Selenium is a nonmetal element belonging to the oxygen–sulfur–tellurium group, and is ranked 70th among the 98 elements that form the earth’s crust. Se is found in sulfide ores such as pyrite, where it partially replaces the sulfur. Oxidation of pyritic parent material is an important natural source of Se in soil where human activities, such as mining, groundwater drawdown, and wetland drainage, have exposed pyritic materials to a more oxidizing environment (Strawn et al., 2002). Se content in most soils ranges from 0.01 to 2 mg·kg−1, but can vary from ≈0 to >10 mg·kg−1 in certain regions (Fordyce, 2005). Se is distributed in the environment through natural processes of weathering; disposal of human, animal, and plant wastes; and emission of volcanic ash (Oldfield, 2002).
Se has been recognized as an essential trace element for animals and humans (Oldfield, 2002). Adult humans have a daily requirement of 55 to 70 μg Se. Se-deficiency diseases have been recognized in some regions: Keshan disease, an endemic cardiomyopathy, and Kashin–Beck disease, a deforming arthritis, were first identified in the Keshan region of China, where the soil is extremely low in Se (Chen et al., 1980; Tan and Huang, 1991). Diet is the main source of Se for humans and animals. Therefore, increasing Se concentrations in the tissues of edible crops by Se-fertilization strategies would improve the overall contribution of Se to human and animal diets (Carvalho et al., 2003). Plants play a unique role in recycling and delivering Se from the soil into the food chain, even though Se has not been yet confirmed as an essential plant micronutrient. In Finland, for example, selenate has been added to fertilizers since 1984 to increase the Se in soils (Alfthan et al., 2010; Wang et al., 1998), where the geochemical soil conditions are relatively uniform, two decades of supplementation of soils nationwide with fertilizers containing inorganic Se were safe and effective way of significantly increasing Se concentrations in most crop plants grown for human consumption (Alfthan et al., 2010). Great Britain has also undertaken efforts to develop soil amendment practices with inorganic Se to increase dietary Se intake through the Se biofortification of food crops (Rayman, 2012). Similarly, vegetables rich in Se contribute as much as 28% to 32% of humans’ daily Se intake in northern Mexico (Kopsell et al., 2009). Malorgio et al. (2009) investigated the effects of Se fertilizer in a hydroponic system on growth of lettuce and chicory and Se content in the plant tissues. Addition of 0.5 and 1.0 mg·L−1 Se in the nutrient solution had a positive effect on plant yield and increased Se content in the crops’ leaves. Kopsell et al. (2009) reported a linear accumulation of Se up to 56 mg·kg−1 in leaves of basil after foliar fertilization with three applications of 32 mg·L−1 Se. In that study, daily Se application in the irrigation system seemed to be more efficient than foliar application.
Contrary various industrial activities, such as oil refineries, electrical utilities, and waste from glass, synthetic pigments, and semiconductor devices can contaminate soil and water bodies with Se (Mirbagheri et al., 2008; Terry et al., 2000). In addition, irrigation of semiarid farmlands in seleniferous regions is a common source of Se contamination, particularly in the presence of an impermeable subsurface layer, where leached Se can accumulate to toxic levels. This phenomenon has been well documented in the San Joaquin Valley of California, where high concentrations of Se (≈300 μg·L−1) in the subsurface agricultural drainage water caused a high incidence of deformity and mortality in waterfowl hatchlings at the Kesterson National Wildlife Refuge (Deverel and Millard, 1988; Fio et al., 1991; Fujii et al., 1988; Ohlendorf et al., 1986; Spallholz and Hoffman, 2002). Anthropogenic Se contamination of groundwater was documented in the Shimron wells located in the Yizre’el Valley in northern Israel (Michelson, 1990). A high concentration of Se (up to 37 μg·L−1) in the well water caused shutdown of two wells in the surrounding area (Michelson, 1990). This high Se concentration could enter the food chain and injure humans and animals. In humans, daily intake greater than 900 μg Se may result in toxicity, termed selenosis (Kopsell et al., 2009).
Plants accumulate selenate against its electrochemical potential gradient by active transport. Among the factors that affect Se status in the plant, species is the most important. Plants can be classified into three main groups according to their Se uptake: primary, secondary, and non-Se accumulators. The Se toxicity threshold for nonaccumulator plants varies from 2 to 330 mg·kg−1 DW in rice and white clover, respectively (Terry et al., 2000). In contrast, Se-accumulator plants can hold Se concentrations of >4000 mg·kg−1 with no toxic effects (Terry et al., 2000). Beath et al. (1937) found a Se level of 14,990 ppm in a sample of Astragalus racemosus, which is a primary accumulator. Also, most plants, even when grown in seleniferous soils, only contain ≈10 ppm Se, or less. Se can accumulate in plant tissues to levels that are toxic to the plant itself. In this case, high Se contents in the plant tissue can cause growth inhibition, yield reduction, chlorosis, and even plant mortality (Terry et al., 2000). Hurd-Karrer (1937) was the first to describe Se phytotoxicity (snow-white chlorosis) in wheat plants that were exposed to 20 mg Se/kg soil in a pot experiment. Se phytotoxicity in wheat was also investigated under field, glasshouse, and laboratory conditions by Lyons et al. (2005), In that study, no Se toxicity symptoms were observed in the field trials with rates of up to 120 g Se/ha as selenate, and in pilot trials with up to 500 g Se/ha applied to the soil or up to 330 g Se/ha applied to the foliage, with soils containing low sulfur (S) concentrations (2–5 mg·kg−1). The critical tissue level for Se toxicity was 325 mg·kg−1 on a DW basis, attained by adding 2.6 mg Se/kg to the growth medium as selenate. Solution concentrations above 10 mg Se/L inhibited early root growth of wheat in laboratory studies (Lyons et al., 2005).
The narrow margin between beneficial and harmful levels of Se has important implications for human health and crop production. Most studies have focused on either supplementation or toxicity aspects of Se, mainly through Se soil amendment or foliar fertilization. Se supplementation via fertigation could provide a practical and efficient method for crop fortification. Therefore, it is important to detail the relationships between Se concentrations in the nutrient solution, plant growth, and Se content. Using tomato and basil as model plants for crops with edible fruits and leaves, respectively, the specific objectives of the present study were to a) examine a wide range of Se concentrations in the irrigation water to determine the concentrations that can enrich basil and tomato plants with Se without damaging yield and b) assess and study Se phytotoxicity threshold values and underlying mechanisms.
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