented by salt treatment in leaves and roots of both accessions. Salinity decreased the catalase activity in Col leaves and in roots, and increased this activity in NOK2 organs. In conclusion, when aggressed by salt, NOK2 was able (1) to produce more leaves than Col, and (2) to efficiently protect its photosynthetic apparatus, perhaps by developing more efficient antioxidative defense through increased catalase and peroxidase activities. Consequently, the overall photosynthetic activity was higher and more robust to salt aggression in NOK2 than in Col.
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