Lipoic acid and redox status in barley plants subjected to salinity and elevated CO2

Future environmental conditions will include elevated concentrations of salt in the soil and an elevated concentration of CO2 in the atmosphere. Because these environmental changes will likely affect reactive oxygen species (ROS) formation and cellular antioxidant metabolism in opposite ways, we analyzed changes in cellular H2O2 and non-enzymatic antioxidant metabolite [lipoic acid (LA), ascorbate (ASA), glutathione (GSH)] content induced by salt stress (0, 80, 160 or 240 mM NaCl) under ambient (350 mu mol mol-1) or elevated (700 mu mol mol-1) CO2 concentrations in two barley cultivars (Hordeum vulgare L.) that differ in sensitivity to salinity (cv. Alpha is more sensitive than cv. Iranis). Under non-salinized conditions, elevated CO2 increased LA content, while ASA and GSH content decreased. Under salinized conditions and ambient CO2, ASA increased, while GSH and LA decreased. At 240 mM NaCl, H2O2 increased in Alpha and decreased in Iranis. When salt stress was imposed at elevated CO2, less oxidative stress and lower increases in ASA were detected, while LA was constitutively higher. The decrease in oxidative stress could have been because of less ROS formation or to a higher constitutive LA level, which might have improved regulation of ASA and GSH reductions. Iranis had a greater capacity to synthesize ASA de novo and had higher constitutive LA content than did Alpha. Therefore, we conclude that elevated CO2 protects barley cultivars against oxidative damage. However, the magnitude of the positive effect is cultivar specific.