80 H. Guo et al.
peroxide and molecular oxygen, while CAT decomposes hydrogen peroxide into
water (Dionisio-Sese and Tobita 1998 ). For higher plants, the induction of POD ac-
tivity is a comprehensive reaction to harmful stress, which may be associated with
oxidation reactions of membrane (Lin and Kao 2002 ). The most important reducing
substrate for hydrogen peroxide detoxification is ascorbate, APX uses two mol-
ecules of ascorbate to reduce hydrogen peroxide to water (Noctor and Foyer 1998 ).
In this study, at mid-tillering growth stage, the activities of enzymes (CAT, APX,
POD, and SOD) in leaves of rice grown on FACE plots with Cu and Cd added
groups were lower than that in leaves of rice grown on ambient plots. We speculate
that elevated CO 2 levels might alleviate oxidative stress in leaves of rice polluted
by Cu and Cd. Some research suggested that elevated concentrations of CO 2 caused
a significant reduction in the activities of SOD and CAT in leaves of plant, and
the oxidative stress of plant was alleviated to a certain extent (Polle et al. 1993 ;
Schwanz et al. 1996 ). It was suggested that the activity of SOD in leaves of beech
( Fagus sylvatica L.) was inhibited with elevating CO 2 levels as a result of increase
of NADPH which was the intermediate of photosynthesis and the activity of CAT
decreased with elevating CO 2 levels because the respiration rate of plant slowed
down and the concentration of hydrogen peroxide which was the product of respira-
tion decreased (Polle et al. 1997 ).
In Cu treatment groups, the activities of enzymes in the leaves of wheat grown
on FACE plots at mid-tillering and panicle-initiation growth stages were lower than
that in leaves of wheat grown on ambient plots, while the trend was opposite in Cd
treatment groups. We surmise that the absorption of Cu and Cd was different in
wheat under different atmospheric conditions. Elevated CO 2 levels increased the
absorption of Cd, resulted in the increase of oxidative stress. Increase in ROS prob-
ably served as an inciting factor that increased the activities of antioxidant enzymes.
4.3.2 Copper Concentration in Plants
In this 2-year study, elevated CO 2 levels significantly led to lower Cu concentration
in both rice and wheat (Figs. 4.5 and 4.6). At the mid-tillering stage of the first rice
season, the Cu concentrations in shoots of rice grown on FACE plots with 50 and
400 mg kg−1 Cu in the soil were 23.0 and 22.9 % lower, respectively, than in shoots
of rice grown on ambient plots (Fig. 4.5a, p < 0.05). At the panicle-initiation stage,
the Cu concentration in shoots of rice grown on FACE plots with 50 mg kg−1 Cu was
22.2 % lower than that in shoots of rice grown on ambient plots (Fig. 4.5b, p < 0.05).
At grain maturity during the first rice season, the Cu concentration in the shoots of
rice grown on FACE plots was 34.1, 16.1, and 19.7 % lower (Fig. 4.5c, p < 0.05) than
their counterparts grown on ambient plots with 0 (control), 50, and 400 mg kg−1 Cu,
respectively, and the Cu concentration in the grains of rice grown on FACE plots
with 400 mg kg−1 Cu (Fig. 4.5d, p < 0.05) was 8.8 % lower than in grains of rice
grown on ambient plots. A similar trend was detected in samples from the second
year. At grain maturity during the second rice season, the Cu concentration in the