Tracking the metabolism of selenium compounds in Spinacia oleracea using multiple stable isotope tracers and...

Understanding the behaviour and metabolic transformations of different selenium species in plants requires conducting as many experiments as there are chemical forms to study. This approach does not allow for the evaluation of the coexistence between different selenium forms. Using multiple selenium isotopic tracers enables the simultaneous comparison of different selenium forms and their effects on plant metabolism in a single experiment. In this context, this study employed a mixture of two selenium isotopic tracers, ⁷⁶SeMet and ⁷⁷Se(IV), along with chitosan-modified selenium nanoparticles (Ch-SeNPs), to assess their metabolic transformations in Spinacia oleracea under foliar exposure.

Selenium supplementation significantly increases chlorophyll a (800 ± 50 mg kg^-1 DW) and total chlorophyll (1200 ± 80 mg kg^-1 DW) content. ID-ICP-MS analysis showed that Ch-SeNPs and ⁷⁶SeMet were most readily incorporated into the aerial part (40 % and 31 % of total selenium), while ⁷⁶SeMet and ⁷⁷Se(IV) were rapidly transported to the roots (43 % and 36 %). ID-HPLC-ICP-MS revealed that SeMet was the predominant species in both the aerial part and root for ⁷⁶SeMet and ⁷⁷Se(IV). However, for Ch-SeNPs, SeMet and an unknown selenium compound were predominant in the aerial part, while Se(VI) and SeMet dominated in the roots. In the phloem sap analysis, the main species for selenium sources were SeMet for ⁷⁶SeMet, Se(IV) for ⁷⁷Se(IV) and unknown compound for Ch-SeNPs. Finally, selenium increased Zn and Fe levels in both plant parts, while reducing Mo concentration in the roots.

This study demonstrates the use of selenium isotope tracers to evaluate and compare the accumulation and metabolic transformation of different selenium forms in a single experiment. By combining post-column IDMS with HPLC-ICP-MS, all selenium species in spinach were quantified without the need for commercial standards. This innovative methodology offers a comprehensive approach for exploring selenium metabolic pathways and holds potential for advancing research in plant fertilization, particularly when paired with advanced mass spectrometry techniques.