Foliar Nourishment with Different Zinc-Containing Forms Effectively Sustains Carrot Performance in Zinc-Deﬁcient Soil

: Among the essential micronutrients, zinc (Zn) affects vital functions in crop plants. The inﬂuences of foliar nourishing with certain Zn-containing forms on the growth, productivity, and physiology of carrot plants (cv. Fire wedge F1) and their nutritional contents when grown in Zn-deﬁcient soil were examined in both 2019/2020 and 2020/2021 ﬁeld trials. Two doses of zinc oxide nanoparticles (ZnO-NPs(1) = 20 and ZnO-NPs(2) = 40 mg L − 1 ), zinc–EDTA (Zn–EDTA(1) = 1 and Zn–EDTA(2) = 2 g L − 1 ), or bulk zinc oxide (ZnO-B(1) = 200 and ZnO-B(2) = 400 mg L − 1 ) were applied three times. The data outputted indicated, in general, that ZnO-NPs(2) were the best treatment that conferred more acceptable plant growth (measured as shoot length and fresh and dry weights), physiology (measured as cell membrane stability index, SPAD readings, and nutrient uptake), and nutritional homeostasis (e.g., P, Ca, Fe, Mn, Zn, and Cu contents). All these positive attributes were reﬂected in the highest yield, which was measured as fresh weight, dry matter, length, diameter, volume, and total yield of carrot roots. However, there were some exceptions, including the highest membrane stability index in both seasons, the highest Cu uptake and Mn content in the ﬁrst season, and root fresh weight in both seasons obtained with ZnO-NPs(1). Moreover, the maximum P uptake and root dry matter were obtained with ZnO-B(1) and the highest content of root P was obtained by ZnO-B(2). Based on the above data, foliar nourishing with ZnO-NPs(2) can be recommended for the sustainability of carrot cultivation in Zn-deﬁcient soils.


Introduction
Most developing countries depend on agriculture as their main source of food and other essential uses. Therefore, agriculture is the backbone of the national income of these countries, including Egypt. In addition, the excessive growth of the world's population, which is expected to approach 10 billion by 2050, requires an increase in agricultural productivity by at least 50% to face the problem of steadily increasing population and achieving food security [1]. In this context, among food crops, carrot (Daucus carota L.), as the most economically important vegetable crop for exportation and local consumption in Egypt in recent years, may contribute to food security. Worldwide, carrot production is approximately 24 million tons [2]. Attempts have been made to grow this crop successfully

Location, Climatic Conditions, and Plant Material for Experiments, and Soil Properties
The current study was accomplished at the field level utilizing a sandy loam soil located at Fayoum University Farm Station (29 • 170 N; 30 • 530 E), Egypt. The seasons of 2019/2020 and 2020/2021 were the two trial seasons, which started on 30 September 2019 and 23 September 2020, respectively, by sowing seeds (cv. Fire wedge F1, supplied from Takii Co., Kyoto, Japan) of carrot (Daucus carota L.) and terminated on 11 January 2020 and 20 January 2021, respectively, by harvesting carrot roots. Average weather data for the studied region for the two seasons from September to January are offered in Table 1. Day • C = Average day temperature, Night • C = Average night temperature, RH = Average relative humidity, AWS = Average wind speed, AM-PEC-A = Average of measured pan evaporation class A, and AP = Average precipitation.
The soil texture tested in both seasons was sandy loam. Soil samples were collected from the upper surface layer with a depth of 30 cm to analyze the most important properties of soil physics and chemistry [16] as offered in Table 2.
All sprays were performed three times at 28, 48, and 68 days after sowing.
from Sigma-Aldrich, St. Louis, MO, USA. Plants were foliarly nourished with all spray-ing solutions three times; 28, 48, and 68 days after sowing (DaS) using a 20-L back sprayer. All plants (n = 140) in each plot were sprayed to runoff with 4.25, 5.10, and 6.80 L of spraying solution (e.g., 25, 30, and 40 mL plant −1 ) in the three spraying times, respectively. The description of all experimental treatments is offered in Table 3. ZnO-NPs were subjected to transmission electron microscopy (TEM), and the morphological structure of ZnO-NPs (≤100 nm) is illustrated in Figure 1. Three times in three plots of 10.5 m 2 each.
All sprays were performed three times at 28, 48, and 68 days after sowing.
The area assigned to the experiments was divided into 21 plots (3.0 m × 3.5 m = 10.5 m 2 for each) that were allocated to 7 treatments (Table 3), and each treatment was repeated three times in three plots. There were four rows of 3.0 m length in each plot and 42-43 hills for sowing seeds in each row. Three seeds were sown in each designated hill with a distance of about 7 cm between each two hills. Four weeks after sowing, the plants were thinned to one hill −1 . The carrot plants were provided with different fertilizers (N, P, and K) based on the recommendations of the Egyptian Ministry of Agriculture. N, P, and K fertilizers were added in the form of (NH4)2SO4 (20.6% N), CaH6O9P2 (15.5% P2O5), and K2SO4 (48-50% K2O) at levels of 200, 300, and 400 kg ha −1 , respectively. All fertilizers were applied at planting, 30, and 60 DaS in three equal portions in the two growing seasons. The area assigned to the experiments was divided into 21 plots (3.0 m × 3.5 m = 10.5 m 2 for each) that were allocated to 7 treatments (Table 3), and each treatment was repeated three times in three plots. There were four rows of 3.0 m length in each plot and 42-43 hills for sowing seeds in each row. Three seeds were sown in each designated hill with a distance of about 7 cm between each two hills. Four weeks after sowing, the plants were thinned to one hill −1 . The carrot plants were provided with different fertilizers (N, P, and K) based on the recommendations of the Egyptian Ministry of Agriculture. N, P, and K fertilizers were added in the form of (NH 4 ) 2 SO 4 (20.6% N), CaH 6 O 9 P 2 (15.5% P 2 O 5 ), and K 2 SO 4 (48-50% K 2 O) at levels of 200, 300, and 400 kg ha −1 , respectively. All fertilizers were applied at planting, 30, and 60 DaS in three equal portions in the two growing seasons.

