| Summary: | Soil salinity is one of the most important environmental factors threatening agriculture on a global level. Plants show differences in resistance to salt stress, both in terms of species and variety. The aim of the study was to determine the effect of salt stress on photosynthetic efficiency and the activity of plants of two barley varietie—KWS Irina and RGT Planet. Plants grown in a pot experiment were subjected to soil treatment with sodium chloride (NaCl) at concentrations of 0, 50, 100, and 150 mmol∙(dm<sup>3</sup>)<sup>−1</sup>. Measurements were made four times at intervals of 7, 14, 21, and 28 days after the application of NaCl. The relative chlorophyll content in leaves (CCl) and selected chlorophyll fluorescence parameters (F<sub>v</sub>/F<sub>m</sub>, F<sub>v</sub>/F<sub>0</sub>, and PI) and gas exchange parameters (<i>P</i><sub>N</sub>, <i>E</i>, <i>g</i><sub>s</sub>, and <i>C</i><sub>i</sub>) were assessed. In the final stage of the experiment, a visual assessment of the plants’ condition was carried out and the amount of fresh mass (FM) of the above-ground part was determined. The content of sodium and potassium in the vegetative parts of plants was also analysed. Salinity significantly influenced the values of the measured parameters in both of the tested barley genotypes. High salt concentrations in the soil at levels of 100 and 150 mmol NaCl (dm<sup>3</sup>)<sup>−1</sup> negatively affected the growth and development of plants by disturbing the process of photosynthesis and other plant gas exchange parameters. The antagonistic effect of sodium in relation to potassium resulted in a decrease in the K<sup>+</sup> content in the plants, along with an increase in the salinity level.
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