Elevated atmospheric CO2 concentration leads to different salt resistance mechanisms in a C3 (Chenopodium quinoa) and a C4 (Atriplex nummularia) halophyte

• NaCl salinity leads to oxidative stress in C. quinoa (C3) and A. nummularia (C4).
• C. quinoa and A. nummularia combat oxidative stress via different mechanisms.
• In C. quinoa elevated CO2 reduces oxidative stress by enhancing net assimilation.
• In A. nummularia elevated CO2 has no effect on net assimilation.
• In A. nummularia elevated CO2 reduces oxidative stress by lowering electron transfer.

This study aimed at investigating the effects of elevated atmospheric CO2 concentration on the salt resistance of the C3 halophyte Chenopodium quinoa and the C4 halophyte Atriplex nummularia. Plants were irrigated with different salinity levels according to their individual range of resistance (0, 100, 300, 500, and in the case of A. nummularia additionally 750 mol m−3 NaCl) under ambient and elevated (540 ppm) CO2. In C. quinoa, NaCl salinity led to a decreased stomatal conductance, Ci, and net CO2 assimilation rate (stomatal limitation of photosynthesis) and consequently to a higher risk of ROS production, indicated by an increased ETR/Agross ratio. Due to its C4 metabolism, A. nummularia exhibited higher net photosynthetic rates and a lower threat of oxidative stress (lower ETR/Agross ratio), leading to a distinctly higher salt resistance.Elevated atmospheric CO2 supported the photosynthesis of both species; however, the salt resistance of quinoa stayed at a distinctly lower level than the one of A. nummularia. In C. quinoa (C3), the stomatal limitation of photosynthesis was ameliorated (indicated by increased Ci values and Anet), so that the threat of oxidative stress was reduced (decrease in ETR/Agross; direct CO2 effect). In A. nummularia (C4), CO2 enrichment did not stimulate Anet. However, the generation of ROS could be avoided by a reduction in electron transfer (indirect non-stomatal effect), resulting in a lower ETR/Agross ratio.The results imply that both species will be suited as cash crop halophytes in a future CO2-rich world.