Abstract: Effects of exogenous melatonin on accumulation and chemical form of Cd in rice seedlings were investigated. The results showed that Cd stress significantly decreased the biomass of shoots and roots, and the chlorophyll content in rice leaves. The addition of exogenous melatonin could remarkably increase the biomass of rice and significantly decreased Cd content in the shoots and roots. At Cd concentration of 5 μmol·L^-1, addition of 20 μmol·L^-1 exogenous melatonin could decrease Cd content in rice shoots and roots by 48.4% and 16.9% respectively, compared with the control. When the concentration of exogenous melatonin rose to 100 μmol·L^-1, Cd content in rice shoots and roots decreased by 67.5% and 47.9% respectively. Furthermore, the addition of exogenous melatonin obviously decreased the Cd-translocation efficiency of rice seedlings. When the Cd concentration was 20 μmol·L^-1, addition of 20 and 100 μmol·L^-1 exogenous melatonin could decrease the Cd-translocation efficiency by 24.4% and 46.8% respectively. The chemical form of Cd in rice seedlings was analyzed by using sequential extraction. The results showed that exogenous melatonin addition increased the proportion of NaCl-extractable Cd, while decreased the proportion of ethanol-extractable Cd and water-extractable Cd in rice seedlings. It was suggested that the addition of exogenous melatonin could promote the transfer of Cd from strong to weak mobility form, thereby reducing the uptake and transport of Cd in rice seedlings.

Key words: rice, melatonin, Cd, chemical form