Effects of zinc level on low dose cadmium transport in rice plant

The interaction of zinc (Zn) and cadmium (Cd) in soil is critical for the uptake and transport of Cd in rice plants. However, the effect of Zn levels on the interactions of Zn and Cd or on the rate of transport of Cd in rice plants is still not entirely clear. In this study, rice plant biomass, and Cd transport and distribution in rice plant were investigated in hydroponic experiment of mild Cd pollution with exogenous Zn addition. In the experiment, Cd concentrations were 0.01 mg·L1 (low dose), 0.03 mg·L1 (medium dose), 0.09 mg·L1 (high dose), and 0 mg·L1 (control); and exogenous Zn were 0.025 mg·L1, 0.05 mg·L1, 0.1 mg·L1, 0.2 mg·L1. The aim of the study was to determine the mechanism of interactions between Zn and Cd and to identify the optimal Zn concentration that effectively reduced Cd pollution in rice. The results showed that biomasses of different parts of rice plant increased significantly with increasing concentration of Zn. The increase of biomass was highest in 0.05 mg·L1 exogenous Zn treatment. However, there was no significant increase when Zn concentration exceeded 0.05 mg·L1. Due to the low dose of Cd in the experiment, there was no significant variation in rice plant biomass with increasing Cd concentration without exogenous Zn. Under Zn-free condition, the ratio of cytoplasm Cd to cell-wall Cd in rice root reduced from 2.88 to 1.04 with increasing Cd concentration. With the applying of exogenous Zn, the ratio of cytoplasm Cd to cell-wall Cd showed increasing tendency, especially under 0.03 mg·L1 Cd. Under medium-to-low Cd dose (0.01–0.03 mg·L1), Zn significantly inhibited the uptake and transport of Cd in root, which significantly reduced Cd concentration in rice cytoplasm and cell wall. Compared with Zn-free treatment, 0.05 mg·L1 Zn significantly reduced Cd content in rice. The concentration of Cd in plant root, stem and leaf reduced by 38%, 71% and 65% under low-dose Cd and by 44%, 79% and 69% under medium- dose Cd, respectively. The rate of transport of Cd between roots and stems, and between roots and leaves decreased by 53% and 44% under low-dose Cd, and by 62% and 40% under medium-dose Cd, respectively. When Zn concentration exceeded 0.05 mg·L1, there was no obvious change in the concentration of Cd in roots, stems and leaves along with the rate of Cd transport in rice. Under high concentration of Cd (0.09 mg·L1), there was no obvious decline in Cd concentration in roots, stems and leaves due to addition of exogenous Zn. Addition of Zn promoted the absorption of Cd by rice stem and leaf. There was no obvious antagonism between Zn and Cd under high Cd concentration, although the synergies were obvious. Thus under medium-to-low dose Cd conditions, Zn controlled the phyto-availability of Cd due to obvious antagonistic effects between Zn and Cd. Application of 0.05 mg·L1 Zn significantly reduced Cd phyto-availability and migration in rice and maximally increased plant biomass under low concentrations of Cd in hydroponic cultures.