Iodine geodynamics and plant availability

Humphrey, Olivier S.

Iodine geodynamics and plant availability

Publication Date

December 13, 2019

Creator

Humphrey, Olivier S.

Abstract

Iodine is an essential micronutrient for human health. However, approximately 1.9 billion people worldwide are at risk of developing an iodine deficiency disorder. The uptake of iodine by crops and subsequent consumption can be a vital component of dietary intake: further understanding of soil and crop iodine dynamics will determine the sustainability of iodine phytofortification. The aim of this thesis was to improve the understanding of soil-plant-iodine dynamics by measuring in-situ soil solution-solid phase interactions and identifying specific plant-iodine mechanisms to better inform iodine phytofortification strategies. Experiments were designed to demonstrate that microdialysis is a suitable means for extracting iodine from soil solution. By using microdialysis it was possible to observe iodine interactions in soils at significantly improved temporal resolution. Online liquid chromatography was coupled with ICP-QQQ-MS to analyse the speciation and molecular weight distribution of iodine in soil solution. Greater instantaneous adsorption rates of iodate (129IO3–) compared to iodide (129I–) were observed in soil with the complete reduction of iodate occurring within 5 hours. Soluble organically bound iodine was identified with a low molecular weight (<5 kDa), with a slower time-dependent formation of larger iodinated compounds (12-18 kDa). Modelling with simultaneous ordinary differential equations revealed that iodine present in soil solution, not instantaneously adsorbed, had an average half-life of <2 hours. Spinach was used to assess iodine uptake, storage and translocation mechanisms. The results confirmed that roots can reduce iodate to iodide in solution. Within root solutions, iodine was present as organically bound iodine or iodide with significantly greater concentrations in the apoplast compared to the symplast, indicating that whilst active pathways exist, passive uptake is the primary absorption mechanism. Iodine applied directly to leaves was successfully adsorbed and stored, however, translocation was severely limited. As such, foliar application is unlikely to significantly increase the iodine content, via phloem translocation, of fruits, grains or tubers. Soil-to-crop transfers are significantly limited by the short window of opportunity in which roots can absorb iodine from soil solution prior to it becoming unavailable. The findings in this thesis contribute to a better understanding of the soil-plant-iodine dynamics and therefore, efficacy of phytofortification strategies.

Item Type

ethesis

Thesis Type

PhD

Supervisors