In:
Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, Vol. 109, No. 37 ( 2012-09-11)
Abstract:
This study did not evaluate plant responses to the metals in plant tissues, the speciation of MNM metals in plant tissues, or the definitive source of nitrogen in plant tissue. Nevertheless, the implications of this study are profound: we provide evidence that two commonplace metal oxide MNMs can change soybean growth, thereby suggesting the eventual necessity for shifts in current agricultural practices. Our results reinforce the importance of the idea of making MNMs sparingly bioavailable by design and thus more environmentally compatible (i.e., likely to cause minimal impact on the environment, and hence agriculture) ( 5 ), and of managing waste streams to prevent the crop-damaging soil buildup of toxic MNMs. Our results prove that these MNMs are indeed bioavailable to plants, and have the potential to dramatically affect food crops. Although aboveground biomass yield was not affected by nano-ZnO, plants grown with high nano-ZnO produced more belowground biomass (i.e., root and nodule). Most strikingly, Zn bioaccumulated throughout the plants, with the amount increasing according to nano-ZnO dose. Although the amount of Zn in soybean pods was high, the highest concentrations were in the leaves. In comparison, plants grown with low nano-CeO 2 had reduced leaf cover, were shorter, and grew more slowly, whereas plants grown with high nano-CeO 2 yielded less soybean biomass. Although Ce was not translocated into aboveground plant tissue, it did bioaccumulate in the roots and root nodules. Then, although potential N 2 fixation rates were not affected by nano-ZnO, the potential rates were near zero in medium and high nano-CeO 2 treatments. The root nodules of plants grown with high nano-CeO 2 appeared to be devoid of nitrogen-fixing bacteria. However, these plants grew faster and larger than plants with low nano-CeO 2 , implying that, with the loss of atmospheric nitrogen fixation capacity, plants were able to successfully compensate by using the soil as an alternative nitrogen source. One implication of this shift is that, under nitrogen-limited field conditions, the application rate of synthetic fertilizer would need to be increased. The broad implication is that food quality, as related to metal content, is significantly altered by nano-ZnO in soils. Furthermore, nitrogen fixation is impaired in plants grown with nano-CeO 2 . In this study, we demonstrated the bioaccumulation of MNMs in a soil-grown food crop and highlight the potential risks to soil fertility and the harvestable food supply. Soybean is an important crop worldwide, with nearly all tissues and extracted oil used for food, feed, and commerce. Soybean is a plant that is able to increase available nitrogen by fixing nitrogen gas from the atmosphere via root nodule symbioses (i.e., interactions) with nitrogen-fixing bacteria. This characteristic of leguminous crops is important to soil fertility because it reduces the use of energy-intensive, and therefore polluting, synthetic fertilizers. Soybean plants were cultivated in greenhouses with three concentrations of nano-ZnO (i.e., zinc oxide) or nano-CeO 2 (i.e., cerium oxide) added to organic farm soil. These metal oxide MNMs, each with a primary particle size of ∼10 nm, are in high production worldwide, and thus are probably accumulating in soils via land-applied wastewater biosolids, which are the final residues from wastewater treatment. Previous research has indicated that these metal oxides may affect hydroponic soybean ( 1 ), and that nano-ZnO alters soil microbial community composition and biomass ( 2 ). Several plants grown hydroponically are affected by myriad MNMs, raising concerns regarding the long-term effects of these materials on the food supply ( 3 ). However, MNMs may not be bioavailable (i.e., accessible to organisms) in soil ( 4 ), and hydroponic studies do not provide information on bioavailability constraints or the effects on symbiotic nitrogen fixation. Therefore, we evaluated plant growth responses, the effects on symbioses related to soil fertility, and the propensity for MNM bioaccumulation, i.e., the uptake of MNMs from the surrounding soil into plant tissues. We used EM and X-ray microscopy to visualize MNM-associated metal accumulations in plant tissue, and energy dispersive spectroscopy to identify elements in electron-dense micrograph features. Manufactured nanomaterials (MNMs), manmade materials with at least one dimension less than 100 nm, are increasingly used in consumer goods (e.g., sunscreens, fuels, and paints), and thus are entering the atmosphere and soil. Some MNMs stress the cells of plants, causing growth inhibition, MNM uptake, DNA damage, and death. For instance, hydroponically grown plants (i.e., plants grown in mineral nutrient solutions) are damaged when they accumulate some MNMs. Also, some MNM-exposed soil microbial communities become less diverse. Therefore, soil-grown food crops could be impacted by MNMs, although there is a lack of direct evidence. We studied soybean growth from planting through seed production in farm soil to which MNMs were added. For two high-production MNMs, we observed the bioaccumulation of MNM metals and the cessation of symbiotic nitrogen fixation.
Type of Medium:
Online Resource
ISSN:
0027-8424
,
1091-6490
DOI:
10.1073/pnas.1205431109
Language:
English
Publisher:
Proceedings of the National Academy of Sciences
Publication Date:
2012
detail.hit.zdb_id:
209104-5
detail.hit.zdb_id:
1461794-8
SSG:
11
SSG:
12
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