Iodine uptake, storage and translocation mechanisms in spinach (Spinacia oleracea L.)
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Iodine is an essential micronutrient for human health; phytofortification is a means of improving humans’ nutritional iodine status. However, knowledge of iodine uptake and translocation in plants remains limited. In this paper, plant uptake mechanisms were assessed in short-term experiments (24 h) using labelled radioisotopes; the speciation of iodine present in apoplastic and symplastic root solutions was determined by (HPLC)-ICP-QQQ-MS. Iodine storage was investigated in spinach (Spinacia oleracea L.) treated with I− and IO3−. Finally, translocation through the phloem to younger leaves was also investigated using a radioiodine (129I−) label. During uptake, spinach roots demonstrated the ability to reduce IO3− to I−. Once absorbed, iodine was present as org-I or I− with significantly greater concentrations in the apoplast than the symplast. Plants were shown to absorb similar concentrations of iodine applied as I− or IO3−, via the roots, grown in an inert growth substrate. We found that whilst leaves were capable of absorbing radioactively labelled iodine applied to a single leaf, less than 2% was transferred through the phloem to younger leaves. In this paper, we show that iodine uptake is predominantly passive (approximately two-thirds of total uptake); however, I- can be absorbed actively through the symplast. Spinach leaves can absorb iodine via foliar fertilisation, but translocation is severely limited. As such, foliar application is unlikely to significantly increase the iodine content, via phloem translocation, of fruits, grains or tubers.
KeywordsFoliar fertilisation Inhibitor Iodine Spinach (Spinacia oleracea L.) Translocation Uptake mechanisms
Funding for O. S. Humphrey was provided by the British Geological Survey-University of Nottingham, Centre for Environmental Geochemistry, BGS University Funding Initiative (contract number: BUFI-S314). The authors would like to thank Mark Meecham and the Sutton Bonington glasshouse staff for assistance with growing the plants and David Gardner for assistance with osmolality measurements. Special thanks to BGS laboratory staff with sample preparation and analysis and to Dr Louise Ander for initial comments for the overall study. This work is published with the permission of the Executive Director, British Geological Survey.
All authors contributed to this work. O.S.H, S.D.Y, E.H.B, N.M.J.C and M.J.W conceived the ideas and designed the research; O.S.H and E.M.H performed the experiments and analysed the data; O.S.H wrote the manuscript with feedback from all co-authors.
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Conflict of interest
The authors declare that they have no conflict of interest.
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