*4.3. Iodine*

Iodine (I) is essential for humans; it is required in the synthesis of the hormones thyroxine and triiodothyronine that are produced in the thyroid gland and are responsible for regulating growth and development, besides maintaining the basal metabolic rate [68]. The RDA of I is 150 μg day−1, whereas the UL for adults is 1100 μg day−<sup>1</sup> [69]. Typical I concentration in soils is between 0.5 and 20 mg kg−1, and even though not essential to plant growth, it can be absorbed and translocated within the plant tissues. Plant leaves absorb I through stomata (60%) and leaf surface (40%), but I losses can occur too, due precipitation, wind, and tissue decay; the remaining can be partially transported via phloem to the other plant organs, including roots [70]. According to Smolèn et al. [71], leaves absorption occurs due the organophilic nature of I and its interaction with cuticular waxes or oxidation of I- (iodide) to I2 (iodine), facilitating I penetration into the cuticle. It is known that root absorbs iodide better than elemental I or iodate, especially in plants grown in hydroponic systems. This I is majorly retained into the roots, but when in nutrient solution with concentrations higher than 0.01–10 μM it can also be translocated to the shoots [70,72]. In fact, I is efficiently transported into the xylem, transport in plants is analogous to chloride movement, I– uptake being catalyzed by H+/anion symporters and released into the xylem by anion channels [27]. Concentration of I in plants can be zero or extremely low, about 30– 100 μg kg−<sup>1</sup> FW [73]. Depending on plant species, a nutrient solution with concentrations higher than 10–100 μM can already be phytotoxic and inhibit plant growth [70]. In general, the different I chemical forms present the following phytotoxicity order: (I2) > (I−) > iodate (IO3 −) [74]. Horticultural crops are the best candidates for I biofortification, because of their ability to absorb and accumulate exogenous I into the edible fractions [75]. As reported in Table 1, once submitted to different biofortification protocols, leaf species such as basil and Chinese cabbage, showed an average I increase higher than 100-fold in their edible tissues, while cabbage, lettuce, mizuna, mustard, and spinach resulted in increases varying from 5 to 91 times. Average accumulation of I in vegetable fruit species was higher than 100-fold in both tomato and cowpea. Tuber species such as potato, showed a 13-fold average increased in I content, while root vegetables such as carrot presented a much higher average increase (greater than 100-fold). Biofortification of I through repeated foliar spray has been successfully performed in carrot and mustard plants [76,77]. Higher efficacy of lettuce iodine biofortification was noted Smolèn et al. [71] after foliar application, rather than adding the element to the nutrient solution. On the contrary, Caffagni et al. [75] demonstrated that, even though it is possible to enhance the I content of tomato fruits through KIO3 foliar spray, better results were observed through fertigation with a 5 mM solution of KI; this allowed to achieve a 249-fold I increase in this vegetable. When grown in water culture, lettuce plants grown with 90 μgIL−<sup>1</sup> as potassium iodide (KI) showed better biofortification results than plants submitted to the same amount of I as potassium iodate (KIO3), with the result consisting of 30-times more I in leaves than untreated plants [78]. Low doses of I, such as 0.25 mg L−<sup>1</sup> (KI) or 0.50 mg L−<sup>1</sup> (KIO3) in the nutrient solution are enough to achieve around 7 mg kg−<sup>1</sup> DW of I in strawberry fruits, compared to 0 in the control, improving plant growth too [79]. Analogous results were observed in several leafy vegetables (e.g., broccoli raab, curly kale, mizuna or red mustard) when submitted to low doses of iodine (0.75 mg <sup>L</sup>−1, 5.9 μM KIO3) through fertigation, showing an increase ranging from 390 to 471 μg kg−<sup>1</sup> FW [80]. However, high I levels (50 mg <sup>L</sup>−1) in the nutrient solution, proved to increase the I content in carrot up to toxic amounts for humans (9 mg kg−<sup>1</sup> FW) showing also phytotoxic effects on plants [76]. In addition, I biofortification should be carefully evaluated, since there is evidence that I can decrease Cu uptake by plants [73]. However, even though insufficient phloem loading and high volatilization rates could limit I accumulation, both K iodate and K iodide have successfully increased the I content in horticultural products.