Evaluation of Growth and Physiological Parameters
Average plant height and root length were measured (in cm) using a graduated ruler. Leaf fresh and dry weights plant −1 (g) were assessed using a digital electric balance. For the rapid, accurate, and non-destructive measurement of leaf chlorophyll concentration, the SPAD-502-meter handheld device was utilized. Measurements with the SPAD-502 meter produce relative SPAD meter values that are proportional to the amount of chlorophyll present in the leaf. Relative SPAD meter values were evaluated using uniform leaves on five selected plants from each treatment.
After excluding leaf midrib, a duplicate 0.2 g leaf sample was taken in test tubes with 10 mL of deionized water to determine leaf membrane stability index (MSI) [18]. At 40 • C, Agronomy 2021, 11, 1853 5 of 18 a sample was heated for 30 min with a water bath. Solution electrical conductivity (EC 1 ) was taken. At 100 • C, the other sample was boiled for 10 min. Solution conductivity (EC 2 ) was also measured. The percentage of membrane stability index (MSI) was calculated by using the following equation: All previous measurements were performed using five randomly selected plants.

Evaluation of Leaf and Root Nutrient Contents
Random samples of the upper fourth leaf and root were collected using five selected plants from each treatment, washed with distilled water, and oven dried at 70 • C to constant weight to determine the contents of P, Ca, Fe, Mn, Zn, and Cu. Inductively coupled plasmaoptical emission spectrometry (ICP-OES, Perkin-Elmer OPTIMA-2100 DV, Norwalk, CT, USA) was used to determine those elements in the Laboratory of Soil Chemistry, Faculty of Agriculture and Natural Resources, Aswan University, Aswan, Egypt.
Anti-radical power (ARP) content of roots was determined according to the widely used decolorization assay of Brand-Williams et al. [19]. An appropriate volume of the root extract was mixed with 90 M DPPH • in methanol, making up final volume of 3.0 mL. The mixtures were shaken vigorously and were stored in the dark for 30 min at room temperature. The decrease in absorbance of the reaction mixtures regarding the control was monitored spectrophotometrically at 515 nm. To determine β-carotene content, the root samples were thawed in the dark at 4 • C to avoid carotenoid oxidation. A total of 2 mL of acetone: hexane (4:6) solvent was added to 0.1 g of root tissue homogenate and mixed in a test tube; then, the mixture was centrifuged at 10,000 rpm for 15 min at 40 • C. Automatically, 2 phases separated and an aliquot was taken from the upper solution, and the absorbance readings (A) were taken at 663, 645, 505 and 453 nm using UV-Vis spectrophotometry. β-Carotene content was calculated using the following equation [20]: All previous measurements were performed using five randomly selected plants.

