Arsenic (As) is a toxic and carcinogenic element present naturally in low quantities in the environment that is harmful to plants and human beings. Arsenic is a natural component of the earth’s crust and is widely distributed throughout the environment in the air, water, and land. According to the World Health Organization, it is highly toxic in its inorganic form, a confirmed carcinogen, and the most significant chemical contaminant in drinking water globally. People are exposed to elevated levels of inorganic arsenic by drinking contaminated water, using contaminated water in food preparation and irrigation of crops, industrial processes, eating contaminated food and smoking tobacco. Arsenic has various natural mechanisms of environmental introduction, for example, weathering aquifer rocks and geothermal activity, irrigation with As-bearing groundwater, run-off from mines, smelting, As-based insecticides, herbicides, and fertilizers. Arsenic is highly toxic in its inorganic form and is naturally present at high levels in the groundwater of several countries. Contaminated water used for drinking, food preparation, and irrigation of food crops poses the greatest threat to public health from arsenic. Long-term exposure to arsenic from drinking water and food can cause cancer and skin lesions, and it has also been associated with cardiovascular disease and diabetes. In utero and early childhood, exposure has been linked to negative impacts on cognitive development and increased deaths in young adults.

Groundwater abundance and natural availability allow farmers to utilize groundwater irrigation and free farmers of rainwater dependence. Furthermore, rainwater cannot meet the high demands of present-day agriculture and the growing population. Although there are no recommended limits for vegetables, the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) have set thresholds for As in drinking water (10ug/L) and irrigation water (100ug/l). Although irrigation water is limited to 100ug/L, As can still build up in crop soil which, in turn, increases the As density within the plant. Ultimately, the As build-up in the plant reaches the consumer’s plate and contributes to dietary health hazards. Hyper accumulation of As in the soil potentiates a change in overall soil quality, and these trace elements in soil and plants alter food quality and safety.

Accumulating As in soil from irrigation water presents either inorganic arsenate or arsenite, which are heavily affected by soil pH, redox potential, and soil organic matter (SOM). All bioavailable As in the soil is not phytoavailable; uptake depends on plant type and species, plant ability to incorporate and translocate As, and the predominant As species. Arsenic accumulation in plants is usually in the order: roots > stem>leaves>edible parts. Low concentrations of As have been reported to increase growth in plants, but As is a metabolic inhibitor, negatively impacting the plant at high concentrations. Excessive As inhibits germination, reduces root and shoot growth, lowers the yield, produces reactive oxygen species (ROS), affects photosynthesis mineral nutrition, and causes necrosis, thus posing a threat to agricultural productivity. Mobility of As in soil is dependent on the soil characteristics; soils rich in Al or Fe oxides bind As strongly; thus, sandy soil has the lowest As retention, while silt soil tends to have a high As concentration retained by the clay fraction and OM content. Sandil et al. (2021) show that a low dosage of As increases chlorophyll content and improves photosynthetic rate due to increased P and N uptake as the plant tries to combat stress from As within the plant system.

Source: Abbas et al., 2017

Figure 1. The plant pathways that activate upon uptake, transportation, and detoxification of arsenic are not well understood. However, there are some studies that have been done to try to understand these pathways better. One study found that plants have an enzyme called arsenate reductase, which helps them detoxify arsenic by converting it into less harmful forms. Another study found that plants use a transporter called AtABCG32 to help them take up arsenic from the soil and transport it to their roots, where they can then use arsenate reductase to detoxify it.

Source: Abbas et al., 2017

Figure 2. Plant toxicity due to arsenic includes metabolic, physiological, biochemical, and genetic damage, which activates plant stress. The toxic effects of arsenic on plants are well documented. The most common plant toxicity symptoms due to arsenic are chlorosis and necrosis. Chlorosis is a yellowing of the leaves that can be caused by a lack of chlorophyll or iron in the plant. Necrosis is the death of cells in the leaf tissue that can be caused by various factors, including disease or injury. Arsenic has been shown to cause plants metabolic, physiological, biochemical, and genetic damage. This damage activates plant stress responses such as oxidative stress and heat shock proteins (HSPs).