We also studied the production of H2O2 and the activity of
the antioxidant enzymes catalase and APX in sunflower leaves.
As shown in Fig. 6A, H2O2 production increased with leaf aging,
especially under elevated CO2 concentrations, suggesting that high
levels of CO2 may play a role in regulating leaf senescence in
sunflower plants by increasing ROS production. Catalase and APX
activities increased during early leaf development, reaching their
maximal levels at 22 days and decreasing later in senescent leaves.
Also, these antioxidant enzymatic activities were lower in the
plants grown at elevated CO2 concentrations throughoutleaf devel-
opment (Fig. 6B and C).