Mechanisms Of Alleviation Effects Of Selenium Application On Cadmium Toxicity In Rice

Cadmium (Cd) is one of the most deleterious heavy metals to both animals and plants, which has no beneficial biological function in the aquatic or terrestrial organism. High accumulation of Cd in plants not only seriously affects crop yield and quality, but also endangers human health via food chain. Although several approaches have been proposed to reduce soil Cd level, including the use of hyper-accumulating plants, none have been effectively applied. Concerning medium or slightly polluted farmlands, it might offer a practically acceptable and cost-effective strategy for the complete utilization of natural resources and ensure safe food production by application of chemical regulators to ease Cd toxicity and reduce plant Cd uptake. Selenium (Se) is an essential element for humans and animals. The present study was carried out to elucidate the mechanism of mitigative effect of Se application on Cd toxicity by examining growth parameters, nutrient uptake and translocation, reactive oxygen species (ROS) metabolism and other physiochemical responses of the Cd-sensitive rice cultivar xiushui63. The main results are as follows:1. Pot experiment was undertaken, using Xiushui63(Cd-sensitive cultivar), to study the effect of exogenous Se on yield and Cd uptake and translocation in rice in response to Cd stress. There were4treatments:(1) Control, basic nutrition solution (BNS);(2) Cd, BNS+100μM Cd;(3) Se, BNS+6μM Na2Se03; and (4) Cd+Se, BNS+100μM Cd+6μM Na2Se03; at1L per pot. The pots were artificially contaminated with Cd20d before seeding by adding BNS solution containing100μM Cd (20mg kg-1soil) as CdCl2. After adding1L per pot of BNS solution with or without Cd, the soil was allowed to equilibrate for20d in a greenhouse. Se was applied at both the seedling and elongation stages. The results indicated that6μM Se addition to100μM Cd-contaminated soils significantly alleviated the negative effect of Cd on plant height and yield per plant, and markedly reduced Cd concentration in leaves, stems and grains under Cd stress.2. Hydroponic experment was carried out to elucidate the mechanism of mitigative effect of Se application on Cd toxicity via examing SPAD value, plant height, dry weight, root length, biomass accumulation and nutrient uptake and translocation. The were6treatments:(1) Control, basal nutrient solution (BNS);(2) Se, BNS+3μM Na2SeO3;(3) Cd, BNS+50μM CdCl2;(4) Cd+Se, BNS+50μM CdCl2+3μM Na2SeO3;(5) pre-Se, BNS+3μM Na2Se03(24h pretreatment); and (6) pre-Se+Cd, BNS+3μM Na2Se03(24h pretreatment before Cd exposure)+50μM CdCl2. The results showed that addition of 3μM Se to50μM Cd (Cd+Se and pre-Se+Cd) elevated Cd-induced drastic reduction in plant growth as indicated by SPAD value, plant height, root length, and biomass. Supplemented Se markedly down-regulated Cd-induced increase in root K, Mg, Fe and Zn and leaf/root S and Cu concentations and tended to counterbalance Cd-induced decreases of leaf Mn and Zn contents.3. The effect of exogenous Se on Cd accumulation and the vigor of root cells in rice seedlings under Cd treatment were investigated. The results elucidated that plants grown in Cd+Se and pre-Se+Cd medium showed lower Cd concentration both in shoots and roots compared with Cd alone treatment. Similarly, Cd fluorescence localization and fluorescence intensity indicated that Se markedly decreased Cd accumulation in roots of Cd-treated rice plants. Leaf and root Se concentrations were markedly decreased by50uM Cd. The results of FDA-PI assay experiment indicated that Se supplementation can reduce Cd-induced oxidative damage in cells and promote cell viability.4. The effect of exogenous Se on ROS metabolism in rice seedlings under Cd stress was studied. The results demonstrated that Se alleviates Cd-induced oxidative stress as reflected by reduced O2·-，H2O2, and MDA. Additionally, the protective role appears to be responsible for beneficial effects of Se in terms of activation of leaf CAT during the entire treatment and leaf APX after15d treatment. In addition, Se addition to culture medium decreased Cd-induced high leaf and root SOD and leaf POD activity.5. The effect of exogenous Se on ATPase in rice seedlings under Cd stress was studied. The results showed that50μM Cd significantly increased H+-and Total-ATPase activities in leaves, but no effect on Ca2+-ATPase activity. Cd+Se and pre-Se+Cd treatments counteracted Cd induced a dramatic reduction in H+-ATPase, Ca2+-ATPase and Total-ATPase activities in roots.