The poor translocation of the selected PPCP/EDCs from roots to leaves may be attributed to several factors. The compounds considered in this study have moderately high hydrophobicity with log Kow from 3.35 to 4.48 . Translocation of organic compounds within plants generally decreases with increasing hydrophobicity . Also, roots have higher lipid content than most other plant tissues, and neutral compounds have been shown to be preferentially distributed in tissues with high lipid content . In addition, the rapid conversion of 14C residue to the non-extractable form, as discussed above, may be another important factor for the negligible transfer from roots to other plant tissues.The use of 14C labeling, while giving unique information such as the total chemical accumulation in plant tissues, did not provide insights on the chemical composition of the accumulated residue. It is likely that some of the PPCP/EDCs were transformed in the nutrient solution before they were taken up by plants. The used nutrient solution from hydroponic cultivation was subjected to fractionation on HPLC to characterize the portions of 14C existing as parent compound and transformation products . It is evident that different PPCP/EDCs were transformed to different degrees in the nutrient solution and the presence of plants generally enhanced the transformation. In the no-plant control of DCL and NPX, the majority of 14C was in the form of the parent compound , while the percentage of 14C in the SPE aqueous filtrate or eluted on HPLC prior to the parent compound was very small . The presence of lettuce or collards did not increase the transformation of DCL or NPX, with the exception of the DCL-collards treatment,stackable flower pots where 93.8 ± 6.2% of the recovered activity was detected in the SPE aqueous filtrate.
In contrast, BPA and NP were extensively transformed, even in the absence of plants, and transformation was accelerated in the presence of a plant. For example, 50.3 ± 24.3% of the recovered 14C was identified as the parent in the BPA no-plant control, but collards and lettuce treatments had no detectable BPA. In the presence of a plant, 14C was detected in the aqueous filtrate and in HPLC eluent prior to the retention time for BPA . Extensive transformation of NP was also observed; all of the 14C from lettuce or collards cultivation was found in the aqueous phase of the extraction . The fraction of activity in aqueous phases may be attributed to transformation products that were not retained by the HLB cartridge or solvent phase during solvent extraction . Preliminary experiments showed that an average of 93.6% of 14C-BPA, 84.5% of 14C-DCL, and 92.0% of 14C-NPX were recovered from the HLB cartridges and 97.8% of the spiked 14C-NP was recovered in the solvent phase, while the activity in aqueous phases were below detection. Therefore, 14C in the SPE aqueous filtrate for BPA, DCL, and NPX, or in the aqueous phase for NP, was likely from polar transformation products containing the 14C label. The detection of transformation products in used solution suggests that some of the 14C found in plant tissues may be from transformation products formed in the nutrient solution prior to plant uptake. The demonstrated accumulation of PPCP/EDCs into leafy vegetables suggests a potential risk to humans through dietary uptake. To assess whether the concentrations detected in plant tissues in this study may present a potential human health risk, an individual’s annual exposure was estimated using values from the U.S. Environmental Protection Agency for average daily consumption of leafy vegetables . The annual exposure values ranged from 0.32 × 10−3 mg for BPA-lettuce to 2.14 × 10−2 mg for DCL-collards for an average, 70 kg individual residing in the United States.
To place these amounts in context, the values were then converted to either medical dose or 17β-estradiol equivalents. Both DCL and NPX are commonly available non-steroidal anti-inflammatory pharmaceuticals. Based on typical doses and the observed plant concentrations, an average individual would consume the equivalent of much less than one dose of these medicines in a year due to consumption of leafy vegetables, representing a very minor exposure to these PPCPs. However, it should be noted that DCL has proven ecotoxicity and NPX has shown toxicity in mixture with other pharmaceuticals , so a simple estimation may not encompass all possible human health effects. Both BPA and NP are industrial products known to have endocrine disrupting activity. Bonefeld-Jørgensen et al. calculated the Relative Potency of these compounds as compared to 17β-estradiol , an endogenous estrogen hormone, at activating estrogenic receptors. In Table 4, the exposure values of BPA and NP were estimated as E2 equivalents by dividing by their Relative Potency . When the calculated E2 equivalents of BPA and NP are compared with the Lowest Observable Effect Concentration for E2 , it is obvious that the even the highest expected annual exposure to these compounds by consuming leafy vegetables would not reach the LOEC. This rough calculation suggests that consumption of vegetables would be unlikely to influence an individual’s overall endocrine activity, though caution should be used when considering risk to susceptible population groups. Moreover, it must be noted that the use of hydroponic cultivation likely resulted in greater plant accumulation of these PPCP/EDCs, in relation to soil cultivation, due to the absence of chemical sorption to soil organic matter and minerals.
