Role of Photorespiration and Cyclic Electron Transport in C4 Photosynthesis Evolution in the C3-C4 Intermediate Species Sedobassia sedoides

Plants from two Sedobassia sedoides (Pall.) Aschers populations (Makan and Valitovo) (Chenopodiaceae) with C-2 photosynthesis (precursor of C-4 photosynthesis in phylogenesis) and photorespiratory CO2-concentrating mechanism were studied. Genetic polymorphism and isotope discrimination (delta D-13) levels of the plants were determined under natural conditions, and their morpho-physiological parameters such as fresh and dry biomass of the above ground parts of plants, functioning of photosystem I (PSI) and photosystem II (PSII), intensity of net photosynthesis (A), transpiration (E), photorespiration and water use efficiency (WUE) of plants were calculated under control and salinine conditions (0 and 200 mM NaCl). Results of the population-genetic analysis showed that the Makan population is polymorphic (plastic) and the Valitovo population is monomorphic (narrowly specialized). There were no significant differences between the populations based on delta D-13 values or growth parameters, PSII, A, E and WUE under control conditions. Under saline conditions, dry biomass accumulation decreased in the Makan population by 15% and by more than 2- fold in the Valitovo population. Population differences were revealed in terms of photorespiration intensity and P700 oxidation kinetics under control and saline conditions. Under control conditions, Makan plants were characterized by a higher photorespiration intensity, which decreased by 2-fold under saline conditions to the photorespiration level of Valitovo plants. Cyclic electron transport activity was minimal in the control Makan plants, and it increased by almost 2-fold under saline conditions to the level of that in Valitovo plants under control and saline conditions. Under control conditions, photosynthesis in Makan plants can be specified as the proto-Kranz type (transitional type from C-3 to C-2) and that in Valitovo plants can be specified as the C-2 type (C-4 photosynthesis with photorespiratory CO2-concentrating mechanism), based on their photorespiration level and cyclic electron transport activity. Under saline conditions, Makan plants exhibited features of C-2 photosynthesis. Intraspecific functional differences of photosynthesis were revealed in different populations of intermediate C-3-C-4 plant species S. sedoides which reflect the initial stages of formation of a photorespiratory CO2-concentrating mechanism during C-4 photosynthesis evolution, accompanied by decrease in salt tolerance.