CO2 gas exchange, transpiration, stomatal conductance, water use efficiency and chlorophyll content were investigated at the leaves of three species of the family Plantaginaceae: Plantago maritima L., Plantago subpolaris Andrejev and Plantago schrenkii C.Koch under natural conditions of the habitat on high and low tide areas of the White Sea’ coasts. The high rate of photosynthesis at saturating CO2 (PNmax) at P. maritima (85.0±4.8 μmol m-2 s-1), as compared to P. subpolaris and P. schrenkii (45.2± 7.5 and 36.9±3.2 μmol m-2 s-1) was caused by high activity of ribulose-1,5-bisphoshate carboxylase/oxygenase (RuBPCO), the rate of electron transport, the rate of triose phosphate utilization, TPU), as well as high efficiency of carboxylation. The rates of photosynthesis at ambient concentration of CO2 (PN) at P. maritima were 1.4 and 1.7 times higher compared to P. subpolaris and P. schrenkii. Plants of P. schrenkii are characterized by lower values of stomatal conductance and water use efficiency compared to P. maritima and P. subpolaris. In natural habitat, the limiting factor of CO2 assimilation P. subpolaris is the rate of photosynthetic electron transport, the activity of RuBPCO in P. schrenkii
