5.2.6. Iodine

As explained in Chapter 1 and 2 iodine is essential in human nutrition and billions of people have insufficient iodine intake. Lower plant species, including marine algae require iodine as an essential element. Higher plant species, however, don’t require iodine. Therefore, is it not part of standard nutrient solution in horticulture nor is it part of fertilisation schemes in open field agriculture. Nevertheless, biofortification of vegetables with iodine be a means to increase human iodine intake. Voogt et al (2010) found that an increase in iodine concentration in the nutrient solution significantly enhanced iodine content in the plant. Iodine contents in plant tissue were up to five times higher with I⁻ than with IO₃⁻. On examination of the lettuce plants, iodine was mainly found to be in the outer leaves. Addition of 90 or 129 ug iodine L^-1  resulted respectively in 653 and 764 µg iodine kg⁻¹ of total leaf fresh weight. The authors concluded that 50 grams of iodine‐biofortified lettuce would provide around 25% of the recommended daily intake of iodine for adolescents and adults.

Landini et al (2011) found in tomato that a considerable amount of iodine was stored after a iodine leaf treatment, suggesting that iodine transport through phloem also occurred. These results were confirmed in lettuce by Smolen et al (2014). Tomato plants can tolerate high levels of iodine, both in the vegetative tissues and fruits (Landini et al. 2011). The concentration found are more than sufficient for the human diet and from a plant physiological point of view tomato is an excellent crop for iodine‐biofortification programs.

Voogt et al (2014) preformed an iodine experiment with cucumber, sweet pepper and round tomato. They used three Iodine (I) levels: 5, 23.4 and 148 ppm in the nutrient solution, and established 9.1, 38.9 and 171.8 ppm in the drainage. The results show that iodine concentrations in plant material had a strong correlation with Iodine supply and the majority of the Iodine was found in the vegetative plant parts. Average concentrations in fruits (mg I/kg fresh weight) for the 12.5 and the 125 ppm level were: 0.02 and 0.12 (cucumber), 0.01 and 0.04 (sweet pepper), 0.01 and 0.05 (round tomato), 0.03 and 0.12 (cherry tomato), respectively. Total biomass, yield and fruit quality were not affected by Iodate application. The outcomes demonstrate that a portion of 80 grams of these fruiting vegetables, grown with fertilizers containing 125 mg I/kg fertilizers, constitutes 3-10 μg of iodine intake, i.e., 2-7% of the daily iodine requirement for an adult.

In a study of Jerše et al (2018)with Pea (Pisum sativum L.) plants similar results were found. When plants were sprayed at blooming stage with solutions iodine (I^-) and selenium (Se). The results showed elevated concentrations of both elements in all parts of pea plants. In seeds, I content was more than 6-fold higher, while Se content was up to 12-fold higher than in control plants. Although the plants were in good condition, some differences in pod characteristics and electron transport system activity were observed.

All the studies discussed in this and the previous section show that fertilisation is a promising strategy to increase Iodine and Selenium concentration in edible plant parts. Additionally, there appears to be enough genetic variability to breed for crops with increased I and Se concentrations. However, there is little variation in Se concentration in grain from modern bread and durum wheats. Nevertheless, wild wheats (T. dicoccum and T. spelta) and their relatives (A. tauchlii) have significantly higher Se concentrations than cultivated wheat does (Lyons et al. 2005).