The Cd sequestration effects of rice roots affected by different Si management in Cd-contaminated paddy soil

The application of exogenous silicon (Si) reportedly is one of the eco-friendly practices to mitigate cadmium (Cd) phytotoxicity and regulate the chemical behaviors of Cd in the soil-rice system. But the efficiency of Si on the Cd retention by rice root varies with the Si fertilizer management. The objective of this paper was to interpret the differences in Cd immobilization by rice roots and relevant mechanisms under different ways of Si application (T-Si, supplied at transplanting stage; TJ-Si, split at transplanting and jointing stage with the ratio of 50 % to 50 %; J-Si, supplied at jointing stage and CK, none of Si application) in Cd-contaminated paddy soils. The results showed that the Cd-retention capacity of rice root was increased by 0.60 % ~ 3.06 % under different Si management when compared to CK. The concentrations of monosilicic acid in soils and in apoplast and symplast of roots were increased significantly by Si application, while Cd concentrations in apoplast and symplast of root were decreased by 28.50 % (T-Si), 40.64 % (TJ-Si) and 30.26 % (J-Si), respectively. The distribution of Cd in rice cell wall was increased significantly by TJ-Si. The Cd concentrations of inert fractions (F3, F4 and F6) in root of TJ-Si were raised obviously. Si application downregulated the expression of OsIRT2 and OsNramp5 while upregulated OsHMA3, and the expression of OsHMA3 treated by TJ-Si was obviously higher than CK and J-Si. The distributions of the passive Cd in roots bound with thiol compounds (NPT, GSH and PCs) and polysaccharide components (pectin, hemicelluloses 1 and hemicellulose 2) were raised much more by TJ-Si than by T-Si and J-Si. On the whole, compared with T-Si and J-Si, TJ-Si could more easily replenish soil available Si and enhance Cd sequestration in roots as the result of the decrease of Cd transport factor in roots. This study unravels some mechanisms about different Si management on increasing Cd retention and decreasing Cd migration in rice roots, and TJ-Si is worthy of being recommended.