Selenium is a beneficial element for plants that acts as an antioxidant and plant prooxidant agent. In addition, it confers tolerance to different abiotic stresses such as those caused by different toxic metals/metalloids [1]. The correlation of bioavailability and toxicity of selenium with its chemical form triggered interest in selenium speciation in plants. Therefore, the use of multiple selenium isotopic tracers is a powerful tool to perform speciation and metabolic studies of different selenium species in plants [2]. This strategy would allow simultaneous comparison of the effect of different species on factors such as selenium uptake, translocation and transformation.

In this work, two stable isotopic tracers of selenium (⁷⁶selenomethionine (⁷⁶SeMet), ⁷⁷selenite (⁷⁷Se)) and chitosan-stabilized selenium nanoparticles (Ch-SeNPs) have been used to jointly evaluate their metabolic transformations after exposure to Spinacia oleracea and Raphanus sativus species. Plants were grown in hydroponic medium using perlite as substrate and Hoagland 0.5 solution as nutrient, and selenium was applied via foliar by enriching the nutrient solution with 1.5 mg Se L^-1 in the form of ⁷⁷Se, SeNPs and ⁷⁶SeMet. After 45 days of treatment, plants were collected, weighed and divided into root and aerial parts to determine the accumulation of selenium from each species, the distribution of species for each chemical form of selenium supplemented and the effect of this metalloid on the incorporation of essential metals such as Fe, Zn, Cu and Mo.

The analysis by ICP-MS of the digested extracts allowed to determine the total accumulated selenium content. The quantification of the total accumulated selenium content of each of the chemical forms of this element by each plant species was performed by isotope dilution, using a selenium standard enriched in isotope 78 as a tracer. The results show that the Ch-SeNPs were the species most easily incorporated by both plants followed by ⁷⁶SeMet and ⁷⁷Se. However, ⁷⁶SeMet was the species that translocated the most to the root, while Ch-SeNPs the least. The biotransformation of the different chemical forms of supplemented selenium was evaluated by HPLC-ICP-MS using two separation mechanisms (anion exchange, PRP-X100, and reverse phase, C18) after enzymatic extraction with protease XIV. The quantification of the different species was performed by post-column IDA. The results showed that in spinach the main species both in the aerial part and in the root was SeMet. The analysis by isotopic dilution allowed to conclude that this species came mostly from ⁷⁶SeMet. In the case of radishes, apart from SeMet, the other main species was selenomethylselenocysteine (SeMeSeCys). Subsequently, the extracts were analysed by HPLC-ESI-MS/MS to confirm the species. Finally, selenium supplementation increased the accumulation of Cu, Fe and Zn by plants.

References

[1] M. Hasanuzzaman et al Environmental and Experimental Botany, 2020, 178, 104170.

[2] P. Di Tullo et al Analytical and Bioanalytical Chemistry, 2015, 407, 9029-9042.