Interaction between drought and elevated CO2 in the response of alfalfa plants to oxidative stress

Alfalfa (Medicago sativa L.) plants were grown for 2 years in open top chambers both at ambient (540 ppm) and at high CO2 concentrations (600 ppm). During the second-year regrowth cycle, plants were subjected to drought for 5 days. A chamber effect on the oxidative status of cells, total ascorbate contents, glutathione reductase, and Ca(2+)ATPase and Ca(2+)ITPase activities occurred. Ascorbate was found to be more affected than glutathione by CO2 enrichment and an interaction between CO2 and drought prevented the decrease in ascorbate/dehydroascorbate ratio and increased the reduced glutathione/oxidised glutathione ratio as compared with ambient-CO2 plants. In the microsomal fraction the presence of multiple Ca2+ pumps. i.e. the plasma membrane-type and endoplasmic reticulum-type pumps, was highlighted by their differential substrate preference for ATP and ITP, though only one major peptide of 130 kD was recognised by an antibody against Ca(2+)ATPase. The pump was degraded in CO2-enriched plants and its preference for ATP was reduced. The activity of the Ca2+ pump was also less susceptible to oxidation in drought conditions. CO2-enriched plants were less affected by drought as shown by a higher reducing status of their cells and by a reduced requirement for Ca(2+)ATPase activity to maintain Ca2+ homeostasis.