**2. Iodine, selenium, and silicon in agricultural systems**

The contents of iodine, selenium, and silicon in the Earth's crust range from <0.1 to 150 mg kg−1 and 0.05 to 1.5 mg kg−1 and 540,000 mg kg−1 (54%) [1, 2]. This data establishes iodine and selenium as trace elements and silicon as the second most abundant element. Although silicon reaches 23–46.5% of the mass of the parent rocks in the soil, most of this element is present in mineralized form as crystalline or amorphous SiO<sup>2</sup> [3]. It has been found that the concentration of Si in the soil solution can be as low as 0.09 mg L−1, reaching a maximum of 23.4 mg L−1. The amount of bioavailable silicon depends mainly on the composition of the parent rock of the soil, as well as on factors such as the content of organic matter, the pH, and the oxidationreduction potential (Eh) [4].

Regarding iodine and selenium, uniformly low concentrations have been found in most of the minerals in the parental rocks. However, there is a positive correlation between the amount of organic matter and the availability of I and Se in soils derived from sedimentary rocks [5].

The availability of iodine and selenium in the soil seems to be based on factors other than the geology itself. In iodine, the most significant influence is exerted by the distance to the ocean, because the ocean is the primary reservoir of iodine on the planet [6]. However, in the soil, the amount of organic matter is the most studied factor regarding the dynamics of the I and Se. In general, it has been established that in the presence of a high content of organic matter, low volatility of I and Se is found, and that the presence of metal oxides and hydroxides such as aluminum, iron, and manganese plays an essential role in retention, and this process is directly related to pH and Eh [7].

In the soil, the predominant chemical species with reducing conditions and low pH (<7) are the iodides (I<sup>−</sup> ) and selenite (Se4 ), species with a great affinity for organic matter; additionally under these conditions, the Se4 can be reduced to Se<sup>0</sup> by precipitating and thus becoming less available [8, 9]. On the contrary, under basicity conditions (pH > 7) and soils with oxidizing conditions, the predominant forms will be IO<sup>3</sup> − and Se6 , which have been shown to have binding affinity with the metal oxides and hydroxides present in the organic matter, through weak electrostatic attractions, thus allowing availability for mobilization and absorption by plants [10]. On the other hand, the Si available in the soil depends on the type of parent rock, since this comes from the weathering of the original material. Greater solubilization has been found from granite rocks than from basalt rocks [11]. The bioavailable form of Si is monosilicylic acid (H<sup>4</sup> SiO4 ) which is found in the liquid phase of the soil [12]; it has been established that it remains in a non-charged and bioavailable form in a pH range of 4.5 to <8, and it is de-protonated to H+ + H3 SiO4 at pH > 9, forming polymers of different molecular weights [13, 14]. A high degree of polymerization of H4 SiO4 has been found under conditions of high concentration of aluminum in the mineral fraction of the soil, as well as during the processes of evaporation of soil water and freezing [15]. The application of acidic solutions in the soil favors solubilization, whereas liming reduces it [16].

programs and projects, to increase their intake by improving their concentration and bioavailability in food. On the other hand, it has been found that by using I, Se, and Si in crop plants, applied in seeds, plants, or fruits, favorable responses are obtained such as increased growth and tolerance to stress. Tolerance to stress is associated with a higher concentration of antioxidants. Thus the use of these elements is a useful technique for the nutritional improvement of crop plants, both in antioxidant level and biofortification. This chapter presents the advances in the last 10 years about the use of I, Si, and Se both for mineral biofortification and for the increase in the concentration of antioxidants in plants, with an emphasis on redox metabolism adjustments and antioxidant chemical species studied. The scope of the chapter is on horticul-

The contents of iodine, selenium, and silicon in the Earth's crust range from <0.1 to 150 mg kg−1 and 0.05 to 1.5 mg kg−1 and 540,000 mg kg−1 (54%) [1, 2]. This data establishes iodine and selenium as trace elements and silicon as the second most abundant element. Although silicon reaches 23–46.5% of the mass of the parent rocks in the soil, most of this element is present in

tion of Si in the soil solution can be as low as 0.09 mg L−1, reaching a maximum of 23.4 mg L−1. The amount of bioavailable silicon depends mainly on the composition of the parent rock of the soil, as well as on factors such as the content of organic matter, the pH, and the oxidation-

