Nano zinc oxide coated with urea helps to increase crop yield
Nano zinc oxide (ZnO-NPs) hold promise as new fertilizer nutrients for plants. However, their ultra-small size can impede large-scale field application due to the potential for untimely drift, dissolution or agglomeration. In this study, urea was coated with ZnO-NP (1%) or bulk ZnO (2%) and evaluated in wheat (Triticum aestivum L.) in a greenhouse, under arid conditions (Triticum aestivum L.) 40% field moisture holding capacity; FMC) and no drought conditions (80% FMC), compared with uncoated urea (control) and urea supplemented with ZnO-NP (1%) or ZnO (2%) ). Plants that were exposed to ≤ 2.17 mg/kg ZnO-NP and ≤ 4.34 mg/kg bulk ZnO , showed higher rates of Zn exposure from bulk ZnO . ZnO-NP and ZnO bulk showed similar urea coating efficiency of 74–75%. Drought significantly (p ≤ 0.05) increased flowering start time, reduced seed yield and limited the uptake of Zn, nitrogen (N) and phosphorus (P). Under drought conditions, nano zinc oxide significantly reduced the mean time to flowering by 5 days, regardless of coating and compared with the control. In contrast, bulk ZnO did not affect the initiation time of cotton formation. Compared with control, grain yield increased significantly, 51 or 39%, with urea coated with ZnO-NP or uncoated. The yield increase from bulk or uncoated ZnO-coated urea was insignificant, compared with both the control and ZnO-NP treatments. Zn uptake by plants increased to 24 or 8% with coated or uncoated ZnO-NPs; and 78 or 10% with or without coated bulk ZnO. Under non-drying conditions, Zn treatment did not significantly reduce the time to flower initiation, except for uncoated bulk ZnO. Compared with the control, nano zinc oxide (regardless of coating) significantly increased grain yield; and coated zinc oxides enhance Zn absorption 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 zinc oxide nanoparticle enhances crop yield and Zn accumulation, thereby promoting field-scale nanoscale micronutrient deployment. Notably, the lower Zn input from zinc oxide enhanced crop yields, comparable to the higher input from bulk ZnO. This highlights a key benefit of nano-fertilizers: reducing nutrient inputs into agriculture without compromising yields.
Copyright by NanoCMM Technology
INTRODUCTION
Nano zinc oxide (ZnO-NPs; ≤100 nm in at least one dimension) are incorporated into a wide variety of industrial, medical and household products to improve quality and functionality (Piccinno et al., 2012). ). However, ZnO is a biological reactant, causing beneficial and harmful effects on the environment. Compared with ZnO bulk particles, such effects can be accentuated if exposed to ZnO-NPs. This is the result of enhanced reactivity of the nanoparticles arising from their small size and larger surface area, compared with bulk particles. Such height or nanoscale-specific effects have been observed in bacteria, plants and other terrestrial species ( Dimkpa et al., 2012b ; Dimkpa, 2014 ; Anderson et al., 2018 ; Rajput et al., 2018). In addition to the reactivity at the larger nanoscale, the extent of the effect of ZnO-NPs also depends on the dose, plant species and age, route and duration of exposure as well as environmental conditions such as degree of 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 ). The contrasting (toxic versus. beneficial) effects of ZnO-NPs suggest that they can be used as plant fertilizers if provided in reasonable doses. Accordingly, in the context of agriculture and human and environmental health, ZnO-NPs are being systematically evaluated in plants to enhance their ability to regulate yield and nutrient utilization efficiency. ; create tolerance to biotic and abiotic stresses; and fortification of 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 ).
One of the potential benefits of nanoscale fertilizers is the ability to reduce nutrient fertilization rates without reducing yields ( Kottegoda et al., 2017 ), thereby saving input costs and reducing impacts. environment of chemical fertilizers in a sustainable way. As discussed previously ( Dimkpa and Bindraban, 2018), despite these benefits, the use as nano-fertilizers of Zn and other essential trace elements in large-scale field crop production is currently not feasible due to some potential complications. In the case of the application of powder nanoparticles, the suspension of the nanopowder in the air will result in great loss and potential for human inhalation and subsequent health hazards to handlers and objects. unintended biodegradation. While the deep placement of nanopowder into the soil can minimize the risk of handling, adhesion of the particles to the fertilizer surface, especially in wet conditions, can interfere with effective distribution. Similarly, suspensions of nanoparticles in water, especially unstable products (i.e. bare nanomaterials with no surface function), for use as soil trenches, foliar spray, or fertilizer irrigation there are at least two potential problems. In the aqueous medium, the particles can dissolve into ions; or they can agglomerate into non-nanoscale particles. Particle dissolution at high rates disturbs the efficiency of nanofertilizer treatment due to the ubiquitous ionic activity (García-Gómez et al., 2017 ; Qiu and Smolders, 2017). In contrast to dissolution, the aggregation of nanoparticles to produce larger particles negates the definition of “nano” and the reactivity decreases. Thus, in both cases, the particle modification counteracts the basic function of the nanofertilizer. Furthermore, the large weight difference between urea granules and ZnO powder causes size-dependent segregation when they are mixed together and packaged for transport. This particle separation problem is made worse when zinc oxide is used to mix with urea. Therefore, there is a need to develop effective and safe methods of delivering nano-sized nutrients to plants, and to rationalize fertilization events in a macronutrient regime. Dimkpa and Bindraban, 2018 ) . Previous studies have described the coating of urea with zinc oxide and studied the kinetics of Zn dissolution from urea ( Milani et al., 2012 ; Milani et al., 2015); Notably, the effect of zinc oxide nano-coated urea on crop yield was not evaluated.
