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The small size of nanomaterials gives them unique properties which differ from their larger counterparts. For example, zinc oxide is more soluble, cerium oxide displays enhanced antioxidant property, silicon exhibits electrical conductivity, and gold becomes chemically reactive at nanoscale.
Such peculiar properties are being exploited and tailored for different applications that can spur scientific discoveries, promote economic growth, create jobs, improve human health, prevent/cure diseases, and safeguard the environment. The wider use of these materials has increased their release into the environment through soil, water, and air, which may lead to unintended contamination of terrestrial and aquatic ecosystems.
Nanomaterials Entering the Environment
One route in which nanomaterials enter the environment and humans is through agriculture. As many nanomaterials enter into widespread use, huge amounts of nanomaterials are expected to accumulate in sewage sludge in wastewater treatment plants. A significant portion of sludge is added to agricultural fields as fertilizer.
Alternatively, nano-agriculture - the application of nanomaterials in agricultural practices - could present a more serious and direct threat to environmental and human risks. Nano-enabled agriculture is particularly attractive because it offers beneficial improvements exceeding those of farm mechanization and the green revolution.
Nanoformulations are claimed to boost the efficacy of agricultural chemicals, improve delivery systems, promote plant nutrient uptake and yield, and enhance food quality at minimal impact to the environment. In fact, nano-agricultural inputs like nanopesticides and nanofertilizers have been commercially available for several years already, and new products are expected to inundate the market as thousands of patent applications are currently in the pipeline.
As such, it is highly possible that edible plants could be exposed to high levels of nanomaterials from direct purposeful application of nano-enabled agricultural inputs. Unfortunately, the difference between the potential benefits and harm from nano-enabled products may be quite subtle and a large knowledge gap exists on the long-term impacts of nanomaterials to the environment, crop production, and human health.
Assessing Crop Responses to Nanomaterials
Agronomic assessments of crop responses to nanomaterial exposure have been performed. The body of literature provides sufficient proof that nanomaterials can alter plant growth, phenological development, grain formation, and crop yield, which may have serious implications in agricultural productivity.
Current knowledge highlights the contradictory effects of nanomaterials in plants, which is not surprising given the complex processes involved in plant-nanomaterial interactions. For example, researchers from the University of Texas at El Paso reported that nanoceria (cerium oxide nanoparticles) exposures to wheat and barley under similar soil and environmental conditions induced a tremendous increase in shoot biomass in barley but only minor changes in wheat. They also found that nanoceria was detrimental to grain production in barley, but improved grain yield despite the delay in grain formation and maturity in wheat. Similarly, related studies reveal that nanomaterials impose unknown risks to plant-associated microorganisms, enzyme activity, and microbial compositions/processes in soil, all of which may elicit critical changes in soil health, nutrient cycling, and bacteria-plant symbiotic function. While nanomaterials may benefit agriculture, available evidence is inconclusive to support widespread nanomaterial application in agricultural practices.
Nanometerials in the Food Chain
Nanomaterial entry in to the food chain is a serious concern because it represents a pathway for human exposure. The accumulation of nanomaterial in plants depends on crop species and type of nanomaterial; however, there is overwhelming evidence showing the storage of nanomaterials and/or component metals (e.g. zinc from zinc oxide nanoparticles) in the edible portions of food crops such as tomato, cilantro, cucumber, rice, barley, maize, beans, green peas, and peanut.
Once nanomaterials are absorbed by plants, they can move through trophic levels and compromise the food web. In fact, there are studies demonstrating the trophic transfer and biomagnification of nanomaterials in aquatic and terrestrial organisms. Other issues raised are the possible compromise of nutritional quality since nutrient uptake can be altered in crops exposed to nanomaterials and also increased accumulation of already existing soil contaminants in plants.
Knowledge Gap
Understanding food safety issues associated with nano-agriculture is a critical aspect in gaining public acceptance of this technology. Unfortunately, it is an area where a huge knowledge gap exists. The fate of nanomaterials and the resulting implications for organisms that consume nanomaterial-contaminated food crops are not well understood.
The present state of knowledge in nano-agriculture is still in a foundational stage. Not only is data limited and inconclusive regarding nanomaterial impacts in agricultural productivity and food safety, but more information is needed on properties that control nanomaterial effects in plants.
It is evident that particle size, coating, and surface charge, application dose, and whether nanomaterials are applied as foliar or soil-based has an influence on nanomaterial impacts and bioavailability to plants. The interplay of these factors gives confounding results making it almost impossible to predict nanomaterials impacts in plants.
Unlike herbicides and pesticides wherein mechanisms of action can be established based on the functionality of the active ingredients, chemical characteristics that can predict environmental behavior and impacts of nanomaterials are unknown. Hence, plant responses to nanomaterials treatments have been reported but the mechanism of action are not yet understood.
Other shortcomings in current nanophytotoxicity studies are the lack of assessments on materials developed for agricultural applications and use of dosage that reach up to several orders of magnitude higher than predicted environmental concentration.
Summary
In summary, nano-agriculture is an emerging technology with potential risks that should be understood in order to fully realize its economic benefits.
References
- Gardea-Torresdey et al. (2014) Environmental Science and Technology 48:2526-2540.
- DeRosa et al. (2010) Nature Nanotechnology 5:91.
- Kah et al. (2013) Critical Reviews in Environmental Science and Technology 43:1823-1867.
- Rico et al. (2014) Journal of Agricultural and Food Chemistry 62:9669-9675.
- Rico et al. (2015) Environmental Science and Pollution Research DOI10.1007/s11356-015-4243-y.
- Hawthorne et al. (2014) Environmental Science and Technology 48:13102-13109.
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