Regarding iodine and selenium, uniformly low concentrations have been found in most of the minerals in the parental rocks. However, there is a positive correlation between the amount of organic matter and the availability of I and Se in soils derived from sedimentary

The availability of iodine and selenium in the soil seems to be based on factors other than the geology itself. In iodine, the most significant influence is exerted by the distance to the ocean, because the ocean is the primary reservoir of iodine on the planet [6]. However, in the soil, the amount of organic matter is the most studied factor regarding the dynamics of the I and Se. In general, it has been established that in the presence of a high content of organic matter, low volatility of I and Se is found, and that the presence of metal oxides and hydroxides such as aluminum, iron, and manganese plays an essential role in retention, and this process is

In the soil, the predominant chemical species with reducing conditions and low pH (<7) are

available [8, 9]. On the contrary, under basicity conditions (pH > 7) and soils with oxidiz-

binding affinity with the metal oxides and hydroxides present in the organic matter, through weak electrostatic attractions, thus allowing availability for mobilization and absorption by plants [10]. On the other hand, the Si available in the soil depends on the type of parent rock,

− and Se6

can be reduced to Se<sup>0</sup>

), species with a great affinity for organic matter; additionally

by precipitating and thus becoming less

, which have been shown to have

[3]. It has been found that the concentra-

tural species in the open field and under greenhouse or tunnels.

mineralized form as crystalline or amorphous SiO<sup>2</sup>

reduction potential (Eh) [4].

156 Antioxidants in Foods and Its Applications

directly related to pH and Eh [7].

under these conditions, the Se4

) and selenite (Se4

ing conditions, the predominant forms will be IO<sup>3</sup>

rocks [5].

the iodides (I<sup>−</sup>

**2. Iodine, selenium, and silicon in agricultural systems**

There are differences with respect to the hydrological mobilization of these elements. For the Se and Si occur by the dragging of sediments or dissolved chemical species through continental aquatic flows, with an estimated 14,000 t per year toward the ocean in the case of Se [17]. Iodine mobilization presumably occurs in reverse to that of Se and Si, that is, from the ocean to continental waters, mainly through rainfall. The rainfall has an iodine concentration of 0.5–5 μg L−1, and probably such concentration is a reflection of the gaseous dynamics of iodine in the atmosphere [18]. In surface water, iodine has been reported in ranges of <20 μg L−1, while in groundwater the reported concentrations have been higher (from 430 to 4100 μg L−1), probably due to the desorption of organic matter rich in iodine, sediment leaching, or concentration by evaporation in arid zones [19]. Silicon solubilized from the parent rock and converted to its bioavailable forms, both by the physicochemical processes and by the metabolism of different organisms such as plants, has been estimated to be 240 ± 40 μg L−1 [20].

The natural uptake of these elements by the plants will be conditioned to the different growth conditions. The low biodisponibility is the typical situation; thus, the resulting level of I, Se, and Si in the plants will be similarly low (**Table 1**). In the case of soilless crops grown under protected conditions, both the availability of these elements and the resulting concentration in the plants will be very low, since in this case, the primary source would be irrigation water, which contains a small amount of I, Se, and Si.



**Table 1.** Concentration of I, Se, and Si in soils, irrigation waters, and crop plants.

In regard to the concentration of I, Se, and Si, the use of soilless crops results in plants with a lower level of these elements than in soil crops. Hydroponic production of crops for human consumption has increased substantially in recent years, mainly due to the efficient use of water and nutrients from the crop. However, as far as commercial production is concerned, nutrition only considers the application of the elements deemed essential for plants [29], leaving aside those that are beneficial as I, Se and Si. These beneficial elements raise the antioxidant content in plants, giving an advantage against oxidative stress, in addition to its use allows obtaining biofortified crops with high nutritional value for human consumption.