When developing nano-assisted fertilizers for soil application, the product’s effectiveness in field crop production can be affected by natural phenomena such as drought, which reduces the mobility of nutrients. nutrients in the soil and thus absorbed by plants. Indeed, drought continues to decimate different regions of the globe, with devastating consequences for soil nutrient bioavailability and crop yields ( Lesk et al., 2016 ; Moreno-Jiménez et al., 2019 ). Mechanistically, Zn can mitigate the effects of drought on plants ( Karim et al., 2012 ; Dimkpa et al., 2017a ; Dimkpa et al., 2019b), due to the role Zn plays in metabolism. water dynamic regulator. For example, under water stress conditions, plants produce high amounts of abscisic acid (ABA) to optimize stomatal closure to conserve water ( Karim and Rahman, 2015 ; Yang et al., 2018 ) . Zn is known to increase ABA production in plants ( Zengin, 2006 ; Yang et al., 2018 ), thereby enhancing stomatal regulation of ABA under water-restricted conditions.
Little is known about whether coating ZnO-NPs on urea leads to better results, in terms of performance and nutrient uptake, for crops grown under difficult environmental conditions and there is no challenge, or this new material will compare with other fertilization regimes such as with urea and ZnO-NP applied separately. The objectives of this study were to: i) understand the difference in yield of wheat using ZnO-NP vs. urea separates from ZnO-NP; ii) determine whether ZnO-NP coated on urea can mitigate the impact of drought stress on wheat yield; and iii) evaluate whether using a lower dose of ZnO-NP could produce the same effects as bulk ZnO at a higher dose. Collectively, all effects were compared with those of bulk ZnO to determine the significance of the nanoscale.
materials and methods
Chemistry
Commercial Nano zinc oxide (18 nm) were purchased from US Research Nanomaterials, Inc., Houston, Texas, USA. ZnO powder (≥1,000 nm) was purchased from Sigma-Aldrich, St Louis, Missouri, USA. For in vitro characterization, a suspension of zinc oxides in water is sonicated, followed by precipitation. The supernatant was filtered with a pipette (20 µm pore) and diluted 1:1 in methanol. One drop (3 μl) of suspension was mounted on a 300 mesh carbon-coated Cu grid. The nanoparticles were imaged using a transmission electron microscope (TEM; Hitachi 7800) in high resolution mode at an accelerating voltage of 80 kV. Furthermore, the solubility of both types of oxide particles in the experimental soil was evaluated by adding 10 mg of each powder separately to 20 g of soil and incubating at room temperature for 24 h. The spiny soil was extracted in diethylenetriaminepentaacetic acid (DTPA) extract solution ( Lindsay and Norvell, 1978), shaken, filtered, and then centrifuged at 10,000 rpm for 10 min. The supernatants were collected and analyzed for dissolved Zn by inductively coupled plasma optical emission spectroscopy (ICP-OES; model Spectro Arcos, SPECTRO Analytical Instruments GmbH, Kleve, Germany). The ZnO-NPs were not further characterized in soil because of the size-related properties. This is because the complexity of the soil environment, such as the presence of natural nanoscale colloids, will disturb the results of the NP size characterization in the soil.
