Effect Of Sulphur (S) Supply And Root Structure On Iron Plaque Formation And Chromium (Cr) Uptake And Accumulation In Rice (Oryza Sativa L.) Plants Grown In Solution Culture

The contamination of heavy metal(HM)in agricultural lands and the subsequent pollution of food chain is one of the most serious agro-environmental problems.Chromium(Cr)is one of the eight primary HMs in agricultural lands,and rice is the major crop in China;it is of significant importance to control Cr contamination in rice grain.Iron plaque(IP)on rice root is a critical barrier to control Cr transfer from soil to rice.Sulphur nutrition affects the composition of IP,and probably influences the uptake of Cr in rice.Moreover,the contrast root structures between mature root section and root tip results into the different uptake mechanisms of Cr.However,it is still unknown whether S nutrition and different root sections have distinct mechanisms to prevent HM uptake via the establishment of chelation and exclusion mechanisms.Therefore,in the thesis,three hydroponic experiments were designed(i)to examine the effect of S supply(+S/-S)on bioaccumulation of Cr in two rice cultivars(Hashemi and Lemont)after treatment with 1.0 mg Cr(III)and Cr(VI),(ii)to explore the influence of different levels of S supply(S0: 0 m M/ limited,S1: 1.75 m M /normal,S2: 3.5 m M /moderate and S3: 7 m M/ excessive)on Cr accumulation and bioavailability in rice plants,and(iii)to investigate the subcellular localisation and/or molecular bonding environment of Fe and Cr at the micro-scale in the root tip and mature root regions using combined imaging techniques.The main results of the conducted experiments are as follows:S addition significantly promoted the root and shoot growth of rice cultivars under Cr stress.A significant correlation was found between ACA-extractable Fe and Cr(r =0.73,p <0.05)in IP.Compared to + S treatment,Cr(1.49 to 0.69 mg kg-1)and Fe(38.03 to 13.07 mg kg-1)contents in IP without S treatment were markedly reduced,especially for Cr(VI).Cr content in roots and shoots was indicated to be at par between cultivars;however,it significantly differed for S and Cr treatments.Rice seedlings treated with Cr(III)showed a lower capacity for Cr accumulation in aerial tissues than Cr(VI)in the presence of IP,and the transport of Cr within plant cells was more evident in Cr(VI)treatment with IP than Cr(III)treatment.For both +S/-S seedlings,the concentration of both Cr species followed the sequence Cr-ACA(ascorbic citrate acetic)extract > Cr-root > Cr-shoot,with less Cr accumulated in shoots of +S seedlings to that of-S seedlings.S additions at rates of 1.75(S1)and 3.5(S2)m M resulted in the least decline in root(S1:10.7% and S2:12.7%)and shoot(S1:1.1% and S2:3.6%)growth of rice seedlings under Cr stress.The subcellular distribution of Cr in roots and shoots differed with varying S supply levels.Iron concentration in shoots was consistent with the level of Cr uptake.Our results also revealed that S treatment at a moderate level(S2)was more effective in suppressing the bioavailability of Cr in rice shoots than were the other levels.S-induced reduction in shoot Cr concentration,particularly from S1(4.08 mg kg-1)to S2(2.89 mg kg-1),was more likely attributed to the enhanced biosynthesis and copresence of GSH and PCs compounds in rice roots than the enhanced physical resistance of IP-induced by S on the root surface.The distribution pattern of Cr and micronutrients differed between the examined root regions and was characterised by element-specific ?-XRF analysis.XANES spectra further showed that domination of Cr accumulation in root epidermis and sub-epidermis layers over root surface IPs and shoots could be attributed to Cr(III)sorption/co-precipitation with crystalline ferrihydrite(Fh)minerals and organic acid(OA)anions.Based on the linear combination fitting(LCF)results,the Fh composition of Fe oxyhydroxide competed with OA anions for complexation with Cr(III)ions in outer cell layers of rice roots;this competition differed in and between the tip and mature regions.The epidermis layer of the mature root region contained the most abundant proportion of Cr(III)-OA species(>70%),which was well aligned with signals of co-location of 13C-enriched phytometabolites with Cr taken up from solution medium.In addition,Cr(III)-Fh complexes dominated over Cr(III)-OA complexes in subepidermis layer of both root regions.These results indicate that increased immobilisation of Cr in the IP of +S seedlings and its subsequent reduction in aerial tissues may likely shed some light on the barrier feature of IPs in the uptake of both Cr species by rice seedlings.Importantly,a sufficient amount of S supply to Crcontaminated paddy fields may be considered as a practical approach in reducing the potential health risk associated with Cr accumulation in rice grain.The poor barrier capacity of IP to Cr absorption in rice plants primarily ascribed to the level of applied Cr concentration and partly to the competition between Cr and S at the absorbing sites.Our finding suggested that a combination of chelation and exclusion mechanisms,involving the formation of Cr(III)-OA and Cr(III)-Fh complexes in the mature section,was responsible for Cr immobilisation in the roots.