Use of Iodine to Biofortify and Promote Growth and Stress Tolerance in Crops

Abstract

Iodine is not considered essential for land plants; however, in some aquatic plants, iodine plays a critical role in antioxidant metabolism. In humans, iodine is essential for the metabolism of the thyroid and for the development of cognitive abilities, and it is associated with lower risks of developing certain types of cancer. Therefore, great efforts are made to ensure the proper intake of iodine to the population, for example, the iodization of table salt. In the same way, as an alternative, the use of different iodine fertilization techniques to biofortify crops is considered an adequate iodine supply method. Hence, biofortification with iodine is an active area of research, with highly relevant results. The agricultural application of iodine to enhance growth, environmental adaptation, and stress tolerance in plants has not been well explored, although it may lead to the increased use of this element in agricultural practice and thus contribute to the biofortification of crops. This review systematically presents the results published on the application of iodine in agriculture, considering different environmental conditions and farming systems in various species and varying concentrations of the element, its chemical forms, and its application method. Some studies report beneficial effects of iodine, including better growth, and changes in the tolerance to stress and antioxidant capacity, while other studies report that the applications of iodine cause no response or even have adverse effects. We suggested different assumptions that attempt to explain these conflicting results, considering the possible interaction of iodine with other trace elements, as well as the different physicochemical and biogeochemical conditions that give rise to the distinct availability and the volatilization of the element.

Environmental Parameters Affecting the Concentration of Iodine in New Zealand Pasture

“…Sheep and cattle require feed I concentrations of 0.05 to 0.2 and 0.3 to 0.5 mg kg −1 respectively, for which pasture is the predominant source in many grazing systems (Suttle, 2005; Flachowsky, 2007; Grace and Knowles, 2012). The prevalence of I deficiency is due to low concentrations in inland soils, low soil bioavailability, and nonessentiality for terrestrial plants (Fuge, 1996; Zimmermann, 2009; Medrano‐Macías et al, 2016). Iodine deficiency is exacerbated by goitrogens, which are thyroid I‐limiting compounds commonly found in brassicas ( Brassica spp.)…”

“…In contrast, seawater, which contains 0.06 mg I L −1 (Wright, 1995), is a major reservoir of the element in the biosphere, with seaspray and subsequent wet and dry deposition of volatilized I being important processes for supplying the element to soils. It follows that elevated I concentrations are often found in soils close to the sea (Fuge and Johnson, 2015; Medrano‐Macías et al, 2016). Johnson (2003) reported geometric means of 11.6 and 2.6 mg kg −1 <50 and >50 km from the sea, respectively.…”

“…However, there was no significant relationship between pH and total I in an extensive Northern Irish database (Smyth and Johnson, 2011), with reports of high‐I soils formed from heavily weathered limestone and other high pH areas (Fuge, 1996). Solubility increases with waterlogging and decreasing redox potential when iron (Fe) and manganese (Mn) oxides dissociate and IO 3 − is reduced to I − (Medrano‐Macías et al, 2016; Qian et al, 2017). Iodine is firmly retained by clay‐sized particles and particularly organic matter (Shetaya et al, 2012; Medrano‐Macías et al, 2016; Humphrey et al, 2018), with Johnson (2003) finding that peat followed by clay soils contained the most I.…”

“…Solubility increases with waterlogging and decreasing redox potential when iron (Fe) and manganese (Mn) oxides dissociate and IO 3 − is reduced to I − (Medrano‐Macías et al, 2016; Qian et al, 2017). Iodine is firmly retained by clay‐sized particles and particularly organic matter (Shetaya et al, 2012; Medrano‐Macías et al, 2016; Humphrey et al, 2018), with Johnson (2003) finding that peat followed by clay soils contained the most I.…”

“…Plant roots take up I − more rapidly than IO 3 − , as I − likely follows the same pathway as chloride (Cl − ) (Whitehead, 2000; White and Broadley, 2009), with recent evidence that plants convert IO 3 − to I − (Humphrey et al, 2019). Iodine enters the leaves via stomata and cuticular waxes, though it is often fixed by the waxes and hence retained in exposed leaves with reduced translocation throughout the plant (Humphrey et al, 2019; Medrano‐Macías et al, 2016). Solubility and atmospheric concentrations affect plant uptake.…”