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.
AslamM.HarbitK.B.HuffakerR.C.1990Comparative effects of selenite and selenate on nitrate assimilation in barley seedlingsPlant Cell Environ.13773782
AlfthanG.P.AspilaP.EkholmM.EurolaM.HartikainenH.HeroH.2010Nationwide supplementation of sodium selenate to commercial fertilizers: History and 25 year results from the Finnish selenium monitoring program p. 312–337. In: B. Thompson and L. Amoroso (eds.). Combating micronutrient deficiencies: Food-based approaches. FAO/CAB International Rome
BanuelosG.S.MeekD.W.HoffmanG.J.1990The influence of selenium, salinity, and boron on selenium uptake in wild mustardPlant Soil127201206
BeathO.A.GilbertC.S.EppsonH.F.1937Selenium in soils and vegetation associated with rocks of Permian and Triassic ageAmer. J. Bot.2496101
CarvalhoK.M.Gallardo-WilliamsM.T.BensonR.F.MartinD.F.2003Effects of selenium supplementation on four agricultural cropsJ. Agr. Food Chem.51704709
DekokL.J.KuiperP.J.C.1986Effects of short-term dark incubation with sulfate, chloride and selenate on the glutathione content of spinach leaf-disksPhysiol. Plant.68477482
DeverelS.J.MillardS.P.1988Distribution and mobility of selenium and other trace-elements in shallow groundwater of the western San Joaquin Valley, CaliforniaEnviron. Sci. Technol.22697702
FioJ.L.FujiiR.DeverelS.J.1991Selenium mobility and distribution in irrigated and nonirrigated alluvial soilsSoil Sci. Soc. Amer. J.5513131320
FordyceF.2005Selenium deficiency and toxicity in the environment p. 373–415. In: O. Selinus B. Alloway J.A. Centeno R.B. Finkelman R. Fuge U. Lindh P. Smedley (eds.). Essentials of medical geology: Impacts of the natural environment on public health. Elsevier Burlington MA
FujiiR.DeverelS.J.HatfieldD.B.1988Distribution of selenium in soils of agricultural fields, western San Joaquin Valley, CaliforniaSoil Sci. Soc. Amer. J.5212741283
GolubkinaN.A.ZhumaevA.A.Dem’yanova-RoiG.B.2003Pattern of selenium distribution in tomato Lycopersicum esculentumMill. Biol. Bull.30468471
KinraideT.B.2003The controlling influence of cell-surface electrical potential on the uptake and toxicity of selenate (SeO42-)Physiol. Plant.1176471
KopsellD.A.RandleW.M.1999Selenium accumulation in a rapid-cycling Brassica oleracea population responds to increasing sodium selenate concentrationsJ. Plant Nutr.22927937
KopsellD.A.SamsC.E.BarickmanT.C.DeytonD.E.KopsellD.E.2009Selenization of basil and cilantro through foliar applications of selenate-selenium and selenite-seleniumHortScience44438442
LyonsG.H.StangoulisJ.C.R.GrahamR.D.2005Tolerance of wheat (Triticum aestivum L.) to high soil and solution selenium levelsPlant Soil270179188
MalorgioF.DiazK.E.FerranteA.Mensuali-SodiA.PezzarossaB.2009Effects of selenium addition on minimally processed leafy vegetables grown in a floating systemJ. Sci. Food Agr.8922432251
MichelsonH.1990Chemical (selenium) and biological contamination of Shimron wells in the southern Nazareth mountainsIsr. J. Earth Sci.39131137
MillerO.R.1997Nitric-perchloric acid wet digestion in an open vessel p. 57–61. In: Y.P. Kalra (ed.). Handbook of reference methods for plant analysis. CRC Press NY
MirbagheriS.A.TanjiK.K.RajaeeT.2008Selenium transport and transformation modelling in soil columns and ground water contamination predictionHydrol. Processes2224752483
OhlendorfH.M.HoffmanD.J.SaikiM.K.AldrichT.W.1986Embryotic mortality and abnormalities of aquatic birds—apparent impacts of selenium from irrigation drainwaterSci. Total Environ.524963
OldfieldE.J.2002Selenium world atlas. Belgium Selenium-Tellurium Development Association (STDA) Grimbergen Belgium
PezzarossaB.PiccotinoD.ShennanC.MalorgioF.1999Uptake and distribution of selenium in tomato plants as affected by genotype and sulphate supplyJ. Plant Nutr.2216131635
ShennanC.SchachtmanD.P.CramerG.R.1990Variation in [75Se] selenate uptake and partitioning among tomato cultivars and wild speciesNew Phytol.115523530
StrawnD.DonerH.ZavarinM.McHugoS.2002Microscale investigation into the geochemistry of arsenic, selenium, and iron in soil developed in pyritic shale materialsGeoderma108237257
WangW.C.MäkeläA.L.NäntöV.MäkeläP.LagströmH.1998The serum selenium concentrations in children and young adults: A long-term study during the Finnish selenium fertilization programmeEur. J. Clin. Nutr.52529535
WhiteP.J.BowenH.C.MarshallB.BroadleyM.R.2007Extraordinarily high leaf selenium to sulfur ratios define ‘Se-accumulator’ plantsAnn. Bot. (Lond.)100111118
WhiteP.J.BowenH.C.ParmaguruP.FritzM.SpracklenW.P.SpibyR.E.MeachamM.C.MeadA.HarrimanM.TruemanL.J.SmithB.M.ThomasB.BroadleyM.R.2004Interactions between selenium and sulphur nutrition in Arabidopsis thalianaJ. Expt. Bot.5519271937