Plasticity of photorespiratory carbon concentration mechanism in Sedobassia sedoides (Pall.) Freitag & G. Kadereit under elevated CO2 concentration and salinity

Rising atmospheric CO₂ (carbon dioxide) concentrations and salinization are manifestations of climate change that affect plant growth and productivity. Species with an intermediate C₃–C₄ type of photosynthesis live in a wide range of precipitation, temperature, and soil quality, but are more often found in warm and dry habitats. One of the intermediate C₃–C₄ photosynthetic type is C₂ photosynthesis with a carbon concentration mechanism (CCM) that reassimilates CO₂ released via photorespiration. However, the ecological significance under which C₂ photosynthesis has advantages over C₃ and C₄ plants remains largely unexplored. Salt tolerance and functioning of CCM were studied in plants from two populations (P1 and P2) of Sedobassia sedoides (Pall.) Freitag & G. Kadereit Asch. species with C₂ photosynthesis exposed to 4 d and 10 d salinity (200 mM NaCl) at ambient (785.7 mg/m³, aCO₂) and elevated (1571.4 mg/m³, eCO₂) CO₂. On the fourth day of salinity, an increase in Na⁺ content, activity catalase, and superoxide dismutase was observed in both populations. P2 plants showed an increase in proline content and a decrease in photosynthetic enzyme content: rubisco, phosphoenolpyruvate carboxylase (PEPC), and glycine decarboxylase (GDC), which indicated a weakening of C₂ and C₄ characteristics under salinity. Treatment under 10 d salinity led to an increased Na⁺ content and activity of cyclic electron flow around photosystem I (PSI CEF), a decreased content of K⁺ and GDC in both populations. P1 plants showed greater salt tolerance, which was assessed by the degree of reduction in photosynthetic enzyme content, PSI CEF activity, and changes in relative growth rate (RGR). Differences between populations were evident under the combination of eCO₂ and salinity. Under long-term salinity and eCO₂, more salt-tolerant P1 plants had a higher dry biomass (DW), which was positively correlated with PSI CEF activity. In less salt-tolerant P2 plants, DW correlated with transpiration and dark respiration. Thus, S. sedoides showed a high degree of photosynthetic plasticity under the influence of salinity and eCO₂ through strengthening (P1 plants) and weakening C₄ characteristics (P2 plants).