Measurement of Yield and Its Components
Average root fresh and dry weights (g) were measured using a digital electric balance. Average root diameter (cm) was assessed using a precision graduated ruler. Average root volume (cm 3 ) was evaluated with the help of a graduated glass cylinder and water. All previous measurements were performed using five randomly selected plants. At the time of harvest, the total root yield was calculated (ton ha −1 ) using all plants in all plots of each treatment.

Statistical Analysis
All experimental data were analyzed applying ANOVA, and Fisher's LSD test was applied to identify whether the differences between means (±SE) of all treatments were significant. The analysis software (SAS ver. 9.2; SAS Institute Inc., Cary, NC, USA) was applied, and differences were identified at p ≤ 0.05.

Length and Fresh and Dry Matter of Plant Shoot
The tested soil was a native alkaline sandy loam (pH 7.77 and 7.59) with available zinc (Zn) of 0.04 and 0.07 mg kg −1 (Table 2) in the 2019/2020 and 2020/2021 seasons, respectively. Shoot length (ShL), shoot fresh weight (SFrW), and shoot dry matter (SDrM) of carrot plants were appreciably influenced by the maximum dose (40 mg L −1 ) of zinc oxide nanoparticles (ZnO-NPs(2)), which produced incremental increases and the highest values of the above-mentioned growth parameters; however, the lowest values were produced by the untreated plants (control) ( Table 4). Values of 52.5 cm and 53.5 cm were Agronomy 2021, 11, 1853 6 of 18 recorded as the highest ShL in the two seasons, respectively. The corresponding SFrW and SDrM were 51.2 and 21.0 g plant −1 , respectively, in the 2019/2020 season and 52.6 and 21.3 g plant −1 , respectively, in the 2020/2021 season. It was noticed that the increasing percentages of the greatest and lowest values were 25.3 vs. 15.5%, 240.3 vs. 175.6%, and 127.5 vs. 115.2% for the above-mentioned growth parameters, respectively. Table 4. Influence of foliar nourishment with some zinc (Zn) forms on some vegetative growth and physiological parameters of carrot plants grown under Zn deficiency stress in two growth seasons.

Relative Chlorophyll Content and Membrane Stability Index
The data shown in Table 4 display that the relative chlorophyll content (SPAD) was improved by all Zn form treatments compared to the control, and ZnO-NPs(2) conferred the highest significant increase in SPAD in both seasons. ZnO-NPs(2) contributed to the SPAD reading with values of 72.8 vs. 77.5 in both seasons, respectively. The increasing percentages amounted to 180.1 vs. 187.2% compared to the comparison treatment, which gave the lowest values (26.0 vs. 27.0) in the growing seasons 2019/2020 and 2020/2021, respectively. The behavior of the membrane stability index (MSI) was the same as that of SPAD with Zn form treatments (Table 4). However, the percentages of increase in MSI were 9.46 and 5.30% in the two seasons, respectively, which were obtained using ZnO-NPs(1) that produced the maximum values (93.7 vs. 91.8) in both seasons, respectively, compared to the control.

Leaf Nutrient Contents
As shown in Table 2, the experimental soil is characterized as calcareous containing 10.5 vs. 10.8% CaCO 3 in both seasons, respectively. It is characterized by low nutrient availability. The results related to Table 5 indicate that ZnO-NPs(2) were the best treatment for carrot leaf iron (Fe), manganese (Mn), and zinc (Zn) contents. This treatment produced the highest contents of 917. the maximum leaf contents of phosphorus (P) and calcium (Ca) were 5.48 vs. 5.47% and 1.52 vs. 1.53% in both seasons, respectively, which were obtained with the higher dose of bulk zinc oxide, 400 mg L −1 (ZnO-B(2)), and the comparison treatment in both seasons, respectively, while the lowest contents of P (4.35 vs. 4.39%) were recorded with 1 g L −1 of zinc-chelated EDTA (Zn-EDTA(1)) and the lowest contents of Ca (1.01 vs. 1.03%) were produced with Zn-EDTA(2) treatment.

