Effect of elevated CO₂ and temperature on the oxidative stress response to drought in Lolium perenne L. and Medicago sativa L.

Studies addressing the combined impact of multiple climate factors on plant abiotic stress responses are still scarce. We investigated physiological and molecular (antioxidant), responses to water deficit, in grassland-model species, Lolium perenne L. and Medicago lupulina L., under future climate conditions, i.e. elevated CO₂ (+CO₂, +375 ppm) and elevated temperature (+T, +3 °C). Elevated CO₂, but not warming, significantly increased biomass (gDW) in L. perenne, but not in M. lupulina. Photosynthesis (A(sat)) and stomatal conductance (g(s)), were differently affected by climate in each species, L. perenne generally being more sensitive. Elevated CO₂ increased lipid peroxidation levels in M. lupulina, but not in L. perenne, and had no effect on protein oxidation and little effect on antioxidant levels. Drought stress caused severe inhibition in biomass and photosynthesis, most severely in L. perenne, and strongly increased oxidative damage. Elevated CO₂ protected against the drought-induced damage. Decreased activities of APX and POX may indicate lower levels of oxidative challenge (relaxation) at the level of H₂O₂ production. Polyphenols, tocopherols and antioxidant capacity, increased under drought stress, in all climate conditions. Elevated CO₂, increased reduced ascorbate (ASC) and reduced glutathione (GSH), and their redox status, in both species, although to different levels. Changes in activities of key ASC/GSH cycle enzymes, under stress and climate treatments, showed weak correlations with ASC and GSH levels, indicating the complexity of this network. Together this work supports the idea that redox changes are involved in responses to climate changes, in the absence and presence of water-deficit stress.