H. morsus-ranae is a true free-floating aquatic plant of lakes, ponds and slow-flowing rivers. Plants are mostly occurring at full light, but also at the relatively low light levels, up to about 30% of diffuse radiation incident in an open area (Pindel and Wozniak, 1998). The goal of our work was to carry out the comparative investigations of the photosynthetic apparatus in H. morsus-ranae plants being under different illumination – full light and shade in the natural conditions. Samples of mature leaves were collected from plants, which grew in the Psel River (Velyka Bagachka, Poltava region) and the Gulf of Venice in Kyiv area at a certain distance from the shore under direct sunlight and along the coast, usually between the plants of Typha latifolia and Phragmites australis in the shading conditions. The methods of light and electron microscopy and biochemistry as well as a principal component analysis (Lever et al., 2017) of photosynthetic apparatus variability were used. It was shown plasticity of the H. morsus-ranae photosynthetic apparatus in adaptation to shading. The leaves of plants growing in shade differed from those at full light in such traits as thickness of the blade, palisade and spongy parenchyma, the content of anthocyanins per unit area of the leaf blade, chlorophylls and carotenoids, the chloroplast ultrastructure, first of all a number of thylakoids in grana, and quantum efficiency of photosynthetic electron transport.
The differences in quantum efficiency had the opposite direction in different parts of the electronic transport chain. By principal component analysis it was found that the first principal component (PC 1) is associated with 43%, and the second (PC2) with 23% of all data variability. PC1 is associated primarily with the content of photosynthetic pigments and parameters Fv / Fm, PC2 with the content of phenylpropanoids, anthocyanins and quantum efficiency of electron transport to the terminal acceptor.