Giant Rice Is a Unique Candidate for the Sustainable Phytoremediation of Cadmium-Contaminated Paddy Fields

The limited availability of ideal cadmium (Cd) hyperaccumulating plants remains a significant challenge for phytoremediating Cd-contaminated soils. This study aims to find out if the high biomass giant rice can be a suitable phytoremediation candidate for Cd-contaminated paddy fields and to elucidate physiological mechanisms relative to low or high Cd accumulation. Soil column and lysimeter tests were conducted with giant and normal species of rice under different water conditions. Cd distribution in different parts of root and shoots were determined. Under sustained flooding in column tests or keeping soil moist after the tillering stage in lysimeter tests, Cd contents in the grain of giant rice were very low. The Cd bioconcentration factor (BCF) of grain of giant rice (0.015-0.034) were significantly lower than those of normal rice (0.088-0.099), while BCF of root was in the reversed trend. Of the Cd in giant rice roots, 23.5% was trapped by the iron plaque on the root surface, compared 11.7% for normal rice. More Fe-O, -COOH functional groups in giant rice roots were confirmed by FTIR spectra. Under semi-dry condition, Cd contents in giant rice were very significantly increased, BCF straw in regenerated giant rice reached 13.4. Cd in shoots were accumulated in basal stems of the giant rice, the phytoextraction rate reached 18.7% of total Cd in topsoil, compared to 9.6% for giant Napier grass (Pennisetum hybridum). Therefore, the giant rice is a unique crop who can highly produce safe rice under wet conditions in the first half year and meanwhile by regenerating it can efficiently phytoextract Cd in the soil under semi-dry condition with a low labor input and carbon emission.