Differences in Acclimation to Constant and Fluctuating Salinity between Two Populations of C2 Species Sedobassia sedoides

This paper investigates the diversity of physiological responses of the C2 halophyte Sedobassia sedoides to constant and fluctuating salinity. Experimental plants, grown from seeds of two populations naturally inhabiting areas with constant (P1) and fluctuating (P2) salinity, were subjected to different salinity regimes: constant salinity, alternating salinity and recovery periods, and a control condition with no salinity treatment. The effects of salinity, recovery and re-salinization on parameters of growth and water-salt balance, CO2/H2O gas exchange, efficiency of photosystem II (PSII) and activity of cyclic electron transport (CET) around photosystem I (PSI), proline content, activity of antioxidant enzymes, and expression levels of genes encoding components of PSI, PSII, and CET proteins were studied. P1 plants were found to produce more biomass and were more tolerant to constant salinity than P2 plants. However, P2 plants demonstrated greater tolerance to fluctuating salinity. Differences in mechanisms of adaptation to constant salinity between the populations were observed. Thus, changes in regulation of stomatal function and C2 carbon concentrating mechanism (CCM) played a significant role in the adaptation of P1 plants, whereas in P2 plants proline accumulation and reactive oxygen species (ROS) scavenging were more important. The two studied populations differed in their recovery processes. After the recovery period, P1 plants demonstrated low stomatal conductance, whereas P2 plants restored control level of stomatal conductance. Apparently, P2 plants possess acclimation mechanisms, allowing them to open their stomata and resume CO2 assimilation during substrate desalination. After re-salinization, P1 plants differed from plants grown under constant salinity solely in the expression of psaA, whereas P2 plants had higher expression of psaA and PnsB5, as well as higher levels of carbon dioxide compensatory point (Γ) and assimilation at high CO2 concentrations than under constant salinity, which suggests different acclimation strategies between populations and a pronounced stress memory possessed by P2 plants.