Comparative Study On Selenium Enrichment Ability Among Different Brassica Vegetables

Selenium（Se） is one of the essential trace elements, known as the protective agent of life. Selenium deficiency can lead to various diseases of the human body. Plants can absorb Se and transfer Se to human and animals through food chain. It is an effective measure to prevent selenium deficiency by eating Se enriched agricultural products for people that living in selenium deficiency area. Vegetables play an important role in people’s daily life. Cruciferous vegetable is one of Se enriched crops. Brassica is the earliest classification of Brassicaceae, and one of the highest economic values and the most widely used plant groups. Brassica vegetables（included Brassica rapa var. perviridis, Brassica rapa ssp. chinensis, Brassica rapa var. purpuraria, Brassica rapa ssp. pekinensis, Brassica oleracea var. capitata, Brassica rapa var. rapa, Brassica juncea var. tumida） as materials, effects of exogenous selenium on crop growth, selenium absorption, transformation and transport were studied. The expression levels of selenium metabolism related genes were also detected by the qRT-PCR analysis techniques. The main results are as follows:（1） Application of exogenous Se in soil(<5 mg·kg^-1) increased the growth of Brassica vegetables, and the total biomass（dry weight） was increased by 8.22%-101.22% compared to control, but high concentration of selenium(5 mg·kg^-1) has toxicity on vegetables. The highest biomass was found in Brassica juncea var. tumida among 7 kinds of vegetables, while the lowest biomass was found in B.rapa var. perviridis. Yield of Brassica juncea var. tumida increased by 1.80% with spraying 1 mg· L-1 Se solution, being unsignificant.（2） Exogenous Se could change the nutritional quality of vegetable. Appropriate Se(≤1 mg·kg^-1) applied in soil decreased the contents of nitrate and free amino acid of most vegetables, while increased nitrate content of Brassica rapa ssp. chinensis and Brassica rapa var. rapa, but decreased Vc content of B. rapa ssp. chinensis, Brassica rapa ssp. pekinensis, Brassica rapa var. rapa, and Brassica juncea var. tumida, while appropriate Se(≤0.5 mg·kg^-1) increased Vc content of B. rapa var. perviridis, B. rapa var. purpuraria and Brassica oleracea var. capitata. Se decreased the reducing sugar contents of Brassica rapa ssp. pekinensis and Brassica juncea var. tumida, and reducing sugar content of B. oleracea var. capitata decreased significantly first, then increased with increasing Se levels, and increased by 13.70% compared to control at 5 mg·kg^-1 Se treatment, and Se could increased reducing sugar of other tested vegetables in a certain extent. Spraying Se decreased the content of nitrate in Brassica juncea var. tumida, and increased the content of Vc, reducing sugar and free amino acid in a certain extent, increased by 40.41%, 13.36% and 46.73% respectively compared to control.（3） Certain amount of Se(31 mg·kg^-1) in soil increased GSH-px activity of leaf and root in most vegetables, and 5 mg·kg^-1 Se treatment significantly reduced GSH-px activity of leaf and root in Brassica oleracea var. capitata. The variation of GSH-px activity in root of Brassica rapa ssp. pekinensis was irregular under different Se treatments. GSH-px activity of B. oleracea var. capitata was the strongest among tested vegetables.（4） Exogenous Se significantly increased the contents of total Se and organic Se in the edible parts of vegetables, and Se content in each organ was positively correlated with the concentration of exogenous Se. Selenium content in Brassica juncea var. tumida was the lowest at the same Se concentration in soil among tested vegetables, and highest conversion of organic Se was found in B. rapa var. perviridis. B. rapa var. purpuraria had high ablity to uptake Se in soil, while B. oleracea var. capitata was the weakest. For the edible parts of vegetables, Se enrichment coefficient（BCF） of Brassica rapa var. rapa was the highest, followed by B. rapa var. perviridis, B. oleracea var. capitata was the lowest. Selenium content uptaked by overground of B. rapa var. perviridis was higher than the root of Brassica rapa var. rapa. However TF value of Brassica rapa var. rapa was less than B. rapa var. perviridis. With increase of the concentration of Se spraying, Se accumulation in root, stem and leaf gradually increased, and Se accumulation in leaf was the highest, followed by the accumulation in stem, and root was the lowest.（5） Significant differences of the expression levels of the 6 genes（gene families） in 7 vegetable crops of Brassica were found in Se treatments. The expression levels of the 6 genes（families） among different sub-species/varieties of B. rapa vegetables are still quite different under the same Se treatments. The expression levels of six genes（families） detected in Brassica rapa var. perviridis showed significant upregulation by Se level 1-2.5 mg·kg^-1, followed by declining to base levels at higher concentration of treatment. The expression levels of SAT, SMT, CysD genes（families） in Brassica rapa ssp. chinensis decreased at Se levels of 1-2.5 mg·kg^-1, but significantly increased and even raised above the base level under se treatment 5mg·kg^-1, showing a U-shaped curve, while the expression levels of APS, APR, MMT genes（families） showed up-regulation by Se level of 0.5 mg·kg^-1, but significantly down-regulated to lower than basal levels under se treatment 1-2.5 mg·kg^-1, showing a double-peak curve. The expression levels of six genes（families） detected in Brassica rapa var. purpuraria showed a continuous down-regulation curve after Se treatments. The expression level of CysD gene in Brassica rapa ssp. pekinensis showed continuing falling trend, but the other genes（families） showed high up-regulation. The expression levels of six genes（families） in Brassica rapa var. rapa and Brassica oleracea var. capitata were in line with double-peak curves after Se treatments. The expression levels of six genes（families） in Brassica juncea var. tumida were in line with single peak curves after Se treatments. The reaction trends among gene members within the same gene family were generally similar among each other.