Urea coating with ZnO . powder
Dry nano zinc oxide (0.4 g) or ZnO powder (0.8 g) was put into transparent plastic bottles. To those were added 0.4 ml of commercial vegetable (canola) oil and 0.08 ml of black food coloring (McCormick, Hunt Valley, Maryland) for ZnO nanopowder, or 0.8 ml of vegetable oil. object and 0.16 ml food coloring for ZnO powder. Urea particles and ZnO powder are both white; therefore, the food coloring creates a contrast signaling the physical binding of the ZnO powder to the urea surface which would otherwise be difficult to observe with the naked eye. The solutions were mixed to produce a gray ZnO slurry. Urea granules were sieved in sieves with a cut-off of 2 mm to obtain particles of uniform size. Approximately 40 g of urea granules were added to the ZnO slurry. With this ratio, vegetable oil and food color accounted for 1.0 and 0.2% by weight, respectively, of urea seeds, for the control drug and nanomedicine mixture; and 2.0 and 0.4% by weight of urea for bulk ZnO powders, respectively. Similarly, bulk ZnO-NPs and ZnOs represent 1 and 2% by weight of urea, respectively. The control urea was only coated with vegetable oil and food coloring and did not add any ZnO powder. Each slurry granular urea mixture was transferred to a low speed mechanical shaker generating approximately 32 rpm and all samples were allowed to mix overnight. Urea coated with ZnO was analyzed for final Zn composition by acid digestion (20 ml 50% HCl), then boiled for 15 min, filtered and diluted. ICP-OES was used to determine the Zn content. Urea initially contains 46% N; After coating, the N content is 45.7 and 45.3%, respectively.
Prepare the land
The experimental soil is sandy loam soil with the following characteristics: pH 6.87; organic matter content 0.92%; N and P bioavailability are 4 and 2 mg/kg, respectively; and the bioavailable Zn (extractable from DTPA) was 0.1 mg/kg, indicating a state of Zn much lower than the soil level important for most crops, 0 ,5–1.0 mg/kg. The soil was supplemented with P (75 mg/kg; from monocalcium phosphate) and potted at a dose of 8 kg/pot, in three replicates. No K was added, as the soil contained just the right amount, at 1,903 mg/kg.
The growth of plants
A greenhouse potting experiment involving winter wheat (Triticum aestivum vector. Dyna-Gro 9522) was conducted in Muscle Shoals, Alabama (34.7448°N, 87.6675°W) for a period of time from November to May 2018–2019 (temperature, 1–33 °C; relative humidity, 25–92%). Three wheat seeds are sown in the pot and are pruned to a single seed upon germination. Two weeks after germination, the gourds were fertilized with Zn-coated and uncoated urea; Specifically, 217 mg of urea was applied per kg of soil by subsurface fertilization about 2 cm from the base of the plant and about 3 cm deep. With 46% N, the urea intake resulted in a nitrogen fertilization rate of about 100 mg/kg (217 x 0.46). Coating urea with 1 or 2% ZnO powder and applying 217 mg of urea/kg of soil resulted in the application of 2.17 mg of ZnO/kg of soil for nanoparticles and 4.34 mg/kg of soil for bulk seeds. These levels of ZnO correspond to ≈1. 7 and ≈3.5 mg Zn/kg soil, respectively. Therefore, the corresponding percentages of Zn used directly for soil amendment for the uncoated urea treatments were 17.4 mg ZnO-NP and ≈34.7 mg ZnO bulk powder, respectively. on each pot. Finally, five urea-Zn treatments were established, each in three replicates: i) control (urea coated with vegetable oil and food coloring); ii) control of urea coated with ZnO-NP (1%); iii) urea control + individual addition of nano zinc oxide ZnO-NP (1%); iv) control of bulk ZnO coated urea (2%); and v) urea control + individual addition of ZnO block (2%). Each of these treatments was replicated for arid and non-arid conditions, resulting in a total of 10 treatments. corresponding to ≈17.4 mg ZnO-NP and ≈34.7 mg ZnO powder in bulk per pot. Finally, five urea-Zn treatments were established, each in three replicates: i) control (urea coated with vegetable oil and food coloring); ii) control of urea coated with ZnO-NP (1%); iii) urea control + individual addition of ZnO-NP (1%); iv) control of bulk ZnO coated urea (2%); and v) urea control + individual addition of ZnO block (2%). Each of these treatments was replicated for arid and non-arid conditions, resulting in a total of 10 treatments. corresponding to ≈17.4 mg ZnO-NP and ≈34.7 mg ZnO powder in bulk per pot. Finally, five urea-Zn treatments were established, each in three replicates: i) control (urea coated with vegetable oil and food coloring); ii) control of urea coated with ZnO-NP (1%); iii) urea control + individual addition of ZnO-NP (1%); iv) control of bulk ZnO coated urea (2%); and v) urea control + individual addition of ZnO block (2%). Each of these treatments was replicated for arid and non-arid conditions, resulting in a total of 10 treatments. iv) control of bulk ZnO coated urea (2%); and v) urea control + individual addition of ZnO block (2%). Each of these treatments was replicated for arid and non-arid conditions, resulting in a total of 10 treatments. iv) control of bulk ZnO coated urea (2%); and v) urea control + individual addition of ZnO block (2%). Each of these treatments was replicated for arid and non-arid conditions, resulting in a total of 10 treatments.
