Abstract <p>A comparative study of structural and functional features of leaves of sea arrowgrass (triostrennik, <i>Triglochin maritima</i> L.) and saline aster (<i>Tripolium vulgare</i> Nees.) growing along a gradient of conditions on the littoral of the intertidal zone of the Pomorsky coast of the White Sea was carried out. The study of leaf morphometric parameters showed that as the gradient of conditions increases from the upper part of the littoral to the conditions of deeper and longer flooding, there is a decrease in the area and leaf structure parameters. The number of stomata decreases, but their area per leaf unit does not change. Higher values of photosynthesis rate, transpiration, stomatal conductance and carboxylation rate were observed in leaves of plants growing near the shoreline and exposed to insignificant level and time of flooding in comparison with plants at the water’s edge under prolonged stay under water. Immediately after emergence from prolonged flooding, plants, in addition to utilizing the major carbon dioxide fixation pathway via RBPC/O, turn on the PEP-carboxylase pathway of CO₂ fixation. Functional differences between species are related to the maintenance of gas exchange under flooding conditions: in triostrennik due to large funds of storage parenchyma, and in aster due to metabolic switching. The revealed differences explain their different degree of development of seaside territories: triostrennik, in comparison with aster, develops a wider range of littoral ecotopes. Additional arguments in support of high plasticity of littoral species in Arctic conditions were obtained.</p>