Uptake Contribution,Translocation,and Phytotoxicity Of Atmospherically Deposited Cadmium By Leaves Of Crops

Recently,atmospheric deposition has gradually become an essential source of cadmium(Cd)pollution in the agricultural ecosystem,which can enter crops via roots and leaves,accumulate in crops,and eventually threaten human health through the food chain.Compared to root uptake,foliar uptake processes of Cd,and the differences of metabolism and phytotoxicity in the plant under root and foliar uptake pathways have not been clearly recognized.In this thesis,atmospherically deposited Cd was simulated using cadmium sulfide nanoparticles(Cd S_N).Typical crops(i.e.,water spinach,pak choi,chilli and wheat)were used as experimental objects to conduct foliar and root pot experiments.Using elemental analysis,the subcellular analysis and a variety of microscopy technologies,we clarify the mechanisms of Cd uptake,transport and accumulation in plants through leaves and toxic characteristics,and compare toxic and metabolic differences between two uptake pathways(i.e.,root and foliar).Contributions of two uptake pathways to Cd accumulation in plant tissues were quantified through Cd isotope analysis.This study will provide theoretical support for the selection of safe production technology of food.Main results are as follows:(1)Stomata are the main pathway for foliar Cd uptake.When Cd enters the plant,part of it is fixed in the leaf cells(mainly in cell wall),and the rest is transported to the roots and stems of the plant,which eventually elevated Cd concentrations of water spinach and pak choi tissues.Under highest concentration treatment level,foliar Cd concentration in spinach and pak choi were 100 and 36 times than those in CK,respectively.Despite of slight changes in fresh biomass,photosynthetic parameters,antioxidant enzymatic activity,and other elements uptake of water spinach and pak choi after foliar Cd exposure,the estimated daily intake of pollutant(EDI)values of the higher concentration treatments of water spinach and pak choi were 2.35 to 8.61 times of the tolerable daily intake(TDI)respectively,and the hazard quotient(HQ)values were above one(>1),both of which suggesting that the consumption of these vegetables would pose a serious health hazard.Furthermore,the growth and Cd accumulation of plants demonstrated no obvious differences between Cd S_N and the corresponding Cd S bulk particles(Cd S_B),which indicated that exposure concentration in the particle size range of this study regulated the foliar uptake and accumulation of Cd.(2)Single root or foliar uptake pathways increased the concentration of Cd in crop tissues and subcellular,but the effect of the two pathways on the concentration of Cd in the plant was antagonistic.Root and foliar uptake pathways dominated Cd concentration changes in tissues and subcellular of roots and leaves,respectively.Under two uptake pathways,the subcellular distribution pattern of roots,stems,leaves and fruits of chilli was consistent.In chilli,Cd was mainly distributed in soluble fraction of roots,stems and fruits(30.8–68.0%),and in organelle fraction of leaves(48.2–68.7%).In wheat,most Cd was present in organelle fraction of leaves(45.6–74.7%),and in organelle and soluble fraction s of roots and stems.In addition,the transport process of Cd in plant was further explored by a variety of microscopic techniques,which showed that the deposited Cd on the surface of chili leaves entered leaf tissue mainly via the stomata,then part was fixed in leaf cells,the rest was mainly through phloem transport to the plant roots,fruits and other organs.(3)Single root or foliar uptake had no significant effect on fresh weight and chlorophyll content of chilli,while concurrent uptake through root and foliar showed additive effect.Single root or foliar uptake affected the enzyme activities of chilli and wheat,but effects of concurrent uptake through root and foliar was antagonistic,with larger effects on chilli and wheat enzyme activities through root uptake than foliar uptake.Based on nontargeted metabolomic analysis,we found that the number of differential metabolites in roots,stems and leaves with root uptake were significantly more than that of foliar uptake,except fruit.The metabolite expression levels in leaves of chilli were significantly upregulated by Cd absorption under both foliar and root pathway,whereas more than 63%and 57%of metabolites in stems and fruits were upregulated after foliar uptake as compared to 69%and 81%downregulated in stems and fruits after root uptake.Both of two uptake pathways interfered with osmotic regulation,energy metabolism and amino acid metabolism of chilli,resulting in significant enrichment of sphingolipids metabolism,starch and sucrose metabolism,and arginine biosynthesis.However,alanine,aspartic acid and glutamate metabolism were upregulated of the stems and fruits under foliar uptake,and purine metabolism was upregulated in leaves.After root Cd uptake,pentose phosphate pathway of fruits and stilbenoid,diarylheptanoid and gingerol biosynthesis of roots were significantly downregulated,and biotin metabolism of fruits were significantly upregulated.(4)Results from Cd isotope show that when root and leaf Cd were exposed simultaneously:Cd of chilli roots,stems,fruits,and wheat roots,glumes and grains were mainly from the root uptake.However,Cd of chilli leaves,wheat stems and leaves were mainly associated with foliar uptake.Cd through root uptake contributed 88.1%,116.6%and 90.4%to root,stem and fruit of chilli,and 58.3%,94.7%and 93.0%to root,glume and grain of wheat.Foliar uptake contributed 73.6%to chilli leaf,78.2%and 111.6%to wheat stem and leaf.