**Abstract**

Selenium is a mineral element that is essential for both animal and humans and can also serve as an environmental toxicant. A narrow margin exists between an ideal and toxic intake of selenium. It is a useful microelement existing in minute amounts in animals, plants, microorganisms and humans. Although it is beneficial to both animals and humans as an antioxidant, it can be toxic at high concentrations as a result of it competing and replacing sulfur in amino acids leading to inappropriate folding of protein and eventually creating a nonfunctional protein and enzymes. Selenium exists in organic forms as SeMet and SeCys and inorganic forms as selenide, selenite and selenite in the environment. It is translocated in plants via the sulfate transporters in the plasma membrane of the plant root. Its translocation and distribution however depends on the plant species, their different developmental phases, forms, concentration and other physiological conditions like pH. Inorganic selenium is first converted to selenite via the action of two different enzymes (ATP sulfurylase and APS reductase), selenite is further converted to selenide by sulfite reductase. Selenide eventually couples with O-acetyl serine via the action of cysteine synthase to form SeCys. SeCys can either be methylated to methyl-SeCys through the action of selenocysteine methyltransferase or to elemental selenium via SeCys lyase or converted by a series of enzymes to selenomethionine. Selenium toxicity or Selenosis can occur when the optimal concentration of selenium is exceeded. Two major mechanism of selenium toxicity exists; either by induction of oxidative stress or malformation of selenoproteins. Selenium uptake, metabolism and toxicity in tropical plants are hereby discussed in this chapter.

**Keywords:** selenium, distribution, toxicity, tropical plants

#### **1. Introduction**

Selenium (Se) is a widely distributed trace metalloid found in the crust of the Earth. Jacob Berzelius, Chemist first isolated selenium in 1817 and it has been known for its toxic effect. However, in 1957, some importance of selenium was discovered. It is mostly linked to sulfur and an essential nutrient for human, animals and microorganisms. Many enzymes such as thioredoxin reductase and glutathione peroxidase are mostly composed of selenium which helps the enzymes to perform roles like reproduction, tumor prevention and antioxidation [1]. Selenium can also

promote growth of lettuce seedlings by delaying senescence [2]. It can be toxic at large concentrations and can lead to pro-oxidative reactions. Also deficiency of selenium can occur in soils where selenium bioavailability is low leading to health risks for animals and humans. Supplementing fertilizers with sodium selenate has been shown to improve the food chain from soil to animals and then to humans [3]. The recommended daily intake of selenium should be adhered to for maximum utilization of its benefits. Within the plant and soil environment, selenium is converted to another chemical form [4]. The metabolism and mechanisms through which plants cope with high selenium concentrations are explained by the transformations of selenium form one form to another. The bioavailability, biotransformation, speciation, metabolism and functions all have great implications for both human and animal health. This chapter presents the physicochemical properties of selenium, its sources in the environment and locations, role of selenium in the body, metabolism, uptake and accumulation in plants.

### **2. Physicochemical properties of selenium**

Selenium being a metalloid from the same family of sulfur and oxygen, has it name derived from the word "Selene" that is, moon goddess since it is mostly linked to tellurium [5]. It has six isotope coexisting in nature with mass numbers 74, 76, 77, 78, 80 and 82 [6]. It is similar to sulfur in terms of bond energies, oxidation state, atomic size and ionization potentials [7]. Selenium possesses properties of both non-metal and metal hence it is referred to as a semi metal. It is considered stable as it does not oxidize at room temperature. It produces selenium dioxide and blue flame when it burns which is followed by an unpleasant smell. Selenium can form compounds with elements (fluorine, bromine, hydrogen and phosphorus) having a close analogy to those of sulfur [8, 9]. It has a lower affinity for oxygen than sulfur with only two oxides known; SeO3 and SeO2. Combustion of selenium in air produces dioxide which dissolves in water to give selenious acid (H2SeO3); a solution that can oxidize most metals except platinum, palladium and gold [10–20].

Selenic acid (H2SeO4) is a hygroscopic diacid with a higher oxidizing potential than H2SO4. It is produced by the reaction of oxidizing agents such as chlorine, fluorine, bromine with Se, SeO2, H2SeO3 in the presence of H2O. Reaction of selenium with hydrogen and reaction of metal selenides with acids (or water) releases hydrogen selenide (H2Se), a highly reactive compound. At about 160°C, it starts to decompose to Se and H2, it also forms a deposit of red selenium in moist air [21].

### **3. The physical and chemical forms of selenium**

Selenium exists in nature and in organisms in organic and/or inorganic forms. The organic form includes selenocysteine (Secys) and selenomethionine (Semet), while the inorganic forms includes selenate (SeO4 −2 ), selenide (Se<sup>−</sup><sup>2</sup> ), selenite (SeO3 −2 ) and selenium (Se) (**Figure 1**) [22].

Selenium exists in a solid state at room temperature and can take up various physical forms [8, 23]. Precipitation from aqueous solution produces amorphous selenium (red brick powder) with a density of 4.26 with photoconductive properties. At very high temperature between 110 and 180°C, the color turns gray, this is a variety of selenium that is thermodynamically stable and it is obtained by cooling liquid selenium hence it is used for its semiconducting properties.

*Uptake, Metabolism and Toxicity of Selenium in Tropical Plants DOI: http://dx.doi.org/10.5772/intechopen.90295*

**Figure 1.** *Selenocysteine and selenomethionine.*
