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

This study aimed at investigating the effects of elevated atmospheric CO2 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) CO2. In C quinoa, NaCl salinity led to a decreased stomatal conductance, C-i, and net CO2 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 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 (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/Agross; direct CO2 effect). In A. nummularia (C-4), CO2 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 CO2-rich world. (C) 2015 Elsevier B.V. All rights reserved.