Soil Inorganic Amendment Produces Safe Rice by Reducing the Transfer of Cd from Rice Root to Grain and Increasing Key Amino Acids in Brown Rice

The digestibility of cadmium (Cd) in brown rice is directly related to amino acid metabolism in rice and human health. In our field study, three kinds of soil inorganic amendments (SIA) at three dosages were applied to produce safe rice and improve the quality of rice in Cd-contaminated paddy field. With the increased application of SIA, Cd content in iron plaque on the surface of rice root significantly increased, the transfer of Cd from rice root to grain significantly decreased, and then Cd content in brown rice decreased synchronously. The vitro digestibility of Cd and amino acids (AAs) in brown rice were estimated by a physiologically based extraction test (PBET) and high-performance liquid chromatography, respectively. The PBET showed that more than 70% of Cd in brown rice could be digested by simulated gastrointestinal juice. Based on the total and digestible Cd contents in brown rice to evaluate the health risk, the application of 2.25 t·SIA·ha-1 could produce the safe rice in acidic slightly Cd-contaminated paddy soils. The increased application of SIA increased the contents of total AAs and 5 key amino acids (KAAs) in brown rice significantly. The structural equation model indicated that KAAs could be affected by the Cd translocation capacity from rice root to grain, and consequently altered the digestibility of Cd in brown rice. The formation of indigestible KAAs-Cd complexes by combining KAAs (phenylalanine, leucine, histidine, glutamine, and asparagine) with Cd in brown rice could be considered a potential mechanism for reducing the digestibility of Cd.