Functional plasticity of photosynthetic apparatus and its resistance to photoinhibition in Plantago media

Morphological and functional characteristics of Plantago media L. leaves were compared for plants growing at different light regimes on limestone outcrops in Southern Timan (62°45′N, 55°49′E). The plants grown in open areas under exposure to full sunlight had small leaves with low pigment content and high specific leaf weight; these leaves exhibited high photosynthetic capacity and elevated water use efficiency at high irradiance. The maximum photochemical activity of photosystem II (F v/F m) in leaves of sun plants remained at the level of about 0.8 throughout the day. The photosynthetic apparatus of sun plants was resistant to excess photosynthetically active radiation, mostly due to non-photochemical quenching of chlorophyll fluorescence (qN). This quenching was promoted by elevated deepoxiation of violaxanthin cycle pigments. Accumulation of zeaxanthin, a photoprotective pigment in sun plant leaves was observed already in the morning hours. The plant leaves grown in the shade of dense herbage were significantly larger than the sun leaves, with pigment content 1.5–2.0 times greater than in sun leaves; these leaves had low qN values and did not need extensive deepoxidation of violaxanthin cycle pigments. The data reveal the morphophysiological plasticity of plantain plants in relation to lighting regime. Environmental conditions can facilitate the formation of the ecotype with photosynthetic apparatus resistant to photoinhibition. Owing to this adjustment, hoary plantain plants are capable of surviving in ecotopes with high insolation.