The aim of this study was to evaluate the effects of elevated CO₂ concentration on acclimation mechanisms related to gas exchange, photochemical activity, photorespiration, and oxidative protection in cashew plants exposed to salinity. Thirty-day-old cashew plants were irrigated with nutrient solution without (control) or with supplemental NaCl (100 mM) for 2 weeks in the greenhouse. Afterward, control and salt-stressed plants were transferred to the growth chamber and supplied with atmospheric (380 µmol mol⁻¹) or high CO₂ (760 µmol mol⁻¹) concentrations for 15 days. The results show that elevated CO₂ alone reduced the CO₂ net assimilation rate (PN) without affecting stomatal conductance (gS) and transpiration rate (E), whereas salinity and NaCl + high CO₂ reduced the PN associated with a decrease in gS and E. The potential quantum yield of photosystem II (Fv/Fm) was not altered, but a slight reduction in electron transport rate and photochemical quenching (qP) in response to high CO₂ alone or combined with NaCl occurred. However, non-photochemical quenching increased due to the effects of high CO₂ and NaCl alone and by their combination. High CO₂ alleviated the toxic effects of Na⁺ favoring the K⁺ /Na⁺ ratio under salinity. High CO₂ coupled with salinity decreased glycolate oxidase activity and the contents of hydrogen peroxide (H₂O₂), NH₄⁺, and glyoxylate. Furthermore, we observed increase in membrane damage associated with increased thiobarbituric acid-reactive substances levels under high CO₂. High CO₂ also decreased ascorbate peroxidase activity, but did not affect superoxide dismutase activity. In general, our data suggest that high CO₂ could induce acclimation processes in plants independent of salinity, revealing a set of responses that are more associated with acclimation than with protective responses.
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