1、1Study of the response of wheat to U and Cd combined stress and their uptake and accumulation charactAbstract. Water culture experiment was adopted to investigate the effects of U and Cd combined stress at different concentrations on the uptake and accumulation of U and Cd and physiological and bioc
2、hemical characteristics in wheat seedlings. The experiment results showed that the U and Cd uptake by wheat seedlings was higher in roots than in aboveground parts when the total concentration of U and Cd was 5 mg/kg or 20 mg/kg， and the transfer factors （TFs） of U and Cd in wheat seedlings were abo
3、ve 1 when the total concentration was 100 mg/kg. But low concentration of U and Cd were rarely accumulated in wheat seedlings， indicating that wheat seedlings have good tolerance to U and Cd combined stress at low concentration. The experiment results also showed that with the increase in the total
4、concentration of U and Cd， the root activity of wheat seedlings first increased and then decreased， and it exhibited a significant increase when the concentration was 20 mg/kg. Also， with the increase in concentration， the chlorophyll content of wheat seedlings 2first increased and then decreased， w
5、hile the SOD activity first decreased and then increased. Key words： Wheat Uranium Cadmium Uptake Accumulation Physiological response CLC X591 Introduction Thanks to the development of nuclear industry and extensive application of nuclear technology， as well as activities in other fields， such as in
6、dustry， agriculture， energy， military， transportation， medical treatment and public health， radionuclide pollution has become one of the important environmental problems difficult to solve nowadays. Radial generated in the process of radionuclide disintegration is the source of such pollution. As th
7、e “death” of short-life nuclide happens， primarily such long-life fission products and nuclear materials as 3H， 137Cs， 90 Sr， 239Pu and U form long-term pollution 1. After these radionuclides enter water and soil， they not only severely damage human beings physical health through food chain2， but al
8、so form big obstacles in eliminating these nuclides， especially the elimination of large-area and low-amount radionuclides in soil， which is far more difficult. In addition， due to 3mining， metal smelting， and agricultural application of industrial sewage and sludge， a large amount of poisonous and
9、detrimental heavy metal elements enter soil system， which will not only reduce the number of microorganism and soil enzyme activity， but also restrain degradation of organic pollutants in soil， as well as respiratory metabolism， ammonification and nitrification of soil. At present， the area of agric
10、ultural acreage being polluted by such heavy metals as arsenic， cadmium， chromium and lead in China is approximately twenty million hectares， which equals one fifth of the total agricultural acreage， and the amount of grain output reduces over ten million tons due to heavy metal pollution every year
11、 in China. The amount of agricultural products with excessive Cd is 1.46109kg3 every year. Such combined pollution of nuclides and heavy metals directly or indirectly brings hidden dangers to the health of both people and livestock， and also has severe adverse influence on ecological environment. Wh
12、eat is a kind of gramineous plants planted in a wide range all over the world and also a kind of food crop whose total output ranks the second in the world. Therefore， whether wheat will gradually transport many elements detrimental to human health from root to grain is extremely important. This exp
13、eriment 4takes wheat as material， simulates soil pollution through short-term water culture experiment and studies the features of wheats absorption and enrichment of U and Cd， as well as its physiological and biochemical reaction under such stress， thus offering certain scientific proof for the saf
14、ety of plant growth in contaminative medium and its available values. 1. Materials and methods 1.1 Experimental materials The plant tested was wheat which come from The Seeds of Mian Yang Corporation. The Uranium was applied with the from of UO2（NO3）26H2O（AR， The chemical regament purchasing dump of
15、 China pharma ceutical company in Beijing） ， The Cadmium was applied with the from of Cd（NO3）24H20（AR， The Kelon chemical reagent factory of Chengdu）. 1.2 Experiment methods 1.2.1 Plant culture This experiment adopts water culture in Life and Science Laboratory Building in Southwest University of Sc
16、ience and Technology， Mianyang， Sichuan. Sow wheat seeds respectively in soil with pearlite， and after the seedlings grow five to six pieces of leafs， transfer them to a 2L plastic bucket with 5Hoagland nutrient solution. This bucket is covered by a cystosepiment with holes on it， set one seedling t
17、o each hole， ten to each bucket， four treatments in total， and change nutrient solution each three days. The whole culture lasts one month. 1.2.2 Experiment design After one month of culture， adopt combined stress of UO2（NO3）26H2O and Cd（NO3）24H20 to deal with wheat seedling separately and conduct a
18、dsorption test. The detailed methods are： take Hoagland nutrient solution as the basis， and add UO2（NO3）26H2O and Cd（NO3）24H20 combined conditioning fluid （Table 1） separately， change nutrient solution culturing wheat seedling to UO2（NO3）26H2O and Cd（NO3）24H20 conditioning fluid. Repeat three times
19、with each ten seedlings. After seven days of treatment， use a part of treated materials to test U and Cd accumulated content of wheat； and the other part to test root activity， chlorophyll content and superoxide dismutase by taking the second piece of plant respectively. Table 1 Concentration of U a
20、nd Cd Note：CK is the control. Each treatment consisted of three replications. mg/kg represents the weight of elements per 6kilogram nutrient solution. 1.3 Test methods 1.3.1 Test of U and Cd content After taking out wheat seedling and washing with deionized water， separate root and aboveground part，