One week after Zn treatment, part of the seedlings were subjected to drought stress by maintaining the soil at 40% of the field moisture capacity (FMC) until harvest. Forty percent FMC was predefined using untested potting soil. Finally, each pot was submerged in water until completely rinsed for 24 hours into a rack below. The pots were then weighed and soil water determined at 100% FMC by subtracting the weight of washed soil from the weight at 100% FMC by weight. With potted plants, individual pots are weighed periodically to determine the amount of water needed for each pot, as plant biomass varies per pot. Then the required amount of water for each pot was added to achieve 40% FMC. This regimen was maintained throughout the remainder of the plant’s growing period to keep the plant arid at 40% FMC. In contrast, stress-free plants were kept at 80% FMC. During vegetative growth, the flowering time (beginning of the flower by the main bud) is monitored; and when the plant is fully mature, the plant is harvested, the seed yield is analyzed, and the aboveground plant parts are analyzed for nutrient content.
Analysis of phytonutrients
The harvested plant tissues were dried in an oven at 60°C until a constant weight was achieved. Dried tissues were pulverized using a Thomas Wiley Model 4 Laboratory Mill (Pennsylvania, USA). Soil tissues were acid-digested in 75% sulfuric acid solution (3 ml acid + 1 ml H 2 O 2 50%), heated for 1 h at 350°C, cooled to ambient temperature and equilibrated by distillation. H2O. Subsamples of the prepared tissues were then subjected to Skalar fractionation flow analysis for N and P, or ICP-OES for Zn as outlined above. Soil samples were also collected from the harvest pots for each treatment, to determine the pH and bioavailability of N (as ammonium and nitrate fractions), P and Zn. So the soil without any root seed was collected for each treatment. Detailed procedures for soil extraction and analysis of these elements have been described previously ( Dimkpa et al., 2018a ).
Data analysis
A two-way analysis of variance (ANOVA; OriginPro 2018) was used to identify significant differences in plant responses to Zn treatments as a function of water state, for each variable, including including the vegetative and reproductive development as well as the nutrient content of the plants and soil samples. Fisher mean comparisons of least significant difference (LSD) were performed to further explore differences with significant ANOVA (p = 0.05). Considering that the actual exposure ratio of Zn between ZnO treatments was different, the values obtained for the phytometric variables differed with significant ANOVA for Zn treatment (namely, time of initiation of Zn treatment). flowering head, seed yield and Zn uptake) were normalized by dividing the values by the respective Zn exposure ratio.
Result
Characterization of zinc oxide nanoparticles
The images of the zinc oxide nanosheets obtained by TEM are presented in Fig. Nano zinc oxide come in many shapes, including rectangular, tubular, angular and circular. However, particles with amorphous shape can also be seen. Particles with sizes less than 100 nm and others larger than 100 nm were present, confirming the presence of both aggregated and discrete nanoscale structures. The soil solubility of the particles after 24 h was similar between ZnO-NPs and bulk ZnO, with a recovery of about 100% when 10 mg of ZnO was applied in 20 g of plant-free soil.
Urea coating with nano zinc oxide and bulk particles
Coating urea with bulk zinc oxide and ZnO nanopowders in vegetable oils and food colorings results in a dark gray color of urea particles, in contrast to uncoated urea which is white. Visual observation shows that the entire surface of each urea particle is covered with a layer of slurry, showing uniform coverage (Figure 2). However, post-coating evaluation of the process indicates that not all sludge is coated with urea; Some of the dry, gray granules adhered to the wall of the plastic barrel used to mix the mud and urea. Accordingly, determining the Zn content in the Zn-coated urea particles, showed that the Zn content in the nano-coated urea was only 0.74 ± 0.002% by weight of urea, contrary to the original 1% norm. Similarly, Zn in bulk coated urea was 1.51 ± 0.003% by weight of urea, in contrast to the 2% target. With initial ZnO amounts of 0.4 and 0.8 g, this finding implies that only ≈0.3 g nano and ≈0.6 g bulk ZnO powder were eventually coated on 40 g urea. This showed similar efficiency, 74 and 75%, of urea coating for Nano zinc oxide and bulk ZnO. From the point of view of exposure to plants, Table 1 ).
