Abstract
Zinc oxide nanoparticles (ZnO-NPs) hold promise as novel fertilizer nutrients for crops. However, their ultra-small size could hinder large-scale field application due to potential for drift, untimely dissolution or aggregation. In this study, urea was coated with ZnO-NPs (1%) or bulk ZnO (2%) and evaluated in wheat (Triticum aestivum L.) in a greenhouse, under drought (40% field moisture capacity; FMC) and non-drought (80% FMC) conditions, in comparison with urea not coated with ZnO (control), and urea with separate ZnO-NP (1%) or bulk ZnO (2%) amendment. Plants were exposed to ≤ 2.17 mg/kg ZnO-NPs and ≤ 4.34 mg/kg bulk-ZnO, indicating exposure to a higher rate of Zn from the bulk ZnO. ZnO-NPs and bulk-ZnO showed similar urea coating efficiencies of 74–75%. Drought significantly (p ≤ 0.05) increased time to panicle initiation, reduced grain yield, and inhibited uptake of Zn, nitrogen (N), and phosphorus (P). Under drought, ZnO-NPs significantly reduced average time to panicle initiation by 5 days, irrespective of coating, and relative to the control. In contrast, bulk ZnO did not affect time to panicle initiation. Compared to the control, grain yield increased significantly, 51 or 39%, with ZnO-NP-coated or uncoated urea. Yield increases from bulk-ZnO-coated or uncoated urea were insignificant, compared to both the control and the ZnO-NP treatments. Plant uptake of Zn increased by 24 or 8% with coated or uncoated ZnO-NPs; and by 78 or 10% with coated or uncoated bulk-ZnO. Under non-drought conditions, Zn treatment did not significantly reduce panicle initiation time, except with uncoated bulk-ZnO. Relative to the control, ZnO-NPs (irrespective of coating) significantly increased grain yield; and coated ZnO-NPs enhanced Zn uptake significantly. Zn fertilization did not significantly affect N and P uptake, regardless of particle size or coating. Collectively, these findings demonstrate that coating urea with ZnO-NPs enhances plant performance and Zn accumulation, thus potentiating field-scale deployment of nano-scale micronutrients. Notably, lower Zn inputs from ZnO-NPs enhanced crop productivity, comparable to higher inputs from bulk-ZnO. This highlights a key benefit of nanofertilizers: a reduction of nutrient inputs into agriculture without yield penalities.
Highlights
Zinc oxide nanoparticles (ZnO-NPs; ≤100 nm in at least one dimension) are incorporated into a variety of industrial, medical, and household products to enhance quality and functionality (Piccinno et al, 2012)
Solubility in the soil of the particles after 24 h was similar between the ZnO-NPs and bulk ZnO, with recoveries around 100% when 10 mg of ZnO was applied in 20 g of soil without plants
ZnO-NPs of similar shapes as obtained in this study have previously been observed, and aggregation of ZnO-NPs when suspended in water is documented
Summary
Zinc oxide nanoparticles (ZnO-NPs; ≤100 nm in at least one dimension) are incorporated into a variety of industrial, medical, and household products to enhance quality and functionality (Piccinno et al, 2012). This is as a result of the enhanced reactivity of nanoparticles arising from their small size and greater surface area, compared to bulk particles Such heightened or nanoscale-specific effects have been observed in microbes, plants, and other terrestrial species (Dimkpa et al, 2012b; Dimkpa, 2014; Anderson et al, 2018; Rajput et al, 2018). In addition to greater nanoscale reactivity, the degree of the effects of ZnO-NPs depends on dose, plant species and age, exposure route and duration, and environmental conditions such as pH and surface interactions with other soil components (Jośko and Oleszczuk, 2013; Watson et al, 2015; Mukherjee et al, 2016; García-Gómez et al, 2017; Dimkpa et al, 2019a). In the context of agriculture and human and environmental health, ZnO-NPs are being systematically assessed in plants for their enhanced ability to modulate productivity and nutrient use efficiency; confer tolerance to biotic and abiotic stresses; and fortify edible plant parts with Zn (Elmer and White, 2016; Raliya et al, 2016; Dimkpa et al, 2017a; Dimkpa et al, 2017b; Elmer et al, 2018; Dimkpa et al, 2018b; Zhang et al, 2018; Adisa et al, 2019; Dimkpa et al, 2019a; Dimkpa et al, 2019b)
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