High carbon dioxide (CO2) concentration in atmosphere has direct effects on plants activities. In order to investigate the effect of increased CO2 concentration from 380 to 700 and 1050 mg/L on some chemical and physiological characteristics of Amaranthus tricolor under three salinity treatments (0, 150, and 300 mM NaCl), an experiment was conducted in Research Greenhouse of Ferdowsi University of Mashhad, Iran. The results showed that interaction of salinity and CO2 in traits such as total dry weight, electrolyte leakage, relative water content and Na+uptake was significant at 1% probability level. However, interaction of CO2 and salt stress in plant traits such as proline and total carbohydrates did not show any significant difference (p<0.01). The highest relative water content was measured in 700 mg/L CO2 treatment, when the plants were irrigated with distilled water (no salinity stress). Application of 700 and 1050 mg/L CO2 treatments, at the highest salinity level, reduced electrolyte leakage by 26 and 19 percent, respectively, as compared to CO2 control treatment.