Effect of Zn-coated Urea on Panicle growth in wheat under drought stress
Compared with adequate water conditions (80% FMC), drought (40% FMC) significantly delayed flowering in wheat by about 4–11 days, depending on treatment ( Figure 3 ; left panel ). Under drought conditions, ZnO-NP nanozinc oxide strongly reduced the delay in flowering initiation, regardless of whether the metal was coated with urea or not. In contrast, the ZnO bulk form irrespective of coating did not affect cotton formation time under drought conditions. Overall, Zn fertilization had little significant effect on flowering time in the non-drying scenario, although there was a clear trend towards reduced flower formation time in all treatments using Zn. Only in the case of uncoated ZnO bulk form was the floc formation time different from that of the control ( Figure 3; left panel). However, when the effects of the actual Zn exposure rate are considered by expressing the values per mg Zn as the change in flower initiation and normalization per unit exposure Zn, it can be seen that ZnO-NPs, regardless of urea coating or not, particularly facilitate plant growth under drought conditions, compared with bulk ZnO treatments ( Figure 3 ; panel). the right). However, as suggested by the data in the left panel, the specific effect of this nano is not evident under non-arid conditions.
Effect of Zn-coated Urea on Wheat Grain Yield under Drought Pressure
Drought drastically reduces grain yields, compared with non-dry conditions, by 59–73%, depending on treatment. Under drought conditions, nano zinc oxide significantly increased grain yield compared with control and irrespective of urea coating or not. In contrast, bulk ZnO resulted in intermediate, insignificant effects on grain yield, compared with both control and ZnO-NPs zinc oxide nanotreatment. As with nano-scale treatment, coating of urea with bulk ZnO did not produce a significantly different effect, compared with the individual application of bulk ZnO (Figure 4; left panel). The effect of Zn on grain yield in the non-drying treatments mimicked the effect of the arid treatments. Unfortunately, the variation was greater in the repeats (as indicated by the large error bars) of the bulk oxide treatments under both growth conditions, furthermore under the non-drying condition, resulted in negligible effects of that treatment, compared with the control ( Figure 4 ; left panel). When the Zn effect was evaluated by normalizing in Zn units, coating with Zn produced an effect similar to no coating on the grain yield, for both ZnO-NP and bulk ZnO zinc oxide nanoparticles under drought. However, the influence of the nanoscale is already clear. Similar to arid conditions, the effect of nanoscale on yield was also clearly demonstrated, independent of Zn coating ( Figure 4; right panel).
Effect of Zn-coated Urea on zinc absorption in wheat under drought stress
Drought significantly reduced Zn uptake into aboveground wheat tissues compared to non-dry conditions, by 29–116 %, depending on treatment. Under drought conditions, coating of nano-sized or bulk ZnO particles on urea significantly increased Zn uptake, compared with the control urea treatment. The effect of the coating with nanoscale oxide particles was not significantly different from the effect of the individual modification of the nanoparticles. In contrast, urea coated with bulk oxide yielded significantly greater Zn absorption, compared with urea with bulk oxide separately added ( Figure 5; left panel). Under non-arid conditions, only urea coated with zinc oxide nanoparticle significantly increased Zn absorption compared with the control; other treatments resulted in mean values of Zn absorption when compared with the control and ZnO-NP zinc oxide nano-coated urea (Figure 5 ; left panel). Normalization data confirmed urea coating with ZnO-NP to be more effective for Zn accumulation, compared with separate modifications. Similarly, the ZnO-NPs are more efficient in distributing Zn above ground than the monolithic ZnO Zn. These effects were consistent in both arid and non-arid growing conditions ( Figure 5 ; right panel).
Effect of Zn-coated Urea on Nitrogen and Phosphorus uptake in wheat under drought stress
Drought strongly reduces N uptake into aboveground wheat tissues (buds and seeds) compared with non-dry conditions by 12–23%, depending on treatment. However, Zn addition did not mitigate the effects of drought stress on N uptake, regardless of Zn type, coating or not on urea, and water status of the plants ( Figure 6 ; left panel) . As with N, drought significantly reduced P uptake into aboveground wheat tissues by 18–37% compared with non-dry conditions. Notably, Zn addition also did not mitigate the negative effects of drought on P uptake, regardless of whether the Zn type was coated with urea, and the water status of the plants ( Figure 6 ; side panel). right).
