| Citrus is the most consumed fruit in the world.China is not only a large citrus growing country but also a large citrus consuming and exporting country.With a total growing area of 1.91 million hectares and an annual production of 20.59 million tons,China took the first place among the world citrus growing countries in 2007.Currently,citrus are cultivated in 20 provinces(municipalities or autonomous regions) in China.Citrus growing is the main source of income for farmers in mountainous areas of southern China.With the increase in living standard and the public awareness of food quality and safety problems,consumers require higher and higher quality of fruits with good appearance and taste and pay more attention to food safety issues such as pesticide residue and heavy metal(HM) pollutions.With excessive application of pesticides and fertilizers,and irrigation of sewage water,soils of citrus orchards have become more polluted with heavy metals.Such pollution is under close monitoring during citrus production in developed countries.However,heavy metal pollution in citrus orchards in china has not received much attention until recently.Scattered reports dealt with environmental quality evaluation of citrus orchards.Few report discussed the absorption and accumulation of heavy metals in citrus trees.In this study,potted young Newhall navel orange trees (Citrus sinensis Osb.) on trifoliate orange(Poncirus trifoliate Raf.) rootstock were used to study the uptake and accumulation patterns of heavy metals by citrus from soils contaminated with arsenic(As), lead(Pb) and mercury(Hg).Calcareous purple soil was used as pot-culture medium.At the beginning, heavy metals were applied into the soil at the following combinations and concentrations:As alone (30,60 and 120mg·kg-1),Pb alone(500,1000 and 2000mg·kg-1),Hg alone(1.0,2.0 and 4.0mg·kg-1), As+Pb+Hg combined(40mg·kg-1As + 500mg·kg-1 Pb + 1.5 mg·kg-1 Hg,and(80mg·kg-1 As+1000mg·kg-1 Pb+3.0mg·kg-1 Hg).Control soil contained basal concentrations of As,Pb and Hg at 7.10,24.19 and 0.16mg·kg-1,respectively.The experiment started in the spring of 2004 with one-year-old Newhall navel orange trees.After 3 years,the results are shown in the followings:1,Arsenic treatment had the most pronounced effect on citrus plant growth.The height,canopy size,weight of the whole plants and the trunk thickness were significantly reduced by As3 treatment as the corresponding measurements were 83.8%,74.44%,74.51%,and 77.77%of the controls.Lead and mercury had apparent retardant effect on trunk thickness although they showed little effect on plant height,canopy diameter and weight.2,During the three-year experiment period,the concentrations of As,Pb and Hg in soils were stable,at about 30%～86%,46%～64%,and 84%～118%of their originally added concentrations, respectively.Results showed that both As and Hg had similar accumulation patterns of roots＞leaves＞sticks＞peel＞pulp,while Pb accumulation was in the order of roots＞sticks＞leaves＞peel＞pulp.As, Pb and Hg accumulations in roots with high concentration treatments were 7.2,773.6 and 0.50 rng·kg-1,respectively.In contrast,accumulations in fruit peel and pulp were low since the highest As, Pb and Hg concentrations detected in fruit peel were only 0.15,0.06 and 0.006 mg·kg-1,respectively, and those in fruit pulp were only 0.06,0.05 and 0.004mg·kg-1,respectively.Therefore,the highest contents of heavy metals in fruit of this experiment were far lower than the maximum residue limits of China National "Green Food" Standard.Combined As+ Pb + Hg treatment did not show antagonistic or synergistic effects on their accumulations in plant organs or tissues.3,Heavy metals accumulated mainly in roots and leaves.The underground parts accumulated more than did the above ground parts.As,Pb and Hg accumulations in roots and leaves accounted for 92.20%～95.25%,87.93%～93.15%and 87.70%～95.82%of the respective total accumulations,while those in controls accounted only for 62.80%,52.55%and 77.19%.
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