The Differences Of Selenium Uptake, Translocation And Bioavailibility Of Four Vegetables When Selenium Applied As Selenite And Selenate

Selenium can be either beneficial or toxic to animals, microorganisms, and humans, depending on the chemical forms, the concentrations, and other environmental regulating variables. It is estimated that between 500 and 1000 million people worldwide may be deficient in Se. Growing plants enriched with selenium could be an effective way to reduce dietary deficiencies and increase health benefits. In this study, pot experiments by adding selenite and selenate were carried out under planting four vegetables (pakchoi, mustard, lettuce and spinach) in a greenhouse. The objective of the study was to investigate effects of different levels of selenite and selenate applicated to soil on selenium uptake, accumulation and translocation in four vegetables. Meanwhile, soil enzyme activity and speciation of selenium were also studied, and discussing the environmental chemistry mechanism of the difference of selenium bioavailability in four vegetables, in order to provide the theoretical basis for selecting vegetables which have higher capacity for selenium enrichment and phytoremediation of soil contaminated by selenium. The main results were as follows:1. Lower selenite (<3.81 mg·kg^-1 )and selenate supply rate (<1.45 mg·kg^-1) could stimulate the growth of four vegetables, but higher selenite(>29.91 mg·kg^-1)and selenate supply rat(e>2.04 mg·kg^-1)exerted toxic effects on vegetables. while in the soil applicatd with selenite,spinach had the maximum tolerance and lettuce had the weakest one. In the soil applicatd with selenate,the shoot biomass of the four vegetables showed the follows sequence: Mustard>pak choi>lettuce>spinach, lettuce had the largest root biomass among the four tested vegetables.2. The uptake of selenium of the four vegetables increased with the increasing of applied selenite and selenate levels (p<0.05). In the soil applicatd with selenite, shoot-Se content within pakchoi reached to peak and the root-Se contents within pakchoi and mastard were higher those within lettuce and spinach. However, in the soil applicatd with selenate, shoot-Se contents within pakchoi and mustard shoots were 5.8⁸.5 times greater than those within lettuce and spinach. The selenium contents in roots of the four vegetables showed the follows sequence : mustard>pak choi>spinach>lettuce. For different treatments with selenate application rates, selenium contents in shoot were 1² times greater than those in roots of four vegetables.When lower selenite (<13.21 mg·kg^-1) and selenate (<5.02 mg·kg^-1)supplied to soil, the selenium accumulations of the four vegetables increased with the increasing of applied selenium levels(p<0.05). Later, shoot-Se accumulations of the four vegetables decreased. Shoot-Se and root-Se accumulations within pakchoi and mastard were higher those within lettuce and spinach, selenium accumulations of shoot were significantly higher than root of four vegetables. In all selenite and selenate treatments, mustard had the largest root selenium accumulations in four vegetables.Overall comparison results illustrated that among the four kinds of vegetables, spinach showed the strongest selenite translocation capacity, followed by pakchoi, mustard and lettuce were at the last. Meanwhile, pakchoi had the greatest level of TF for selenate, while that of spinach showed the smallest. Thus, pakchoi was the most suitable species among four tested vegetables to eliminate soil selenium contamination because of its relatively larger biomass and better selenium translocation capacity.3. After planting spinach, urease activity increased gradually with the increasing of selenium, and soil catalase activity decreased. However, in the soil planted with mustard and lettuce, the urease and catalase activity decreased at first and then increased with the the increase of selenite applied to soil, and the "resistive enzyme activity" phenomenon emerged. The alkaline phosphatase activity was not significantly changed when the soil selenite content was less than 13.21 mg·kg^-1, and it reduced gradually thereafter. After the four vegetables were planted, the soil urease and catalase activity increased at first and then decreased with the increasing of selenate. The alkaline phosphatase activity was not changed significantly when the applicated selenate level was less than 2.04 mg·kg^-1 in pakchoi, mustard and spinach soil. Neverthelessly,the soil alkaline phosphatase activity decreased significantly with increasing concentration of selenium when the applicated selenate level was larger than 2.04 mg·kg^-1.Compared the four vegetable crops, urease activity in selenite treatment was higher than that in selenate treatment after planting lettuce, while an opposite phenomenon was found in the soil planted with pak choi and spinach. There was no significant relationship between valence state of selenium and the soil alkaline phosphatase activity in the soil planted with pak choi and mustard, but for lettuce and spinach, the inhibition of alkaline phosphatase activity in selenate treatment was larger than that in selenite treatment. In conclusion, mustard has the biggest influence on soil enzyme activities and lettuce has the lest influence, except for the selenite treatment, in which spinach has more influence on catalase activity. 4.Four soil selenium speciation content were increased with the increasing of applicating selenite content to four vegetables soil. Because of the high dissolubility of selenate, the highest selenium speciation content in selenate treatments was soluble selenium. On the contrast, the highest selenium speciation content in selenite treatments was exchangeable and carbonate selenium. In the largest selenite content (47.96 mg·kg^-1) treatment, the exchangeable and carbonate bound selenium contents in the four vegetables were in the order of spinach, mustard, cabbage and lettuce. The iron manganese oxide combination state selenium and elemental organic and sulfide selenium speciation content in selenite treatments were higher than those in selenate treatments, and elemental organic and sulfide selenium content of pak choi soil was largest than the other three vegetables in selenate treatments.The result of path analysis showed that each selenium speciation of different vegetables has different contribution rate to selenium contents of plant shoots, mainly due to the different selenium absorption capacity of the four kinds of vegetables and bioavaliability of each soil selenium speciation.