Accumulation Of Cadmium And Arsenic In Different Leafy Vegetables And Tolerant Mechanism

Leafy vegetables are highly enriched species of heavy metals and are more susceptible to heavy metal contamination than other crops.Crops did not show significant physiological disturbances,but cadmium or arsenic of the edible part may well exceed the safety threshold,and the accumulation of cadmium or arsenic in vegetables is threatening human health.It was studied of cadmium（Cd）and arsenic（As）uptake and accumulation characteristics of some leafy vegetables cultured in the mildly heavy metal compound contaminated soil with pot experiment,and the aim was to assess and screen low-accumulated vegetables suitable for cultivation in Cd and As mildly contaminated soils,and provide basic data and theoretical basis for the rational and safe use of contaminated soil.In addition,the mechanisms of heavy metal accumulation and stress resistance of leafy vegetables with different tolerances were discussed from the absorption of trace nutrients such as iron,manganese,copper and zinc by cadmium,the activity of protective enzymes,the balance of plant chelating agents（PCs）and thiol pool.Twelve of ploughed layer soil samples of vegetable arable with different Cd&As pollution levels in Xi’an suburbs were collected,and pot experiments were conducted using7 kinds of leafy vegetables during March 6th to May 26th 2015,in the greenhouse of the College of Resources and Environment of Northwest Agriculture and Forestry University.The tested vegetables were:Spinach（Spinacia oleracea L.）,Cos lettuce（Lactuca sativa L.）,Romaine lettuce（Lactuca sativa L.var.longifolia）,Edible amaranth（Amaranthus mangostanus L.）,Water spinach（Ipomoea aquatica forsk）,Garland chrysanthemum（Chrysanthemum coronarium L.）.It was quantified of the yield,Cd and As content and absorption accumulation of seven leafy vegetables,and it was explored to assess the Cd&As thresholds of soil pollution safety for different vegetables with regression models.In this paper,the Cd accumulation and translocation of garland chrysanthemum,spinach,and lettuce were examined by soil pot culture and hydroponic experiments,and we compared the changes in nutrient element content of vegetables under Cd stress.The responses of oxidative stress markers,antioxidative enzymes activity,and thiol pool（cysteine,γ-glutamylcysteine,glutathione,and phytochelatins）content to Cd stress were assayed.The content of nutrient elements（Fe,Mn,Cu and Zn）in vegetables were also determined.Below are key research findings:（1）In cadmium-contaminated soil(Cd:0.3-2.4 mg kg^-1,As:12.6～28.3 mg kg^-1),Cd contents in the edible parts of spinach,cole,amaranth leaves,and lettuce exceeded the food safety critical value(0.2 mg kg^-1),and spinach and cole Cd even exceeded the standard more than 4 folds;while the water spinach stem&Garland chrysanthemum Cd content did not exceed the standard.Although the As content of the tested leaf vegetables did not exceed the standard,the arsenic content of the vegetables increased as increasing of soil arsenic content.（2）The enrichment coefficient of Cd in the seven kinds of vegetables was 0.083～0.491,and it decreased as follows:cole,spinach,cos lettuce,edible amaranth leaves>romaine lettuce,edible amaranth stems,water spinach leaves>>water spinach stems,garland chrysanthemum.The safety thresholds of soil Cd for spinach,cole,cos lettuce,romaine lettuce,edible amaranth,water spinach and garland chrysanthemum were 0.33,0.38,0.46,1.15,0.59～1.79,1.48～8.16 and 8.98～17.11 mg kg^-1,respectively.Among them,the thresholds of spinach,cole and lettuce were similar to the current standards(0.3-0.6 mg kg^-1),while the romaine lettuce,edible amaranth,water spinach,and garland chrysanthemum were all greater than the soil heavy metal pollution standards.The critical thresholds of soil As in the seven kinds of vegetables were 62.31,70.35,70.21,67.41,67.86～90.43,57.21～75.70 and 72.43～105.06 mg kg^-1,respectively,which all were higher than the current standards(25 mg kg^-1).As enrichment coefficient is 0.002～0.006,and the As enrichment coefficient of leaves of spinach and garland chrysanthemum was significantly higher than that other vegetables.（3）In the hydroponic Cd test,with the increase of Cd²⁺concentration,the total fresh weight,shoot and root fresh weight of garland chrysanthemum,spinach and lettuce were significantly decreased.The total plant tolerance index,shoot and root cadmium tolerance index of all three vegetables also decreased.The concentration of Cd in the shoots of vegetables was spinach>lettuce>garland chrysanthemum;however,the concentration of Cd in plant roots ranged as garland chrysanthemum>spinach>lettuce.The TF and SR ratios of garland chrysanthemum were significantly lower than those of spinach and lettuce.The ratio of shoot Cd absorption to root Cd（SR ratio）in the vegetables was exactly the opposite of that of TI.The growth inhibitory concentrations EC50 of Garland chrysanthemum,spinach and lettuce were 5.1,3.2 and 2.3 mg L^-1,respectively.The oxidative stress markers showed that under Cd stress,the oxidative stress of lettuce was the highest,while that of garland chrysanthemum was small.Garland chrysanthemum is resistant to Cd stress,lettuce is sensitive to Cd stress,and spinach is intermediate vegetable.（4）In the Cd stress treatment,except for the significant decrease ofγ-EC and root GSH content in lettuce shoots,the contents of cys,γ-EC and GSH in the shoots and roots of other vegetables remained stable or increased.The order of phytochelatin（PC）concentration in vegetable roots was consistent with the order of plant tolerance index（TI）:garland chrysanthemum>spinach>lettuce.The total content of phytochelatin（PC）in the upper part of spinach was higher than that of garland chrysanthemum and lettuce.（5）In the roots of lettuce,the activity of SOD,POD and CAT decreased significantly under Cd stress.However,these parameters of spinach and lettuce roots remained stable and even enhanced.The SOD activity and the aboveground CAT activity in the roots of lettuce were the lowest among the three vegetables.The POD and CAT activities of the root system of garland chrysanthemum were the highest among the three vegetables.（6）Under Cd stress,the amount of Cd absorbed by vegetables was positively correlated with the amount of Fe,Zn and Mn,but there was a negative correlation between the transfer of Fe,Mn,Cu,Zn and transport Cd from the roots to the shoots.Cd stress made the significant negative correlation（antagonism）disappeared between the absorption of Fe and Zn in the shoots of vegetables,and also eliminated the difference in the ability of nutrients transport from roots to shoots between different vegetables.The cadmium and arsenic enrichment coefficients of vegetables varied significantly,and so did their responses to moderate-mild cadmium-arsenic compound contaminated soils.There were substantially differences in their growth effects,metal contents,and soil safety thresholds etc.Therefore,screening low enrichment and anti-pollution vegetable varieties were a feasible option to cultivate mild heavy metal contaminated soils.The water spinach,and garland chrysanthemum had low Cd enrichment coefficient and could be recommended for cultivation on mild polluted soil.In conclusion,the Cd translocation and partition in plant,antioxidative defense,and PCs homeostasis played an important role in the Cd tolerance of vegetables.The high tolerance of garland chrysanthemum to Cd may be due to the strong activity of ROS scavenging enzymes（SOD,POD and CAT）,the strong chelation ability of heavy metals in the roots,and the lower Cd transport coefficient from root to the shoots.