Effect of Urea coated Zn on postharvest soil pH and residual N, P and Zn
Plant growth in dry soil with and without Zn supplementation did not significantly change the soil pH from the pre-planting value of 6.87. In contrast, soil pH was significantly altered with adequate watering compared with arid conditions; However, the pH increase under normal watering conditions was not significantly affected by Zn treatment ( Table 2 ). Drought resulted in significantly higher N residues in the soil both in the form of ammonium and nitrate fractions. In both sections, the addition of Zn did not significantly affect the residual N, regardless of the water state. However, N persisted in the soil as nitrate rather than as ammonium in all cases, except for treatment with separately modified bulk ZnO under both growing conditions ( Table 2). The residual P level was affected by the water state, but not by the Zn treatment ( Table 2 ). Residual Zn was not affected by the water state of the soil but was significantly affected by Zn treatment. Under dry conditions, all Zn treatments increased residual Zn, more than with bulk ZnO particles. Under non-dry conditions, only bulk oxide treatments significantly increased the residual Zn in the soil ( Table 2 ). These effects were similar in each case, regardless of whether the urea was coated with Zn . or not
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Discussion
Nano zinc oxide with a similar shape to that obtained in this study were previously observed, and the incorporation of ZnO-NPs when suspended in water was also recorded (see e.g. Dimkpa et al., 2012a ; Zhang et al., 2018). However, the synthesis could also be due to drying of the suspension on the TEM grid. However, we demonstrate in this study that urea coating with a low dose of dry ZnO-NP powder can enhance wheat yield by promoting morphological development (initiation of cotton) and increasing wheat yield. Seed yield and Zn nutrition from Zn-deficient soils with drought. . To our knowledge, this is the first report on the effect of ZnO-NP-coated urea on crop yield under difficult growing conditions, in this case drought. Coating with loose fertilizers such as urea can facilitate fertilizer regimens that require the simultaneous use of macronutrients and micronutrients. However, Santos et al., 2018 ). Accordingly, the physical coating of trace nanoparticles on bulk fertilizers has been recommended as a viable option to solve this problem ( Dimkpa and Bindraban, 2018 ).
Milani et al. (2015) coated urea with Nano zinc oxide (ZnO-NP) and bulk ZnO (at 1.5% each) by spraying a small amount of water as a binder for the particles, followed by drying. They report that low Zn solubility, <1%, is similar from the products in alkaline lime soils, where the high pH induced by urea affects Zn solubility due to the agglomeration of particles dependent into pH. However, unlike alkaline soil, the soil used in this study was slightly acidic (pH 6.87), and the urea-Zn treatments did not significantly change the pH after plant growth ( Table 2) , possibly due to the secretion of organic acids against any alkalinity induced by urea. This suggests that dissolution is the main fate of ZnO for both Zn species, in the coated and uncoated urea system. This is indicated by similarly high levels of residual bioavailable Zn in the postharvest soil ( Table 2 ), which agrees with a previous study ( García-Gómez et al., 2017 ). Irfan et al. (2018) also coated urea with bulk ZnO particles (2%) using a paste consisting of honey wax, gum arabica (5% each), paraffin wax or molasses, without and with heating (60°C) under stirring. They reported that the solubility of Zn after 24 h was greater than in the heated system. Compared with the methods of Milani et al. (2012) and Irfan et al. (2018) , the procedure described in the present study, although equally easy, was different because vegetable oils and food coloring were used. This may provide different binding properties and solubility of Zn, compared with water or other described binding agents, in addition to adding carbon to soils containing low organic matter.
Notably, the percentage of plants exposed to Zn from ZnO-NP coated urea was slightly lower (6%) than that of urea with separate Nano zinc oxide (ZnO-NP) addition (1.6 vs. kg). Compared with bulk ZnO, it is 52–54% lower, depending on whether or not urea is coated (see Table 1). Importantly, the ZnO-NP coated urea not only affected flowering initiation time, seed yield and Zn accumulation to a similar extent as uncoated urea with separate ZnO-NP modification at the rate of ZnO-NP. slightly higher Zn exposure, it performed well compared to bulk ZnO even with a significantly lower Zn ratio (52%). Notably, the effect of Zn on cotton initiation at exposure rates was actually at the nanoscale specific under drought conditions. Previously, we reported an acceleration of sorghum growth induced by ZnO-NPs, where flagellum and seedhead emergence was prolonged by drought, but that delay was alleviated by ZnO-NPs. NPs ( Dimkpa et al., 2019b); ZnO or bulk Zn salts were not co-evaluated in that study. In wheat, Zn salt alone (1–3 kg/ha; ≈ 0.5–1.5 mg/kg) did not affect the number of days of reaction in the original soil containing 0.17 mg/kg Zn. . However, the interaction of using N and Zn significantly reduces the reaction time ( Sher et al., 2018). Therefore, the present study in which bulk ZnO-NPs but not ZnO reduced flowering initiation time in drought-tolerant plants but not in non-stressed plants showed both nanospecificity and the dependence on the water state of these effects. The mechanism surrounding the effect of ZnO-NPs on plant growth under arid conditions may involve Zn influencing hormone induction to regulate root growth to improve plant growth. adapted to a limited water supply. Indeed, the expression of genes related to hormones such as ABA and cytokinin was enhanced by ZnO-NPs in drought-stressed wheat plants, along with regulation of root structure that helps to tolerate stress ( Yang et al., 2018). In addition, soil microorganisms can facilitate hormonal activity in plants, as plants can access hormones produced by microorganisms, which contribute to tolerance to dry stress. term ( Defez et al., 2017 ; Yang et al., 2018 ). While hormones such as indole acetic acid (IAA) can prolong the vegetative growth period of plants, heavy metals can reduce plant IAA levels, as reported in depleted cowpeas growing in heavy metal contaminated soil ( Dimkpa et al., 2009 ). Thus, as with some other metals, ZnO-NP nanozinc oxide could alter the effects of IAA in plants by reducing its levels, reported for bacteria (Dimkpa et al. , 2012c ; Haris and Ahmad, 2017). Thus, the time to the onset of reproductive development in wheat was accelerated in the presence of ZnO-NPs under drought conditions.
