Mitigation Of Cd Accumulation In Rice From Cd-contaminated Paddy Soil By Foliar Dressing

Although many measures have been taken to reduce heavy metal accumulation in polluted farmland,it is a pressing need to to curtain the transfer of Cd from contaminated sources to ecosystems.Foliage dressing has been widely used as nutrition fertilizer in agricultural practices for centuries;however,it is just in recent years that foliage dressing received attention for the alleviation of heavy metal pollution in edible plant parts.Although there exist some limitations for its application such as dependence on climate,inability to deal with high metal concentrations,and uncertainty in mitigation mechanism,the foliage dressing has high public acceptance as being cost-effect and environmentally friendly.The foliar treatment offers a potential for the remediation of the medium/or mild Cd-contaminated paddy soil.Mineral elements play an important role in energy storage,plant structure,cofactor for enzymes and electron transfer reactions.Foliar spray of mineral elements might alleviate oxidant stresses caused by heavy metals,thus reducing the heavy metal accumulation in plants.The mechanism of foliar dressing affect key Cd uptake at cell level or metabolism level have not been fully clarified.This study was to compare the remediation efficiency of different mineral elements,and their applied concentration,applied time to optimize the method of foliar dressing.The application effect and detoxification mechanism were also explored using photosynthetic-chlorophyll fluorescence system,fourier transform infrared spectra analysis and microscopic analysis.The main results include:1.A pot experiment was conducted to optimize foliar composition and dosage.The remediation efficiency of new proposed S,P was assessed by comparing with that of commonly used Si,Se,Mg.The change of plant growth,photosynthesis,lipid peroxidation,antioxidant enzyme activities,antioxidants,indicating that Cd uptake in rice was affected by a wide range of physiological process.All treatments increased rice growth as compared with the control plants.After the comparison of all mineral elements,S and P showed better performance of decreasing rice grain Cd concentration and enhancing grain yield.2.Field experiments in four consecutive rice seasons were performed to investigate the effectiveness and mechanisms of foliage dressing.0.5g/L S supplied as Na₂S,0.5 g/L P supplied as NaH₂PO₄ were confirmed to have better performance of decreasing rice grain Cd concentration?-69%?ahd enhancing grain yield?+31%?.The foliar treatment reduced the amount of MDA as compared with the control.The low level of oxidative of stress with foliar dressing corroborated the protective effect on the cell membrane by reducing the lipid peroxidation in rice under Cd stress.The increase of antioxidant enzyme activities with foliar dressing may be attributed to the decreased MDA content resulted from the reduction of Cd concentration in rice,thereby enhancing the ability of plant to cope with Cd stress.The increased plant growth might be due to the low Cd accumulation after foliar dressing,which may reduce the oxidative stress and direct damage to rice.The mass balance of Cd in double-rice cropping systems during the monitoring period showed that the net amount of Cd in 2017 was lower than that in 2016,indicating the effective reduction of Cd accumulation and ecological risks by foliar dressing.3.The initial pot studies and field studies showed that foliar spray of S and P were effective for reducing Cd concentration in rice and increasing grain yield.However,the physiological mechanisms remain less clear on how the foliar dressing of S and P alleviates Cd toxicity in rice.A hydroculture experiment was conducted to clarify the possible mechanisms related to S/P-induced Cd tolerance in rice.The addition of S and P reduced the oxidative stress by intercepting excessive reactive oxygen species,therefore protecting the macromolecules.The increase of photosynthetic parameters,i.e.the maximum photochemical efficiency of PS? reaction center,the actual PS? photochemical efficiency indicated that the foliar treatment alleviated the toxicity of Cd to PS?.The decrease of non-photochemical quenching coefficient indicated the increase of photochemical reaction efficiency with more absorbed light energy for photochemical reactions.Fourier transform infrared spectra showed that the foliar treatment stimulated the syntheses of lignin,lipids,aliphatic acid,polysaccharides,carboxylate and proteins.Micrographs of transmission electron microscope also revealed the reduced mobility of Cd in cells.This study provided preliminary evidence that the effect of foliar application of S and P likely retained the structure of plant cells and effectively regulated protein expression by alleviating Cd stress without sacrificing yield.4.Cd-contaminated paddy soil has become a global agricultural safety issue.The applicaition of foliage dressing with mineral elements to alleviate Cd toxicity in rice might offer a cost-effective and practical strategy for safe food production.