21、 deactivate enzymes under 105 for 30mins， and dry to constant weight under 75 in oven， weight their dry weights， grind， and nitrify with microwave digestion system. Finally， test U content of root and aboveground parts of the plant respectively， repeat three times with each sample. 1.3.2 Test of phy
22、siological and biochemical indexes Adopt TTC method to test root activity4； at 652nm， adopt 95% ethanol colorimetric method to test absorbance and calculate total chlorophyll content4； adopt nitroblue tetrazolium （NBT） to test superoxide dismutase （SOD）5. 1.4 Data analysis SigmaPlot11.0， Microsoft E
23、xcel 2003（U.S.， Microsoft） ， DPS3.1 Software（China） were used for data analysis and all of the testing data repeated three times. 2.Results 2.1 Characteristics of the U and Cd uptake by wheat seedlings 7With the increase in concentration， the U uptake by wheat seedlings first decreased and then incr
24、eased. At the concentrations of 5 mg/kg and 20 mg/kg， the U uptake in roots was higher than that in aboveground parts， and at the concentration of 100 mg/kg， the U content in aboveground parts and roots was the highest， reaching 77.21 g/g and 76.49 g/g， respectively. This fact suggests that the U up
25、take by wheat seedlings was promoted at high concentration， and U is mostly stored in roots. With the increase in concentration， the Cd uptake increased in aboveground parts but decreased in roots， indicating that wheat seedlings have certain tolerance to low concentration of Cd， and the Cd transfer
26、 from roots to aboveground parts is elevated as the concentration increases. The comparison showed that Cd uptake was higher than U uptake in wheat seedling roots， and at the concentration of 5 mg/kg， the Cd uptake in roots was the highest， reaching 101.50 g/g， 1.81 times U uptake. That might be due
27、 to the competition between U and Cd in uptake by roots and transfer from roots to aboveground parts. Table 2 The element content of Wheat and Pea in the different concentration U and Cd treatment Note： Results represents meansSD. Different small 8letters represents significant difference at p=0.05
28、level. Different capital letters represents significant difference at p=0.01 level. 2.2 Characteristics of U and Cd accumulation in wheat seedlings under combined stress at different concentrations Bioaccumulation factor （BF） is the ratio of element content in the aboveground part of a plant to that
29、 in soil. It indicates the difficulty level for element transfer in the soil-plant system and can be used as an indicator for evaluating the capability of plants to transfer U and Cd to aboveground parts. The higher the BF， the higher the mass fraction of U and Cd in aboveground parts and the uptake
30、 and transfer capability of plants. Table 3 shows that the BF of U in wheat seedlings remained below 1， first decreasing and then increasing； at the concentration of 20 mg/kg， the BF was the lowest， reaching 0.127， and at the concentration of 100 mg/kg， it was the highest， reaching 0.772， 1.27 times
31、 that at the concentration of 5 mg/kg. The fact suggests that the U uptake by wheat seedlings was promoted when the U concentration is high. The BF of Cd changed oppositely， first increasing and then decreasing； at the concentration of 20 mg/kg， the BF was 1.226， while at the concentrations of 5 mg/
32、kg and 100 mg/kg， 9the corresponding BFs were 43.05% and 52.96%， respectively， of the highest value， indicating that wheat seedlings are more capable of accumulating Cd. TF is the ratio of element content in the above-ground part of a plant to that in root. It can be used as an indicator for evaluat
33、ing the capability of plants to transfer U and Cd from roots to aboveground parts. The higher the TF， the greater the capability of plants to transfer elements from roots to aboveground parts. Table 3 shows that the TFs of U and Cd gradually rose with the increase in U and Cd concentration. The TFs
34、of U and Cd were 1.009 and 1.894， respectively， when the U and Cd concentration was 100mg/kg， but were 0.027-0.669 when the U and Cd concentration was 5 mg/kg and 20 mg/kg， indicating that wheat seedlings are more capable of transferring U and Cd from roots to aboveground parts under combined stress
35、 of U and Cd at high concentration. Table3. The BF and TF of U and Cd in wheat 2.3 Effects of U and Cd combined stress on the root activity of wheat seedlings Figure 1 shows that with the increase in the total concentration of U and Cd， the root activity of wheat seedlings first increased and then d
36、ecreased， reaching the peak value of 7.241 mg/（g？h） at the concentration of 20 10mg/kg. The root activity of wheat seedlings under combined stress was significantly different from that of controls； there was no significant difference in root activity between the wheat seedlings treated at total conc
37、entrations of 5 mg/kg and 100 mg/kg， but significant difference was found between the above wheat seedlings and those treated at the total concentration of 20 mg/kg. When the U and Cd concentration was 20 mg/kg， the root activity of wheat seedlings increased sharply to 2.25 times that of the control
38、s. This fact suggests that U and Cd combined stress at a certain concentration can help the growth of wheat seedling roots； the roots maintain high root activity and good uptake and transfer functions when exposed to lower concentration of U and Cd， but are damaged and show decreased activity as the
39、 concentration increases. 2.4 Effects of U and Cd combined stress on the chlorophyll content of wheat seedlings Figure 2 shows that with the increase in the total concentration of U and Cd， the chlorophyll content of wheat seedlings first increased and then decreased. The chlorophyll content of the wheat seedlings treated at the total concentration of 5 mg/kg was significantly different from that of controls， reaching the peak value of 871.82 mg/kg， 1.08
