82 H. Guo et al.
shoots of rice grown on FACE plots with 0 (control) and 400 mg kg−1 Cu was 18.6
and 12.6 % (Fig. 4.5e, p < 0.05) lower than shoots of rice grown on ambient plots,
and the Cu concentration in the grains of rice grown on FACE plots with 0 (con-
trol), 50, and 400 mg kg−1 Cu was 25.5, 20.3, and 14.2 % lower than in grains of
rice grown on ambient plots (Fig. 4.5f, p < 0.05). Similar results were observed for
wheat (Fig. 4.6).
In the previous studies of uncontaminated soils, Manderscheid et al. ( 1995 ) found
that elevated CO 2 levels led to lower concentrations of Ca, S, Mg, Fe, and Zn in the
wheat grain. Fangmeier et al. ( 1999 ) reported that elevated CO 2 levels resulted in
lower Ca, S, and Fe concentrations in spring wheat. Yang et al. ( 2007 ) showed that
the Cu content of milled rice grain under elevated CO 2 levels was 20 % lower than
that of ambient atmosphere. In an OTC experiment with contaminated soils and el-
evated CO 2 levels, Li et al. ( 2010 ) reported that the Cu concentration in rice grains
was significantly lower than that of ambient atmosphere. In the short term, lower
Cu concentrations in crops probably alleviate the Cu toxicity and have important
positive implications for the food quality of crops harvested from soils contami-
nated with Cu. In this study, the SEM images of rice roots showed that exposure to
elevated CO 2 levels alleviated Cu stress and increased the root hair density. When
the plants were grown with 2 mg kg−1 Cd on either FACE or ambient plots, the root
hair density of rice was low (Fig. 4.7). However, when the plants were grown with
400 mg kg−1 Cu on FACE plots, the root hair density was markedly higher than that
of plants grown on ambient plots with 400 mg kg−1 Cu (Fig. 4.7). In the long term,
depending on the magnitude of the effect, Cu deficiency in crops has the potential
to contribute to health problems.
4.3.3 Cadmium Concentration in Plants
Elevated CO 2 levels resulted in higher Cd concentrations in the tissues of both
wheat and rice, especially in those plants grown in soils contaminated with high
levels of Cd (Figs. 4.8 and 4.9). At the mid-tillering stage of the first rice season
(Fig. 4.8a), the Cd concentration in shoots of rice grown on the FACE and ambi-
ent plots did not differ significantly. At the panicle-initiation stage, the Cd concen-
trations in shoots of rice grown on FACE plots with 0.5 and 2 mg kg−1 Cd were
55.7 and 7.8 % higher, respectively, than in shoots of rice grown on ambient plots
(Fig. 4.8b, p < 0.05). At grain maturity of both the first and second rice season, the
Cd concentration in shoots of rice grown on FACE plots with 2 mg kg−1 Cd was 11.3
and 21.5 % higher ( p < 0.05), respectively, than in shoots of rice grown on ambient
plots. But the Cd concentration in shoots of rice grown on FACE and ambient plots
with 0 and 0.5 mg kg−1 Cd did not significantly differ (Fig. 4.8c, e). The Cd concen-
tration in seeds was not significantly affected by elevated CO 2 levels in the first rice
season (Fig. 4.8d). In the second rice season, the Cd concentration in seeds of plants
grown on FACE plots with 2 mg kg−1 Cd was 38.8 % higher than in seeds of plants
grown on ambient plots ( p < 0.05), but the Cd concentration in seeds of rice grown