The important finding that lower Zn ratio from Nano zinc oxide (ZnO-NP) increases grain yield similar to higher ratio from bulk ZnO highlights the value of nanoscale fertilizers as a tool to reduce fertilizer input rates in agriculture, while maintaining the same level or even increasing yields compared to bulk fertilizers. Along these lines, Subbaiah et al. (2016) demonstrated that maize yield could be increased to a greater extent by ZnO-NP at a dose 60-98% lower than the bulk dose of Zn-sulfate. However, unlike our study, the Zn ratio assessed by Subbaiah et al. (2016) was at high levels, 50–2,000 mg/L, and route of administration was foliar, rather than soil. In studies involving urea coated with a bulk form of ZnO or Zn-sulfate (0.5–2.0%), wheat grain yield was increased by 5 to 18 or 8 and 22% compared with controls ( Shivay et al., 2008a ). The present report on grain yield increase due to ZnO-NPs is consistent with previous studies with other crops ( Dimkpa et al., 2017a ; Dimkpa et al., 2017b ; Dimkpa et al., 2018b) ; Dimkpa et al., 2019a ; Dimkpa et al., 2019b ), and confirmed that Zn coating on urea did not reduce the effectiveness of the fertilizer due to its potential effect on solubility, as Milani et al. have shown. (2012).
Accordingly, the aboveground tissue Zn accumulation from nanoscale and bulk ZnO particles was not reduced when the particles were coated with urea. In fact, Zn accumulation was facilitated by coating at a higher Zn ratio in the bulk oxide treatments under dry conditions; and the coating was slightly more efficient than the individual modification to distribute Zn under non-arid conditions in the ZnO-NP system. Furthermore, tissue Zn levels were similar between the nanoscale and bulk ZnO treatments, although Zn exposure was lower at the nanoscale. Plant Zn enhancement through fertilization has been reported ( Joy et al., 2015 ; Dimkpa and Bindraban, 2016). However, studies are limited on the use of Zn coated urea to enhance Zn nutrition for plants; Notably, these studies involved the bulk form of ZnO or Zn salts ( Shivay et al., 2008a ; Shivay et al., 2008b ). Thus, as demonstrated for the first time in the present study for ZnO-NPs, coating urea with Zn represents a clear strategy to facilitate Zn introduction into plants, even with a relatively low Zn ratio, and regardless of the soil water condition. It is clear that the characterization of Zn in plants exposed to NPs, although not evaluated in this study, may be Zn-phosphate, based on other reports involving ZnO-NPs and wheat ( Dimkpa) et al., 2013 ; Zhang et al., 2018). We observed no effect of Zn treatment on N and P accumulation in wheat. This is somewhat surprising, in contrast to previous observations ( Dimkpa et al., 2018b ). For N, the previous observation involving wheat showed that absorption into aboveground plant parts could be increased by up to 21% with 6 mg Zn/kg soil from ZnO-NP (same plots used in the present study). However, in the study mentioned, the source of N was ammonium nitrate, not urea. Clearly, the effect of Zn as an element in stimulating N uptake may depend on both the type of N fertilizer and the dose of Zn. For example, 4 mg/kg Zn (from Zn-sulfate) had no effect on N absorption, but higher doses of 6 and 8 mg/kg resulted in greater N absorption compared with controls ( Abbas et al. , 2009). In addition, the Zn dose-dependence of plant N uptake seems to be species-specific; N uptake is strongly enhanced in soybean when exposed to Zn from ZnO-NPs at low levels of 2–3 mg/kg, under arid and non-arid conditions ( Dimkpa et al., 2017a ; Dimkpa) et al., 2019a ). For P, Zn modification is known to inhibit P uptake ( Zhao et al., 2000 ; Abbas et al., 2009 ; Watts-Williams et al., 2014 ; Bindraban et al., 2020), due to the form an insoluble Zn-phosphate complex. We hypothesize that the temporal and spatial separation of Zn and P applications in the present work may contribute to the lack of antagonistic effects between these nutrients. Plants are known to use significant amounts of P during early root development, which can reduce plant P availability prior to Zn-N application.
