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

Abstract This study aimed at investigating the effects of elevated atmospheric CO 2 concentration on the salt resistance of the C 3 halophyte Chenopodium quinoa and the C 4 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) CO 2 . In C. quinoa , NaCl salinity led to a decreased stomatal conductance, C i , and net CO 2 assimilation rate (stomatal limitation of photosynthesis) and consequently to a higher risk of ROS production, indicated by an increased ETR/ A gross ratio. Due to its C 4 metabolism, A. nummularia exhibited higher net photosynthetic rates and a lower threat of oxidative stress (lower ETR/ A gross ratio), leading to a distinctly higher salt resistance. Elevated atmospheric CO 2 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 (C 3 ), the stomatal limitation of photosynthesis was ameliorated (indicated by increased C i values and A net ), so that the threat of oxidative stress was reduced (decrease in ETR/ A gross ; direct CO 2 effect). In A. nummularia (C 4 ), CO 2 enrichment did not stimulate A net . However, the generation of ROS could be avoided by a reduction in electron transfer (indirect non-stomatal effect), resulting in a lower ETR/ A gross ratio. The results imply that both species will be suited as cash crop halophytes in a future CO 2 -rich world.