The influence of arsenic speciation (AsIII & AsV) and concentration on the growth, uptake and translocation of arsenic in vegetable crops (silverbeet and amaranth): greenhouse study

Abstract

We examined arsenic (As) uptake by vegetable crops (amaranth, Amaranthus gangeticus, and silverbeet, Beta vulgaris) as affected by As speciation (As(III) and As(V)) and their concentrations in nutrient solution. Amaranth and silverbeet were grown in a nutrient solution containing four levels of arsenate (As(V)): 0, 1, 5, and 25 mg As/l and three levels of arsenite (As(III)): 0, 5, 10 mg As/l. Both As(V) and As(III) are phytotoxic to these crops with the latter being five times more toxic. Amaranth treated with As(III) exhibited As toxicity symptoms within 48 h of exposure and was close to death within 1 week. However, As(V) treatment did not show clear toxicity symptoms other than wilting and yield reduction at the highest dose rate of 25 mg As(V)/l. The main mechanism used by vegetable crops to tolerate As(V) is probably avoidance-limiting As transport to shoots and increasing As accumulation in the root system. When As(V) was added to the nutrient solution, the uptake of As in shoots increased and, at the highest dose (25 mg As(V)/l), 60 microg As/g DW (3.6 mg/kg FW) accumulated in the edible portion, which exceeds the WHO recommended limit for food stuffs (2 mg/kg FW) as the water contents of the crops were 94%. It is therefore important to determine the nature of the As species and their bio-accessibility. Iron treatment with 0.5 mg NaFe(III)EDTA/l dose decreased silverbeet As uptake by 45% given its affinity to bind As at the root surface or root rhizosphere and so restrict As translocation to the shoots.

Transcriptome profiling of genes and pathways associated with arsenic toxicity and tolerance in Arabidopsis

“…In many circumstances, it is the As-sensitivity of the root that limits the productivity of the entire plant [11]. Hence, plants exposed to As show inhibited root growth and reduced photosynthetic rate [8,12]. While plant roots are the first organs in contact with As, assaying the processes occurring in the roots could provide potential strategies to determine how plants respond and adapt to the heavy metal stress.…”

Impact of arsenic-contaminated irrigation water on vegetables

“…An increase in As concentration of the groundwater applied for irrigation results in an increase in the As accumulation and toxicity in plants. This has been documented in rice, tomato, radish, bean, amaranth, and silverbeet (Caporale et al, 2013;Carbonell-Barrachina et al, 1999;Burló et al, 1999;Azizur Rahman et al, 2007;Rahman and Naidu, 2009). In tomato, As concentrations of 50 and 100 mg/kg resulted in inhibition of vegetative growth and root system (Miteva, 2002).…”

“…Studies on As in vegetables have generally been carried out in hydroponics (Carbonell-Barrachina et al, 1999;Rahman and Naidu, 2009) or soil media (Tlustoš et al, 2006;Miteva, 2002) applying different concentrations of As to determine the As uptake potential and negative impacts of As on the vegetables. However, these studies applied extremely high concentrations of As in soil and groundwater (up to 10 mg/L in hydroponics and 5-100 mg/kg As in soil), which resulted in visible toxicity symptoms in plants.…”

“…Arsenate and arsenite cause damages in plants by different mechanisms. Arsenite was reported to be five times more toxic than arsenate to amaranth and silverbeet plants, causing visible toxicity symptoms in 48 hours and death in a week (Rahman and Naidu, 2009). Contrarily, in lettuce, at a similar As concentration arsenate produced a more pronounced decrease in root and shoot growth as compared to arsenite.…”

“…Irrigation with As-contaminated groundwater leads to the contamination of the agricultural soil (Meharg and Rahman, 2003;Biswas et al, 2014) and vegetables cultivated in As-contaminated soil and/or irrigated with As-contaminated water uptake and accumulate As (Bhattacharya et al, 2010;Huang et al, 2006;Roychowdhury et al, 2005). Under conditions of elevated As in the growing medium, vegetables can accumulate high concentrations of As, exceeding food safety limits (Rahman & Naidu, 2009). These vegetables thus become pathways for As proliferation in the food chain, and people consuming them are prone to As toxicity (Santra et al, 2013; Azizur Rahman et al, 2008).…”