| Some agricultural soils in China have been seriously contaminated with heavy metals,particularly for cadmium and mercury.The safety problems of agricultural products caused by contaminated soil frequently occurred.The "Hg wheat" and "Cd wheat" have attracted public attention.Soil pollution prevention and remediation must be implemented.Screening and planting low heavy metal accumulating wheat cultivars may be an effective approach to solve the above problem.However,the unified selection criteria for low-accumulating-cultivar remain to be established and the mechanism of low accumulation mercury or cadmium in wheat cultivars is not clear.So we selected generalized winter wheat cultivars in Huang-Huai-Hai Plain to investigate the accumulation characteristics of mercury or cadmium among different wheat cultivars,and screened low mercury or cadmium accumulating cultivars.We further investigated the difference of rhizospheric environment between high-and low-accumulating-cultivar to reveal why some wheat cultivars accumulate lower mercury or cadmium than other cultivars.The generalized winter wheat cultivars in Huang-Huai-Hai areas of China were evaluated for mercury resistance and mercury accumulation in grain under greenhouse conditions.The mercury treatments were assigned to three levels:0.1 and 5 mg kg-1.The responses of 11 wheat traits to mercury stress were investigated,including morphological traits,biomass traits and yield-related traits.The mercury resistance coefficient,resistance index,genetic variation coefficient and broad sense heritability of 11 investigated traits were calculated to establish an evaluation method for mercury resistance of wheat cultivars.Results demonstrated that the plant height and uppermost internode length significantly increased,while spikelet number significantly decreased under low mercury stress.Under high mercury stress,the plant height significantly increased,while yield-related traits,including total grain number,fresh grain yield,and dry grain yield,significantly decreased.An improved membership function was constructed with 11 traits of medium to high heritability(0.31-0.68).and then a mercury tolerance evaluation system for wheat cultivars was established through discriminant analysis.Discriminant analysis integrated the mercury resistance coefficients of effective tiller number,fresh grain yield.and dry biomass into the discriminant function under low mercury stress and the mercury resistance coefficients of dry grain yield and dry biomass under high mercury stress.The correct rates of identification were respective 90.0%and 83.3%under low and high mercury stress.The evalution system broke through the simple identification model with yield as a mere evaluation index.Based on the mercury resistance,mercury concentration and bio-concentration factor in wheat grain,ultimately,wheat cultivars Nongda-3163(ND3163)and Gaocheng-8901(GC8901)were screened for high mercury resistance and low mercury accumulation,and the cultivars Jimai-21(JM21)and Taishan-21(TS21)were mercury sensitive and high mercury accumulation.This research fills the knowledge gap on mercury resistance and mercury accumulation characteristics of wheat cultivars generalized in Huang-Huai-Hai areas,and provides germplasm resources for the safe production of slightly or moderately mercury contaminated farmland.In this study,we further selected two low-mercury-accumulating wheat cultivars(ND3163,GC8901)and two high-mercury-accumulating wheat cultivars(JM21.TS21)to clarify the reason of low mercury accumulation in wheat cultivars:low uptake or less translocation from root to aboveground parts.The difference of rhizospheric organic acids and microorganisms between low-and high-mercury-accumulating cultivars were also investigated to explore the specific rhizosphere microenvironment of low-mercury-accumulating cultivars.Under low mercury stress,low-mercury-accumulating cultivars ND3163 and GC8901 accumulated significantly lower mercury concentrations in their roots than high-mercury-accumulating cultivars JM21 and TS21.Under high mercury stress,no significant difference was detected in mercury concentrations in the roots among four cultivars,but ND3163 and GC8901 had significantly lower translocation factors from roots to leaves,glumes,and grains than JM21 and TS21.This shows that under low mercury stress,the low mercury accumulation in wheat grains is due to low mercury uptake in the roots,while under high mercury stress,the low mercury accumulation in wheat grains is due to lower translocation factor from root to the aboveground parts.Under low mercury stress,ND3163 and GC8901 secreted lower amounts of oxalic acid and citric acid in rhizosphere soil than JM21 and TS21.The abundances of gram positive bacteria,gram negative bacteria and fungi in rhizosphere soil of ND3163 and GC8901 were significantly lower than those of JM21,while abundance of protozoa in rhizosphere soil of GC8901 was significantly higher than in high-mercury-accumulating cultivars.The specific rhizosphere microenvironment of low-mercury-accumulating cultivars was less citric acid and oxalic acid,and the higher abundance of protozoa.Improving the beneficial relationship among rhizospheric organic acids,microorganisms and wheat cultivars is a realizable strategy to ensure wheat safety for mercury.Cadmium concentrations in 72 wheat cultivars generalized in Huanghuaihai area were identified by field experiments(field 1),and the stability of cadmium accumulation in these cultivars was tested in fields 2 and 3.The effects of cadmium on microelements(zinc,copper,iron,manganese,and boron)uptake in grains were also investigated.The correlation between agronomic traits and grain cadmium concentration was discussed to provide a simple and quick method to screen low-cadmium-accumulating wheat cultivars.Cluster analysis based on grain cadmium concentrations divided all wheat cultivars in field 1 into 5 categories:extremely low-cadmium-accumulating cultivar,low-cadmium-accumulating cultivar,medium-cadmium-accumulating cultivar,high-cadmium-accumulating cultivar and extremely high-cadmium-accumulating cultivar.A total