Selenium uptake, speciation and stressed response of Nicotiana tabacum L

•We obtained the optimal dose of Se for the flue-cured tobacco growth.•The effect of Se on the antioxidant capacity of plants depended on Se species.•The percentages of different Se species changed with Se levels in soil.•The excessive application of Se led to the higher proportion of inorganic Se. The research on the function and mechanism of selenium (Se) is of great significance for the development of Se-enriched agricultural products. In this paper, uptake, speciation distribution, the effects on the flue-cured tobacco growth and antioxidant system of Se at different levels (0–22.2mgSekg−1) were studied through a pot experiment, aiming to clarify flue-cured tobacco's response to Se stress and the relationship between Se speciation and antioxidant system. The results showed that the leaf area and number, the biomass and the chlorophyll content reached the maximum at 4.4mgkg−1 of Se treatment. Selenium at low levels (≤4.4mgkg−1) stimulated the growth of flue-cured tobacco by elevating the capability of antioxidant stress and reducing the malondialdehyde (MDA) content to 0.6–0.8 times of that of the control. However, high Se levels (≥11.1mgkg−1) depressed the capability of antioxidant stress and raised the MDA content to 1.5-fold of that of the control, and meanwhile the biomass of the aboveground parts and underground parts declined notably. The Se content in different parts of flue-cured tobacco significantly increased with the growth of Se levels. The range of Se content in roots, leaves and stems at 2.2–22.2mgkg−1 of Se treatment were 16.7–58.6mgkg−1, 2.6–37.3mgkg−1 and 2.2–10.3mgkg−1, respectively. According to the detection of different Se speciation, only selenocysteine (SeCys) was detectable in leaves at 2.2mgkg−1 Se treatment; SeCys, selenite [Se(IV)]and selenate [Se(VI)] were detected in flue-cured tobacco leaves at Se treatment (≥4.4mgkg−1), which accounted for 4.6–10%, 9–18.7% and 71–86% respectively; SeCys, selenomethionine (SeMet) and Se(IV) were detected in roots, and organic selenium(66–84%) was the main Se species at Se≤11.1mgkg−1 treatment; four Se species [SeCys, SeMet, Se(IV) and Se(VI)] were detected in flue-cured tobacco roots, and the main Se species was inorganic Se (60%) at 22.2mgkg−1 Se treatment. That was to say, the percentage of organic Se species (SeCys and SeMet in flue-cured tobacco leaves and root) declined, whereas the ratio of inorganic Se species [Se(IV) and Se(VI)] increased with the growth of Se levels. The correlatio