Iodine [I, 53]
149
Iodine contents of plants are a function of both plant species and growth condition.
However, in general, the phytoavailability of I is low, as, most often, it is strongly
fixed by soil particles. There are some indications that soil properties, especially parent
materials, have a real impact on I contents in plants (Anke 2004). Also, distance of
fields from seas and oceans is of real importance. Thus, there is a great variation in I
contents of various plants. Because I in water is easily phytoavailable, marine plants
may contain greater amounts of I (53–8800 mg/kg) than terrestrial plants (<0.01–
10 mg/kg). Iodine concentration in algae seems to be associated with its pigmentation.
On an average, its contents in various algae are (mean, in mg/kg) green, 58; red, 383;
and brown, 2489 (Fuge and Johson vide Kabata-Pendias and Mukherjee 2007).
Iodine is easily volatized from the soil–plant systems, mainly in the methylated
form (CH 3 I). This is apparently due to methylation processes, activated by roots and/
or microorganisms. Organically bound I in soils is scarcely available to plants, but
after the decomposition of SOM, it becomes phytoavailable. The dominant I species
in plants is apparently iodide, and about 65% of its total content is bound to proteins.
Plants take up I relatively easily from soil solution, and presumably, the iodide,
I – , is much more available than the iodate, IO – . Both I species are more concentrated in roots than in shoots, which could be due to their absorption at the root
surface. However, there are also some reports that its higher contents are in top
plants. Apparently, several other factors have an impact on I distribution within plant
tissues. Plants are capable of absorbing I directly from the atmosphere, both through
the cuticle and as adhesive particles on the surface of hairy leaves. Adequate I levels
in food and feed plants are required in human and animal nutrition. Therefore,
I fertilization or foliar application have been investigated. The most effective in both
treatments was potassium iodide (Strzetelski et al. vide Kabata-Pendias 2011).
Vegetables grown in some EU countries contain relatively high amounts of I. The
highest amounts are reported for (in mg/kg) the vegetables: cabbage, 9–19; onion—bulbs,
8–10; potato—tubers, 3–5. In fodder plants, I contents are relatively similar, and vary
(in mg/kg) as follows: in grass from <1 to 7 and in clover from <0.1 to 0.5. Relatively
high I contents are reported for mushrooms, 5–10 mg/kg (Falandysz et al. 2012).
21.6 HUMANS
Content of I in human body is estimated at 10–20 mg; more than 95% of total I is accumulated in the thyroid gland (Emsley 2011). The only known roles of I in metabolism are
its incorporation into the thyroid hormones, thyroxine (T4) and triiodothyronine (T3),
and into the precursor iodotyrosines. Thyroid hormones, and therefore I, are essential
for mammalian life. Both hormones have multiple functions in the energy metabolism
of cells, in the growth, as a transmitter of nervous stimuli, and as an important factor
in brain development. Iodine deficiency reduces the production of thyroid hormones in
humans and animals, leading to morphological and functional changes of the thyroid
gland, and reduction of the formation of thyroxin. Elemental iodine, I 2 , is toxic, and its
vapor irritates the eyes and lungs. The maximum allowable concentration of I in air, at
work places, is 1 mg/m –3 . All iodides are toxic, if taken in excess.
Iodine deficiency is an important health problem throughout most parts of the
world. Mountainous regions, such as the Himalayas, the Andes, and the Alps, and