Cilantro (Coriandrum sativum) cultivated in soil exhibits enhanced photosynthetic pigments and reduced lipid peroxidation when exposed to ZnO nanoparticles -

Cilantro (Coriandrum sativum) cultivated in soil exhibits enhanced photosynthetic pigments and reduced lipid peroxidation when exposed to ZnO nanoparticles.

Introduction: Nanotechnology has become pervasive in modern life, with engineered nanomaterials (ENMs), synthesized at the nanoscale (dimensions below 100 nm), serving as foundational elements in various applications (Reddy et al., 2014, 2016). As these materials are extensively used, they end up in environmental compartments such as soil, water, and air, prompting investigations into their impact on living organisms, particularly terrestrial plants (Pullagurala et al., 2018a,b; Reddy et al., 2017; Ruotolo et al., 2018; Verma et al., 2018). Studies exploring the effects of ENMs like TiO2, ZnO, CeO2, CNTs, and CuO on edible plants have yielded diverse and sometimes conflicting results (Montes et al., 2017; Rawat et al., 2018; Cota-Ruiz et al., 2018; Adisa et al., 2018). Notably, the global production of N ZnO surpassed 30,000 metric tons/year by 2010, finding applications in coatings, cosmetics, energy, and environmental sectors (Keller and Lazareva, 2013; Ong et al., 2018), with consequential disposal into soil, wastewater, and landfills (Smeraldi et al., 2017). While detrimental effects of N ZnO on plants are commonly reported, a few studies have indicated beneficial outcomes (Liu and Lal 2015; Tripathi et al., 2017a). Recognizing the inconsistencies in findings, this research aims to discern the impact of N ZnO on various aspects of plant growth and development. Chlorophyll, crucial for photosynthesis, and carotenoids, essential plant pigments in stress defense, are particularly important. Previous research has demonstrated N ZnO-induced reductions in chlorophyll content but unchanged carotenoid levels in Arabidopsis plants (Wang et al., 2016). Additionally, N ZnO has been associated with oxidative stress in plants, as evidenced by the reduction of malondialdehyde (MDA) levels in chickpea following foliar application (Burman et al., 2013). This study explores the effects of N ZnO exposure on cilantro, focusing on concentrations of 0, 100, 200, and 400 mg/kg. The impact on chlorophyll, carotenoids, and MDA content is assessed. Additionally, 1H NMR-based metabolomic fingerprinting is employed to analyze variations in metabolic analytes, providing insights into potential metabolic changes induced by exposure to Zn-based compounds. The analytical approach is both reliable and operationally straightforward, offering a comprehensive understanding of N ZnO’s impact on plant metabolism.

Materials and Methods:

Zinc-based Materials and Cilantro Seeds: Nano ZnO (N ZnO), bulk ZnO (B ZnO), and ionic ZnCl2 (I Zn) were obtained from the University of California Center for Environmental Implications of Nanotechnology (UC-CEIN). NanoZnO particles were 24 ± 3 nm in size, and their characteristics were detailed in supplementary material. Bulk ZnO (ACS reagent≥99.0% purity) and ZnCl2 (ACS reagent 97+% purity) were procured from Sigma-Aldrich and Acro Organics, respectively. Cilantro seeds were obtained from Del Norte Seed and Feed.

Plant Growth Conditions and Harvest: Suspensions of the three Zn-based compounds were prepared at concentrations of 100, 200, and 400 mg/kg. After sonication, the suspensions/solutions were mixed with soil to reach the desired concentrations. Cilantro seeds were planted in pots containing the treated soil and incubated in a growth chamber. After 34 days, the roots and shoots were harvested for analysis.

Chlorophyll in Leaves of Cilantro: The relative chlorophyll content in cilantro leaves was measured at harvest using a single-photon avalanche diode (SPAD) chlorophyll meter (Minolta, Japan SPAD, Spectrum Technologies).

Carotenoid Content: Carotenoid content was determined by homogenizing leaf tissue in chilled methanol, followed by centrifugation. The absorbance of supernatants was recorded at 470 nm, and values were expressed as mg carotenoids g−1 fresh weight.

