Main conclusion <p>Moderate concentrations of Na⁺ ions in the growth medium have positive effects on growth and performance characteristics of the C₄ euhalophyte <i>Suaeda altissima</i>, optimizing functioning of both, stomata and photosynthetic apparatus.</p> Abstract <p>Effects of salinity on the morphology, ion relations, and gas exchange of the leaves in the euhalophyte <i>Suaeda altissima</i> (L.) Pall. were investigated with emphasis on the guard and epidermal cells. The presence of NaCl in the nutrient solution (NS) at both growth-stimulating (250&#xa0;mM) and growth-inhibiting (750&#xa0;mM) concentrations resulted in increased leaf succulence, net photosynthetic rate (<i>P</i>_n), and instantaneous water use efficiency (WUE_i), and reduced stomatal conductance (<i>g</i>_s), stomatal density on the leaf surface, and transpiration rate (<i>E</i>). X-ray microanalyses revealed Na and Cl accumulation in the guard and epidermal cells of leaves under salinity conditions. However, Na and Cl contents differed by not much in plants grown at 250 and 750&#xa0;mM NaCl, indicating a mechanism preventing accumulation of Na⁺ and Cl⁻ ions in cells paving leaf surface at high NaCl concentrations. Examination of <i>g</i>_s and <i>E</i> as functions of CO₂ concentration in the leaf gas exchange chamber revealed better ability to regulate these parameters in 250&#xa0;mM NaCl-grown plants than in 750&#xa0;mM NaCl-grown or control plants. The study of <i>P</i>_n dependent on CO₂ concentration in leaf intercellular space revealed direct stimulating effect of NaCl on photosynthesis. We hypothesize that <i>S. altissima</i>, a species having anatomy and ultrastructure features of C₄ plants, improves its performance characteristics under saline conditions, optimizing not only the functioning of the stomata complex but also the process of CO₂ assimilation, including the C₄ fixation pathway.</p>