Role of Stomata in Adaptation of Plantago maritima L. Plants to Tidal Dynamics on the White Sea Coast

Structural and functional traits in leaves of sea plantain ( Plantago maritima L.) inhabiting intertidal and supratidal zones of the White Sea coast were studied. At the intertidal, the plants were found to have smaller and thicker leaves with more frequent stomata and thinner cover tissues in comparison with the plants collected from the supratidal. Leaf fluorescence and stomatal conductance obeyed the circadian rhythm in plants residing in the supratidal. The tested parameters considerably changed according to tidal dynamics in the intertidal plants. Two stable states (high and low tides) and two transition states (while water level ascends or descends) were defined. In both stable states, the plants demonstrated high functional parameters of fluorescence ( F v / F m , Y (II), and ETR ) and open stomata. In the transitional states, the stomata were partly closed, and the functional activity was inhibited. Leaf structural peculiarities, leaf gas films, the inducible CO 2 -concentrating mechanism, and the determining role of the stomata are considered in terms of the hypotheses explaining the responses of sea plantain to tidal dynamics on the White Sea coast.