Other common transformations may also be of great importance to improve our understanding of environmental risks of CECs. For example, halogenated CECs can be produced during the disinfection process that is commonly used in treating wastewater and drinking water, and such halogenated derivatives may have very different biological activity profiles as well as environmental behaviors from their precursors. Conjugation with endogenous bio-molecules has been widely observed for biologically mediated CEC transformations. For example, conjugates of CECs and/or their metabolites are common in higher plants. Enzymes such as glucuronidases, aminoacylases, and dipeptidases in the human gut and intestine may hydrolyze these conjugates, releasing the parent or metabolites in their free form. Future research is needed for these unique TPs of CECs to obtain a more comprehensive understanding of the environmental fate and risks of CECs.Global climate change leads to more variable and extreme weather conditions that decrease total agricultural production of staple food crops. To cope with less predictable growth conditions under temperate climate conditions during spring and summer, an earlier time point of drilling may be chosen. Cereal plants tiller before winter, and due to an established crown root system the tillers are more resistant against temporary drought stress in spring. After the winter period, however, tillering continues in particular with nitrogen fertilization in spring, even though tiller number might already be sufficient for optimum yield. Thus, there is a need to uncouple tillering from nitrogen supply. To investigate the physiological effect of different N forms on tillering, spring barley was cultured in buffered nutrient solution supplied with stabilized nitrogen forms. Plants were harvested at the end of the vegetative growth phase and analysed for biomass, tiller number,vertical farming systems concentrations of mineral nutrients and cytokinins.
To investigate the effect of nitrogen forms on cytokinin translocation, endogenous cytokinins were determined in xylem exudates or benzyladenine riboside was supplied to the nutrient solution and measured in xylem exudates. Cytokinins were determined by radio immuno assays using specific antibodies. For field trials winter wheat was fertilized with stabilized nitrogen forms in the starter dressing and analysed at the end of the vegetation period for nitrogen levels, phytohormone levels and yield components.The vast majority of ENM studies have examined the acute toxicity of nanoparticles and particle forms to determine if they represent a risk to human health and/or the environment . In studies that examined the effects of metal oxide nanoparticles on plants, most studies have shown low to moderate toxicity, even at relatively high ENM concentrations . However, release of ENMs into the environment may have other subtle effects on plant uptake and use of important nutrients, which could alter growth and development. For example, nitrogen is one of the most important nutrients since it is an essential component of amino acids, proteins and nucleic acids, including the carboxylating enzyme involved in photosynthesis . Although many forms of N occur in soils, not all forms are available to plants. In addition, microbial processing of N affects pools and fluxes of N in soils. Understanding the effects of ENMs on factors such as N uptake and metabolism is important not only to understand plant growth and development, but also for understanding how ENMs may affect ecosystem processes. Previous studies showed the impacts of heavy metal on N metabolism in plants, and it is possible that metal oxide nanoparticles could influence N uptake. Sutter et al. reported that Cd, Pb, and Zn decreased 15N abundance in aquatic moss while Schmidt et al. found that As or As significantly decreased 15N incorporation in Silene vulgaris. These researchers found that metals affected N uptake and protein synthesis which resulted in decreased metabolic activity of plants. We also reported decreases in 15N/14N ratio of wheat treated with cerium oxide nanoparticles , but did not find whether the isotopic changes occurred in the soil, the root rhizosphere, or after N uptake through changes in root or shoot metabolism . In order to help isolate the mechanisms underlying changes in N uptake and/or metabolism in response to ENM exposure, we used hydroponic systems to allow us to control the forms and isotopic ratios of N supplied to the roots, and to minimize the influence of soil interactions external to plant roots.
We selected CeO2-NPs since they are widely used in many technological applications that could reach the environment and interact with terrestrial/agricultural plant species . In this study, the influence of CeO2-NPs on nitrogen metabolism of different forms of N in wheat and barley was explored. The hypotheses were 1) CeO2-NPs do not alter uptake of N or growth in wheat and barley regardless of the form of N supplied, i.e., NO3 – , NH4 + or NH4NO3, 2) shifts in the isotopic ratios of N in leaves and roots in response to the different forms of N supplied are not influenced by CeO2-NPs exposure, and 3) wheat and barley show similar isotopic ratios in response to the different N forms and to CeO2-NP exposure. We chose to study N because CeO2-NPs modified N and 15N abundance in wheat , and we chose wheat and barley because these species vary in response to CeO2-NPs exposure, possibly indicating different modes of action . We tested 500 mg CeO2- NPs/L because this exposure level in soil altered roots, shoots, and grains δ 15N in wheat .The nanomaterial suspensions were prepared as previously described in Rico et al. . The nutrient solution was placed in 150-mL plastic jars . CeO2-NPs were added to the solution then sonicated for 30 mins at 20⁰C with occasional stirring. After sonication, the jars were covered with caps that had three holes where cuttings from 3 mL plastic pipette were fitted to hold two plants and air pumps . Air was constantly supplied using air pumps. All materials used for the hydroponic experiment were sterile and soaked in 10% hypochlorite solution before use. Two nine-day-old wheat or seven-day-old barley seedlings were grown in nutrient solution in growth chamber set at 16-h photoperiod, 20/10˚C, 70% humidity, 300 μmol/m2 -s. At harvest, root and shoot were separated, washed thoroughly with Milli-Q water. After drying in the oven, total biomass was measured. Plant materials were ground and subjected to N and 15N analysis. The table of two-way ANOVA with CE and NS as main factors is presented in Appendix C . For wheat, NS was significant for all parameters measured while CE was significant only for every biomass, N contents, and δ 15N measurements . In barley, NS was also significant for all parameters except root N concentration, whereas CE was significant only for total shoot and plant N contents, root N concentration, and root δ 15N . The global mean of CE were calculated and presented in Table 1. Ce-500 did not affect N concentration but decreased biomass, and all N contents and δ 15N in wheat, but increased global mean shoot and plant N, root N concentration, and root δ 15N in barley.