In general, when data on plant growth, grain yield and Zn uptake are normalized, there are several interesting hypothetical scenarios based on plant exposure to the same amount (one grams) Zn. Overall (i.e. under arid and non-arid conditions) these data indicate that there are indeed differences based on particle size. When expressed on a per gram basis of Zn, the floc formation time with ZnO-NP nanozinc oxide increased faster than that of bulk ZnO, and the coating had no effect in each case. However, this effect was found only in arid conditions; while in non-arid conditions, only uncoated bulk ZnO allowed the flower to initiate flowering. With respect to grain yield, a nano-specific effect was found under both growing conditions and without coating effect. For Zn absorption, the size effect of the ZnO-NPs was clearly demonstrated, as was the effect of the ZnO coating, regardless of the growth conditions. Drought increases root exudation in response to stress (Karlowsky et al., 2018 ), potentially affecting the dissolution rate of particles, depending on specific metabolites in the fluid. details ( Martineau et al., 2014 ; Yang et al., 2018 ). Therefore, Zn is sufficiently bioavailable from ZnO-NPs under drought conditions, even at a lower exposure ratio than bulk ZnO. However, additional studies aimed at optimizing coating efficiency to provide Zn-to-urea ratio like separate Zn addition to both Nano zinc oxide (ZnO-NP) and bulk ZnO may provide insight into the implications of these assumptions.
The significantly higher soil N residuals in drought-affected plants compared with normal irrigation may reflect the fact that biomass growth, and thus N assimilation, was lower in plants. drought (data not shown). It has been observed that N is more commonly present in the soil as nitrate than as ammonium, except in the case of uncoated bulk ZnO under both growing conditions. The significance of changing ZnO bulk form on the dynamics of soil N species is currently unclear. However, the presence in soil of more nitrate than ammonium may reflect the metabolic dynamics of different N fertilizers. Furthermore, the pH of the soil and the specific requirements of the plant for ammonium or nitrate will also play a role in the ratio of remaining nutrients. In this case, Maathuis, 2009 ), resulting in more residual nitrate in the form of residual N in the soil. In our previous study with wheat ( Dimkpa et al., 2018b ), ZnO-NPs did not reduce residual N levels in the soil, similar to the present finding. As for N, treatment of Zn with nanoscale or bulk ZnO did not affect the residual P level in the soil, similar to the previous finding with ZnO-NP ( Dimkpa et al., 2018b ). However, these are in contrast to the fact that ZnO-NP increases soil P residuals in sorghum ( Dimkpa et al., 2017b). In terms of Zn residues, the difference in Zn exposure between ZnO-NP soils and bulk ZnO soils directly reflects the exposure ratio, although only doubling the mean obtained for nanoforms will show a higher amount of soluble Zn from the NPs from the bulk product.
As noted, the soil used in the study had low organic matter content. At this point, the role of carbon will be added to the soil by a small amount of oil and food coloring used as mulch in enhancing soil organic matter. It should be noted, however, that all urea, with and without ZnO, was also coated with an oil layer, which made any coating effect uniform across treatments.
The findings reported here indicate that Zn in nanoparticle form can accelerate the morphological development of wheat, reproductive performance and nutritional status of Zn under drought stress, as reported. previously reported for sorghum. The broader implication of this study is that the lower Zn ratio from Nano zinc oxide (ZnO-NP) may be sufficient to enhance crop yield under drought stress, compared with the higher input from bulk ZnO. This clearly demonstrates one of the goals of nano-supported agriculture, which is to reduce the proportion of nutrients entering the biosphere without yield penalty. Coating with urea or other N fertilizers with nano-scale micronutrients such as Zn can increase crop yields; facilitate the use of nano-sized micronutrients in field applications; eliminating the problem of separating smaller and larger nutrient particles in bulk fertilizer mixes; and facilitate the one-time use of Zn-urea. However, coating may not necessarily affect yield to a greater extent than separate applications of Zn and urea, as observed in this study. This is especially true because the Zn particles will eventually dissolve from the urea surface and undergo independent transformation into ions or larger aggregates, similar to the separately applied ZnO particles. With that said, it is likely that improving the urea coating process to increase the coating efficiency of ZnO-NP will further improve the results in terms of crop yield and Zn acquisition. This is especially true because the Zn particles will eventually dissolve from the urea surface and undergo independent transformation into ions or larger aggregates, similar to the separately applied ZnO particles. With that said, it is likely that improving the urea coating process to increase the coating efficiency of ZnO-NP nanozinc oxide will further improve the results in terms of crop yield and Zn acquisition. This is especially true because the Zn particles will eventually dissolve from the urea surface and undergo independent transformation into ions or larger aggregates, similar to the separately applied ZnO particles. With that said, it is likely that improving the urea coating process to increase the coating efficiency of Nano zinc oxide (ZnO-NP) will further improve the results in terms of crop yield and Zn acquisition.
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