Study On The Status Of Soil Cadmium Content In Citrus Producing Areas In Hunan Province And The Influence Of Soil Cadmium Pollution On Citrus Trees And Fruits

Cadmium(Cd)is one of the toxic heavy metals in the world.It is very easy to accumulate crops and then enter food chain.As a heavy metal element,cadmium poisons animals and plants,causing damage to the ecological environment and human health.In recent years,crop safety caused by soil heavy metal cadmium pollution has attracted much attention.This study investigated soil p H,organic matter and cadmium pollution of Citrus unshiu Marc.and Citrus Sinensis(L.)Osbeck orchards in Hunan province,and detected the cadmium content of citrus fruit in cadmium-contaminated soil.In laboratory,CK and 1,1.5,3 and 10mg/kg cadmium fertilization experiments were conducted on three-year-old potted plants.Physiological characteristics,biochemical indicators and.A 50mg/L cadmium-stressed hydroponics test was carried out on Poncirus trifoliata and C.sinensis Osb.×P.trifoliate Raf.,the root changes were observed within 30 days and expression pattern of suspected cadmium-response transcription factor CitMYB1 in the treated plants were studied.The main results were as follows:(1)A total of 278 soil samples from Citrus unshiu Marc.and Citrus Sinensis(L.)Osbeck orchards in different areas of Hunan province were tested.The results showed that the p H value of 84.2% was less than 6.5,and the whole soil was acidic.The organic matter content at 78.8% was in the suitable range for citrus growth(15.0-30.0g/kg)Among 201 soil samples in the main producing areas of citrus,6.77% had excessive cadmium content in the soil.The cadmium content in the exceeded samples ranged from 0.30 mg/kg to 0.90 mg/kg,which was relatively clean.Among the 77 soil samples from Changsha,Zhuzhou and Xiangtan areas,70.13% contained excessive cadmium.The exceeded cadmium content in the samples was 0.31 mg/kg at the lowest and 7.46 mg/kg at the highest,which was considered as heavy pollution.(2)A total of 46 cadmium-contaminated samples of Citrus unshiu Marc.and Citrus Sinensis(L.)Osbeck fruits from Changsha,Zhuzhou and Xiangtan areas were tested,and the cadmium content in all citrus fruit reached the pollution-free food standards of China(<0.05mg/kg).(3)Three-year-old Citrus unshiu Marc.and Citrus Sinensis(L.)Osbeck potted plants were used as materials.Plants were treated with 0(CK)and 1,1.5,3 and 10mg/kg cadmium.The results showed that root neck thickness,plant height and chlorophyll had no different between CK and treatments.Accumulation of cadmium in different organs was significantly different.The general trend showed roots >stems and leaves> pulp.Citrus root had weakest capacity to transport cadmium to above-ground part,cadmium is concentrated in the roots.(4)Sixty-day-old Poncirus trifoliata and C.sinensis Osb.×P.trifoliate Raf.rootstock seedlings were used as the material treated with 0(CK)and 50mg/L cadmium in hydroponics culture for 30 days.Leaves of Poncirus trifoliata in treatment group became red and fell off,and the growth of lateral root of C.sinensis Osb.×P.trifoliate Raf.was inhibited.(5)Three-year-old Citrus Sinensis(L.)Osbeck potted plants were used as materials q RT-PCR analysis indicated that CitMYB1 expression in root was 2.9 times,3.1 times and1.7 times higher than CK after cadmium treatment.Sixty-day-old Poncirus trifoliata and C.sinensis Osb.×P.trifoliate Raf.rootstock seedlings were used as the material.It was found that CitMYB1 expression in two varieties rootstock was 2.4 times and 1.3 times higher than that in control,CitMYB1 is a cadmium-responsive factor and may play potential roles in the plant adaption to cadmium stress.Above all,soil of most citrus orchards in main producing areas of Citrus unshiu Marc.and Citrus Sinensis(L.)Osbeck,Hunan province was not polluted by cadmium.The cadmium content in two varieties citrus fruit reached the pollution-free food standards of China(< 0.05mg/kg)that grown on cadmium-contaminated soil.Cadmium mainly accumulates in root of citrus plant and has a weak transport capacity to the above-ground part.Although it does not affect the safety of fruits,the above-ground part of the trees is still damaged dependently.Gene CitMYB1 plays a role in citrus responding to cadmium stress.