The content of chlorophylls (Chls) and carotenoids was studied in the leaves of 42 species of boreal aquatic plants with different degree of submergence (emergent, floating, and submerged) and isopalisade, dorsoventral, and homogenous types of mesophyll structure. Hydrophytes were shown to have a low Chl content (1-2 mg/g fr wt) and low Chls/carotenoids ratio (2.3-3.5) as compared to terrestrial plants. The pigment content per dry wt unit and unit leaf area was dependent on the type of mesophyll structure. It was a consequence of the changes in the parameters of leaf mesophyll structure characterizing the density of photosynthetic elements. In a sequence emergent --> floating --> submerged forms, the content of Chis and carotenoids decreased, and the photosynthetic capacity decreased due to a reduction in the chloroplast number per unit leaf area. Adaptation of submerged leaves to low illumination and slow CO2 diffusion changed the functional properties of chloroplasts. An increase in the pigment content in the chloroplasts of submerged leaves (7 x 10(-9) mg Chi, 2 x 10(-9) mg carotenoids) as compared to emergent and floating leaves was accompanied by a decline in the photosynthetic capacity per Chl comprising 1.6 mg CO2/(mg Chl h) versus 3.9 and 3.8 mg CO2/(mg Chl h) in emergent and floating leaves, respectively.