Our results led to rejection of all three hypotheses tested in this study. N uptake and metabolism plays a central role in all cellular functions in plants, and the shifts observed here in response to CeO2-NP exposure indicate that ENMs have the potential to alter how important nutrients such as N are utilized in plants, even when toxicity is not evident. Our hydroponic experiments removed the chemical and biological complexity of interactions that occur at the root-soil interface in soils in order to better understand possible mechanisms underlying changes in N uptake and metabolism in the two species. Additional studies will be needed to examine CeO2-NP and N interactions in soil-grown plants, and to evaluate the longer-term consequences of changes in N dynamics in plants exposed to CeO2-NPs. CeO2-NPs did not affect the root influx of inorg from NH4 + since total plant and shoot N contents increased without changes in biomass and N concentration . However,vertical growing systems the very high whole-plant and shoot δ 15N discriminations in Ce-500 coupled with its low root δ 15N compared to Ce-0 strongly suggest low influx of 15N into the roots or high efflux of 15Nenriched inorg to outside roots . A previous study showed that discrimination in NH4 + uptake could result from efflux of 15N-enriched NH4 + from inorg . This could happen when NH4 + gets assimilated immediately in the roots which increases the pool of 15N-enriched NH4 +, then the 15N-enriched NH4 + are transported out of the roots. Due to its toxicity, NH4 +, generally is rapidly assimilated or flushed out of the roots . In contrast, data seem to suggest that CeO2-NPs decreased root to shoot translocation of 15N when the wheat seedlings were grown in NO3 – because root δ 15N increased to similar level with shoot δ 15N despite decreased shoot N content and a lack of net change in whole-plant δ 15N discrimination . Wheat was a good discriminator of 15N when the N source was NH4NO3 as shown by notable whole-plant and shoot δ 15N discriminations in Ce-0. Exposure to CeO2-NPs only increased root δ 15N discrimination despite remarkable decreases in biomass production and N content. The decrease in root δ 15N was probably due to the discrimination against 15N similar to what was observed in wheat in NH4 +. It is also probable that shoot δ 15N from Ce-0 and Ce-500 was from δ 15N of the source NO3 – , and the decrease root δ 15N was due to discrimination against 15N from NH4 +. Previous reports also showed that lower N uptake decreased 15N abundance in Cd-treated aquatic moss . In another report, alfalfa plants that exhibited impaired growth features when subjected to high carbon dioxide concentration and water deficiency had negative leaf δ 15N values . In this study, N concentration in both shoot and root did not differ between Ce-500 and Ce-0 indicating that CeO2-NPs did not affect N uptake and that reduced total N content was due to low plant biomass. It is possible that low biomass was due to reduction in macromolecules such as fatty acids and lignins similar to what was observed in rice seedlings exposed to CeO2-NPs . The synchrotron micro-XRF analysis showed that both wheat and barley translocated CeO2- NPs to the shoots regardless of N source in the growth media . We have shown in previous hydroponic studies the uptake of Ce in wheat and barley seedlings , which corroborates the plant uptake of CeO2-NPs recorded in the current study. Our data on speciation is in agreement with data normally reported in the literature regarding the accumulation of CeO2-NPs in plants grown in hydroponic culture solution . Unfortunately, the data does not allow further speciation analysis to determine which part of the plants the reduction occurred. Our findings also revealed that CeO2-NPs were translocated to the shoots suggesting an uptake of CeO2-NPs in barley plants; however, barley seedlings did not exhibit decreases in biomass or 15N uptake . More studies should be performed to understand why CeO2-NPs markedly disturbed N or 15N uptake in wheat than barley. Copper and its compounds have been known to have the ability to inhibit fungi since ancient times and have been used widely in agriculture as fungicides,algaecides,pesticides,and herbicides.There are at least 209 pesticide products registered in California that use copper oxide as an active ingredient.In addition, due to steady increase of drug resistance of bacteria, synthesis and application of novel antibacterial/anti antifungal Cu nanoparticles has increased.Besides antibacterial applications, Cu NPs also have application as additives of livestock and poultry feed.There is increasing concern about the potential for bio-accumulation and toxicity of Cu NPs after their release to the environment. It has been shown in several studies that nano-Cu triggers reactive oxygen species generation and induces oxidative stress in cells, bacteria, and zebrafish.However, very few studies have focused on the toxicity of Cu NPs on terrestrial plants, especially crop plants. Lee et al.,documented that Cu NPs are toxic to mung bean and wheat at concentrations of 335 and 570 mg/L, respectively. Hong et al.reported that even at the level of 5−20 mg/L, Cu NPs significantly reduced the root length of alfalfa and lettuce and altered their nutrient uptake.