of 24 wheat cultivars were screened for extremely low-cadmium-or low-cadmium-accumulating cultivars,and the cadmium concentrations in grains of these cultivars were below 0.1 mg kg-1,which is the maximum permissible concentration(MPC)for cadmium in wheat grain in China(GB2762-201 7).Nine of the 24 low-cadmium-accumulating wheat cultivars identified in field 1 exhibited unstably low cadmium accumulation characteristics in fields 2 and 3.We further investigated the differences of microelements concentrations in grain between low-and high-cadmium-accumulating cultivars.The results showed that five low-cadmium-accumulating cultivars contained significantly lower zinc concentrations in grain at least two experimental fields than high-cadmium-accumulating cultivars.Another one low-cadmium-accumulating cultivar also had lower zinc,copper and manganese concentrations in grain than high-cadmium-accumulating cultivars.There is no significant difference in grain iron concentrations between low-and high-cadmium-accumulating cultivars.Ultimately,nine wheat cultivars showed relatively stable low-cadmium and moderately high microelements accumulation in grain,which were recommended for planting in slightly and moderately cadmium contaminated farmland for food safety and high quality.The thousand kernel weights and yields of these nine wheat cultivars were relatively high in three fields.The correlations between wheat agronomic traits and grain cadmium concentration showed that grain cadmium concentration correlated positively with the dry weight of stem and root.and negatively with spike length.There was no correlation between grain cadmium concentration and wheat yield.Therefore,screening wheat cultivars with low cadmium concentration and high yield was feasible.Wheat cultivars with longer spike tended to accumulate less cadmium in grains.This study revealed that screening low-cadmium-accumulating wheat cultivars should pay particular attention to the stability of low-Cd accumulation characteristics and grain zinc concentrations.We found that some low-cadmium-accumulating wheat cultivars also have low zinc uptake.In this paper,high-throughput sequencing technology was used to study the differences in rhizobacteria diversity and community structure among wheat cultivars with different cadmium and zinc accumulation abilities,to explore the correlations between rhizobacteria and grain cadmium and zinc concentrations.The dominant phylum of rhizobacteria in wheat were Proteobacteria,Actinobacteria,Acidobacteria,Bacteroidetes,Gemmatimonadetes,Chloroflexi,Verrucomicrobia,Planctomycete and Firmicutes,which accounted for more than 95.0%of the total rhizobacterial communities.Low-cadmium-accumulating cultivars had significantly higher rhizobacteria diversity than high-cadmium-accumulating cultivar.At the phylum level,the relative abundances of Gemmatimonadetes,Chloroflexi,Planctomycete and Firmicutes were higher in rhizosphere soil of low-cadmium-accumulating cultivars than high-cadmium-accumulating cultivar.The relative abundances of Proteobacteria was higher in rhizosphere soil of high-zinc-accumulating cultivar than low-zinc-accumulating cultivars.At the family level,the relative abundances of Gemmatimonadaceae,Sphingomonadaceae,Beijerinckiaceae,Nitrosomonadaceae,Nocardioidaceae,Intrasporangiaceae,Micromonosporaceae,Pseudonocardiaceae.Geodermatophilaceae,Gaiellaceae in rhizosphere soil of low-cadmium-accumulating cultivars were significantly higher than that in rhizosphere soil of high-cadmium-accumulating cultivar.The relative abundances of the family of Xanthomonadales was significantly higher in high-zinc-accumulating cultivars than in low-zinc-accumulating cultivars.The low-cadmium-and high-zinc-accumulating cultivars had significantly higher relative abundances of Alphaproteobacteria and Gemmatimonadaceae than low-cadmium-and low-zinc-accumulating cultivars and high-cadmium-and high-zinc-accumulating cultivars.Tax4Fun function prediction showed that low-cadmium-and high-zinc-accumulating cultivars had higher relative abundance of aerobic_ammonia_oxidation and nitrification functional bacteria in rhizosphere soil than high-cadmium-and high-zinc-accumulating cultivars,and higher relative abundance of aerobic_nitrite_oxidation functional bacteria than low-cadmium-and low-zinc-accumulating cultivars.The higher abundances of Alphaproteobacteria and Gemmatimonadaceae in rhizosphere soil are beneficial to low cadmium and high zinc accumulation in wheat grain.We collected all field papers on the accumulation of cadmium in Chinese wheat cultivars published from 2010 to 2020 for Meta analysis,recording grain cadmium concentration,soil cadmium concentration,grain cadmium bio-concentration factor,and soil physical and chemical properties(pH,organic matter,cation exchange capacity,available potassium,alkaline nitrogen).Correlations between the cadmium bio-concentration factor and the soil physical and chemical properties were analyzed.Results showed that cadmium bio-concentration factor in wheat grain correlated positively with soil cadmium concentration,and negatively with soil pH,organic matter,and alkaline nitrogen.No significant correlations were found between cadmium bio-concentration factor of wheat grain and cation exchange capacity and available potassium.When soil pH>6.5.the proportion of cadmium bio-concentration factor<1 was 99.0%,mainly ranged from 0 to 0.1 with the proportion of 71.9%.When 5.5<pH<6.5,the proportion of cadmium bio-concentration factor<1 was 72.7%,mainly ranged from 0 to 0.6 with the proportion of 72.0%.When soil pH<5.5,the proportion of cadmium bio-concentration factor<1 was only 5.90%.It is not effective to regard cadmium bio-concentration factor<1 as one of the indicators for screening low-cadmium-accumulating wheat cultivars when soil pH>6.5.When soil organic matter and alkaline nitrogen were rich,the proportion of cadmium bio-concentration factor<1 was 95.0%and 91.5%,respectively.Farmland with higher pH,organic matter and alkaline nitrogen is more conducive to ensuring the safety of wheat consumption for cadmium. |
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