 Malondialdehyde (MDA) Content: MDA content in shoots was determined following a method by Wang et al. (2017). Plant tissues were homogenized in 10% trichloroacetic acid (TCA), and MDA content was calculated based on absorbance readings at different wavelengths.

NMR Sample Preparation and Measurements: 1H NMR sample extraction involved freezing and powdering leaves, homogenizing with acetone, and suspending the extract in CDCl3. 1H NMR spectra were recorded on a Bruker spectrometer, and metabolites were identified by comparing shifts with published data.

 Elemental Quantification via Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES):

Dried plant tissues were digested in nitric acid, and elemental analysis, including Zn, Mg, Ca, Mn, Cu, and Fe, was performed using ICP-OES. National Institute of Standards and Technology (NIST) standards and a multielemental standard solution were employed for quality control.

Results:

Chlorophyll and Carotenoid Determination: Chlorophyll content significantly increased for all compounds at all concentrations, with N ZnO and B ZnO at 400mg/kg showing significant differences compared to the control (p ≤0.05). N ZnO at various concentrations increased relative chlorophyll by 41%, 37%, and 58%, respectively, compared to the control. Carotenoid content decreased for most treatments, except N ZnO at 100mg/kg, which had a significant increase compared to the control (p ≤0.05). Similar behavior was reported in literature, suggesting a need for further studies to understand variations in carotenoid responses.

Malondialdehyde (MDA) Content: MDA content in cilantro leaves showed significant results for all treatments compared to the control (p≤0.05). N ZnO at 400mg/kg exhibited a dramatic 72% decrease in MDA levels, suggesting potential induction of anti-stress enzymes. B ZnO, however, increased MDA levels significantly at all concentrations. ZnCl2, while inducing higher MDA levels compared to N ZnO, showed lower elevation than B ZnO. The role of ZnO compounds in triggering lipid peroxidation warrants further investigation.

Analysis of 1H NMR Spectra:1H NMR analysis revealed changes in metabolic fingerprints, particularly in the carbinolic region, indicating variations in biomolecules related to glycerolipids, sterols, and pheophytins. This aligns with findings in GC-MS analysis of basil foliar applied with copper-based compounds.

Zinc Uptake in Root and Shoot: Zn concentration in roots and shoots varied among treatments. B ZnO at 400mg/kg showed the highest Zn concentration in roots. N ZnO treatments exhibited a concentration-dependent increase in Zn in roots, with higher accumulation at 200mg/kg than at 400mg/kg. Shoot-to-root Zn ratios differed significantly among treatments, with ZnCl2 resulting in the highest Zn content in shoots. Stems of plants exposed to N ZnO had more Zn compared to B ZnO, possibly due to easier translocation. Zn uptake in B ZnO and ZnCl2 treatments was concentration-dependent, with B ZnO showing higher uptake than N ZnO at 400mg/kg. Soil properties and pH may influence Zn accumulation, suggesting the need for further investigations.

Conclusion:

This study investigated the impact of ZnO nanoparticles (N ZnO), bulk ZnO (B ZnO), and ZnCl2 (ionic/I Zn) on cilantro plants. All Zn compounds increased chlorophyll content by at least 50%, with N ZnO at 400mg/kg decreasing lipid peroxidation by 70%. 1NMR data revealed significant changes in carbinolic-based compounds for all treatments. B ZnO at 400 and I Zn at 100 showed the highest root and shoot Zn uptake, respectively. Notably, N ZnO, particularly at concentrations <400mg/kg, demonstrated potential as a less toxic option, improving photosynthetic pigments and inducing anti-stress enzyme generation. However, higher concentrations (400mg/kg) affected cilantro's nutritional constitution, emphasizing the need for further studies to explore optimal concentrations and species-specific responses.

Reference:

PULLAGURALA, V.L.R., ADISA, I.O., RAWAT, S., KALAGARA, S., HERNANDEZ-VIEZCAS, J.A., PERALTA-VIDEA, J.R. AND GARDEA-TORRESDEY, J.L., 2018. ZnO nanoparticles increase photosynthetic pigments and decrease lipid peroxidation in soil grown cilantro (Coriandrum sativum). Plant physiology and biochemistry, 132, pp.120-127.