This likelihood, when coupled with the fact that most of the 14C in plant tissues was in the non-extractable form, implies that the actual plant accumulation of these PPCP/EDCs by leafy vegetables grown in uncontaminated fields irrigated with reclaimed water may be negligibly small. On the other hand, bio-solids have been shown to contain some PPCP/EDCs at much higher concentrations than treated wastewater and plant uptake from soil amended with may pose an enhanced human exposure risk. Also, given that many PPCP/EDCs may be preferentially distributed in plant roots as compared to above-ground tissues , the potential risk may be significantly greater for root vegetables such as carrots, radishes, and onions. The occurrence of these and other PPCP/EDCs in leafy and root vegetables should be evaluated in the field under typical cultivation and management conditions. Engineered nanoparticles have attracted great interests in commercial applications due to their unique physical and chemical properties. Increased usage of ENPs has raised concerns in the probability of nanoparticles exposure to environment and entry to food chain. The potential health and environmental impact of ENPs need to be understood. Plants are essential components of ecosystems and they not only provide organic molecules for energy but they can also filter air and water, removing certain contaminants. Definitively, plants play a very important role in uptake and transport of ENPs in the environment. Once ENPs are uptaken by plants and translocated to the food chains,tower garden they could accumulate in organisms and even cause toxicity and bio magnification. Nanoparticles are known to interact with plants and some of those interaction have been studied to understand their potential health and environmental impact, including quantum dots, zinc oxide, cerium oxide, iron oxide, carbon nanotubes, among others. The uptake of various ENPs by different plants was summarized in Table 1. Nanoparticles are known to stimulate morphological and physiological changes in several edible plants. Hawthorne et al. noted that the massof Zucchini’s male flowers were reduced by exposed to CeO2 NPs. Quah et al. observed the browner roots and less healthy leaves of soybean treated by AgNPs, but less effects on wheat treated under same condition. Qi et al. reported that the photosynthesis in tomato leaves could be improved by treated with TiO2 NPs at appropriate concentration. Yttrium oxide ENPs have been broadly used in optics, electrics and biological applications due to their favorable thermal stability and mechanical and chemical durability. One of the most common commercial applications is employed as phosphors imparting red color in TV picture tubes. The environmental effects of yttria ENPs have not been reported. Even though the effects of certain NPs have been studied on several plants, the uptake, translocation and bio-accumulation of yttria NPs in edible cabbage have not been addressed until this study.
This plant species was chosen and tested as part of a closed hydroponic system designed to study nanoparticles movement and distribution in a substrateplant-pest system as a model of a simple and controlled environment. The final test “substrate” used was plain distilled water , in which the tested NPs were mixed. In order to observe the translocation and distribution of ENPs in plants, transmission electron microscopy has been one of the most commonly used techniques to identify the localization at cellular scale in two-dimensions , because it can be used to observe all kinds of ENPs. On the other hand, ENPs with special properties, such as upconversion NPs and quantum dots with a particular band gap can be studied with a confocal microscope with alternative excitation wavelengths to trace the ENPs. Several synchrotron radiation imaging techniques exploiting high energy X-ray have become widely used in plant science, which can measure both spatial and chemical information simultaneously, like micro X-ray fluorescence and computed tomography. In this research, we use synchrotron X-ray microtomography with K-edge subtraction to investigate the interaction of yttria NPs with edible cabbage. By using the KES technique, the µ-XCT can not only detect the chemical and spatial information in 3D, but also analyze the concentration of target NPs. The uptake, accumulation, and distribution mapping of yttria NPs in both micro scale and relatively full view of cabbage roots and stem were investigated. We found that yttria NPs were absorbed and accumulated in the root but not readily transferred to the cabbage stem. Compared with yttria NPs, other minerals were observed along the xylem in both cabbage roots and stem. To the best of our knowledge, few reports have studied the impact of yttria NPs on cabbage plants. In addition, by using µ-XCT with KES technique, the distribution and concentration mapping of nanoparticles in full view of plant root have not been previously reported.The µ-XCT was carried out at Beamline 8.3.2 at the advanced light source, Lawrence Berkley National Laboratory. From scanning energies of 16.5 to 17.2 keV, below and above yttrium K-edge, the X-ray attenuation coefficient sharply increases by a factor of 5. Other elements decrease slightly in their attenuation coefficients over this energy range. The localization of yttria NPs can be identified by the subtraction between two reconstructed image datasets , shown in Fig. 2. The slices collected above and below the K-edge were set with same brightness and contrast settings to fairly compare with each other. The grayscale values of reconstructed slices represent the absorption coefficient; therefore, thebright regions in subtracted slice denote the localization of yttria NPs . Other elements appear dark in subtracted slice marked with a red “▲” . These are inorganic elements which support the growth of cabbage. Some biological structures suffered radiation damage during scanning, resulting in a small amount of shrinkage. The bright regions circled in Fig. 2c were caused by such shrinkage, resulting in a registration mismatch between the images above and below the edge. To identify and map the distribution of yttria NPs, an image segmentation protocol was employed that could highlight regions with yttria without finding these regions corresponding to sample shrinkage. The detailed segmentation process is given in the “Method” section.By using K-edge subtracted image technique with Monochromatic X-ray tomography, the translocation and distribution of NPs in the cabbage root is clear . Figure 3a and b were constructed by 17.2 keV and 16.5 keV reconstructed slice datasets, respectively. Their color maps were based on the transverse slice pixel values/absorption coefficients over the range from 0.2 to 17.8 cm−1 . An obvious difference between 17.2 and 16.5 keV visualization in absorption coefficient of yttria NPs was observed. The distribution of yttria NPs in root was segmented and colored in red . Since yttria NPs were not water-soluble, the water that contained them was kept in constant movement with an air pump working 24/7. However, it seems that the dense roots formed a web-like structure that made the suspended NPs to accumulate and aggregate among the roots.