Screening Of High And Low Cadmium Accumulation Cucumber Varieties And Their Difference Of Cadmium Accumulation

With the rapid development of human industrial and agricultural activities,wanton discharge of industrial "three wastes",irrational application of chemical fertilizers and pesticides,and sewage irrigation occur frequently,the problem of cadmium(Cd)pollution has become increasingly serious.The over-standard rate of Cd contaminated soil in China is as high as 7.0%,ranking first in heavy metal pollution.As a non-essential element with strong migration and high toxicity,Cd is easily absorbed by plants and harms human health through the food chain.At present,screening Cd lowaccumulation crop genotypes to reduce Cd content in edible parts of crops is an economical and effective way to reduce Cd content in foods.Cucumber is one of the largest cultivated vegetable plants in the world.It is sensitive to Cd response and has potential risks.This paper intends to use 32 cucumber varieties as materials to screen high-and low-accumulation Cd fruits through pot experiments,and to analyze their tolerance to Cd and the ability to absorb and transport Cd;We explorate deeply the differences in Cd accumulation and antioxidant system in various parts of Cd high and low accumulation varieties under different Cd concentration treatments,in order to providing theoretical and data support for the safe use of Cd-contaminated soil and the safe production of vegetables.The main research results are as follows:(1)Based on potted plant simulation tests at two concentrations of 1.8 and 3.6 mg/kg,the responses of 32 cucumber genotypes to Cd pollution were analyzed from fruit biomass and Cd content,respectively.Using fruit Cd content and national Cd food safety standards as the main indicators,the Cd low-accumulation varieties were selected by cluster analysis as follows: Koukoutian,Gaochun,Jinyan 4,Tangshanqiugua and Jinyou 1.The high accumulation varieties are Yanbai,Yuyuan 1710,Shengshi 999,Jiaduobao F1,Meiweichunqiu,Jinyan 9 and Sihan.Combined with the biomass index,we obtained a Cd low-accumulation cultivar Koukoutian(KKT)with high biomass and low Cd content in the edible part of cucumber and a Cd high-accumulation cultivar Jinyan 9(JC9)to carry out follow-up studies.(2)In concentration of 0 and 100 ?mol/L Cd-contaminated nutrient solution,8 indicators of seedling stage of 32 cucumber varieties were determined as relative root length,relative root elongation,relative fresh weight,relative dry weight,relative plant height,relative plant height elongation,relative fresh weight above ground,and relative dry weight above ground.Principal component analysis and functional membership analysis were used to establish a com prehensive evaluation system of cucumber seedling resistance to Cd stress,and the tested varieties were divided into 4 levels: 6 Cd-tolerant varieties;18 Cd-tolerant varieties;8 Cd-sensitive varieties.At the same time,the Cd absorption and transport ability of the test seedlings was analyzed and it was found that the Cd content of cucumber fruit was extremely significantly positively correlated with the Cd transfer coefficient at the seedling stage,and was significantly negatively correlated with the Cd tolerance capacity at the seedling stage.(3)The changes of growth and photosynthetic pigments of cucumber with high and low Cd accumulation were investigated under five Cd treatment concentrations of 0,10,50,100,and 200 ?mol / L.With the increase of Cd treatment concentration,the plant height and root length of KKT and JC9 had an effect of first promotion and then inhibition,and the inhibition effect became more significant as the stress deepened.Under the same Cd treatment,JC9 plant height and ro ot length decreased much higher than KKT,indicating that the growth index of JC9 was inhibited more seriously than KKT at a high concentration of Cd,and it was more sensitive to Cd pollution.Cd inhibited the chlorophyll a,chlorophyll b,carotenoid,and total chlorophyll contents of KKT and JC9,and the inhibitory effect was more obvious with the increase of Cd concentration.Among varieties,KKT photosynthetic pigments were less affected by Cd,and leaf senescence was slower.(4)Under hydroponic conditions,with the increase of Cd treatment concentration,the contents of malondialdehyde(MDA),ascorbic acid(As A),and antioxidant enzymes(POD)in KKT and JC9 leaves increased significantly.The activity increased first and then decreased,reaching the maximum at a Cd concentration of 10 ?mol/L;the glutathione(GSH)content did not change significantly;the superoxide dismutase(SOD)activity of KKT leaves increased first and then decreased and reached the maximum value under 50 ?mol /L treatment;however,SOD activity in JC9 gradually decreased.The MDA content and POD activity in the root system were significantly increased;the SOD activity,CAT activity,GSH,and As A contents all showed low promotion and high suppression.The GSH content,As A content,CAT activity of the two varieties,as well as the content of Cd in KKT root system,reached the maximum value at 100 ?mol/L cadmium.The maximum SOD activity in the roots of JC9 reached the maximum under 10 ?mol/L cadmium treatment.Among varieties,the MDA content in KKT plants was significantly lower than JC9,while SOD and CAT enzyme activities were significantly higher than JC9,indicating that the membrane damage to KKT was lighter than JC9 and KKT showed better tolerance.The content of GSH an d As A in JC9 roots was significantly higher than that in KKT,indicating that antioxidant enzymes played a major defense role in KKT;while As A-GSH in JC9 played a major role in scavenging reactive oxygen species.(5)Under hydroponic conditions,as the Cd concentration increased,the Cd transfer coefficients of KKT and JC9 gradually decreased,and the ability of two kinds of cucumbers to transfer Cd from the root system to the ground decreased.The transfer ability results showed JC9>KKT,and the difference was significant when Cd> 50 ?mol/L,which indicated that JC9 had a stronger ability to transfer Cd to the aboveground than KKT,and the damage to aboveground part was more serious.The results of the subcellular distribution of Cd showed that the subcellular distribution of Cd in the leaves and roots of the two varieties was soluble part> cell wall> organelles.The Cd content in subcellular component of JC9 was higher than that of KKT except for the Cd content of the root cell wall.The proportion of Cd in soluble components of JC9(69.34%? 78.15%)in leaves was significantly higher than that of KKT(63.14% ? 74.02%).when Cd> 10 ?mol/L,the proportion of Cd in cell wall components of KKT(30.57% ? 46.42%)was significantly higher than that of JC9(29.32% ? 32.91%).Under high Cd stress,Cd low-accumulation varieties could fix more Cd to the root cell wall,while Cd-high accumulation varieties tended to transfer Cd to the soluble components of the leaves.At the same time,it was found that the effect of the in teraction between exposure time and concentration on the Cd content of plant subcellular components was greater than that of time or concentration treatment separately.