Root Nutrient Contents
All treatments had a significant effect on the contents of root nutrients such as P, Ca, Fe, Mn, Zn, and Cu, as depicted in Table 6. The overall trend of results indicates that the 2019/2020 season outperformed the 2020/2021 season in terms of root nutrient contents. The application of ZnO-NPs(2) produced significant maximum values (0.21 vs. 0.62%) for Ca in the first and second seasons, respectively, whereas Zn-EDTA(2) and ZnO-B(1) conferred maximum contents (0.78 vs. 0.77%) for P in both seasons, respectively. The greatest Mn contents (63.4 vs. 168.6 mg kg −1 ) were recorded with Zn-EDTA(1), but the lowest Mn contents (9.5 vs. 12.3) were produced with the comparison treatment in both seasons, respectively. For Fe, the highest contents (63.4 vs. 168.6 mg kg −1 ) were produced with Zn-EDTA(2), while the lowest contents (23.0 vs. 127.9 mg kg −1 ) were obtained with Zn-EDTA(1) in both seasons, respectively. Regarding Cu, the maximum contents (20.1 vs. 17.8 mg kg −1 ) were produced with Zn-EDTA(1), while the minimum contents (11.1 vs. 15.6 mg kg −1 ) were obtained with the comparison treatment in both seasons, respectively. With respect to Zn nutrients, the highest contents (30.5 vs. 38.4 mg kg −1 ) were produced with ZnO-NPs(2), while the minimum contents (17.3 vs. 16.7 mg kg −1 ) were obtained with the comparison treatment in both seasons, respectively. Mean values (± SE) with different letters in each column are significant (p ≤ 0.05). ZnO-NPs (1)

Root Yield and Its Components
The results regarding the effect of ZnO-NPs, Zn-EDTA, and ZnO-B on the root quality of carrot plants are depicted in Table 7.

Root Yield and Its Components
The results regarding the effect of ZnO-NPs, Zn-EDTA, and ZnO-B on the root quality of carrot plants are depicted in Table 7.   76.8% for root volume, and 28.9 vs. 42.5% for root diameter in both seasons, respectively. In general, the incremental rates in the yield of carrot roots and yield components in the second season were better than those in the first season. The results of variables associated with total root production are also shown in Table 7. For all the treatments tested, significant differences were obtained. ZnO-NPs(2) treatment produced the highest root yield, surpassing ZnO-NPs (1)

Correlation and Regression Analysis
Pearson's correlation coefficients in Table 8   Outputs of stepwise regression analysis in Table 9 clarify that there existed five studied parameters (i.e., Fe, LFW, RD, Mn, and Cu) and four studied parameters (DM, RFW, Mn, and Cu) in the 2019/2020 and 2020/2021 seasons, respectively. Both seasons had a highly significant contribution to TRY.
A wide range of different soil types in many parts of the world suffer from Zn deficiency, especially in sandy-textured soils with high pH and calcium carbonate (CaCO 3 ), as shown in Table 2. The uptake of Zn by plants is related to the soil carbonate content. Calcium and soil pH are two of the main factors that limit the availability of this nutrient. Thus, soil Zn deficiency reduces the Zn content of edible foodstuffs [56], and thus Zn deficiency affects more than a third of the world's population [57]. Therefore, this study was planned to address the problem of Zn deficiency in Zn-deficient soils by treating carrot plants using a foliar spray strategy with different forms of Zn such as ZnO nanoparticles (ZnO-NPs), Zn-EDTA, and ZnO-B in an attempt to identify which one of these Zn forms could be a solution for the successful adaptation of plants to coexist with the problem of Zn deficiency in defective soils. The application of ZnO-NPs, Zn-EDTA, and ZnO-B as a foliar nourishing strategy for carrot plants has rarely been studied at the field level under Zn-deficient soil conditions.
For carrot plants grown in Zn-deficient soil, ZnO-B was applied at both levels of 200 and 400 mg L −1 , Zn-EDTA was applied at both levels of 1 and 2 g L −1 , and ZnO-NPs was applied at both levels of 20 and 40 mg L −1 compared to the control, in which Zn was not applied. Among all these treatments, ZnO-NPs applied at 40 mg L −1 (ZnO-NPs(2)) were the best treatment that collected more acceptable growth and physiological parameters and nutrient homeostasis, which was reflected in the highest yields. The best nutrient homeostasis in plant tissues obtained due to the balanced nutrient uptake with applying ZnO-NPs(2) leads to acceptable plants benefiting from the crucial roles of nutrients in noticeably improving photosynthesis and plant growth in favor of plant productivity [58,59]. Therefore, foliar use of ZnO-NPs(2) due to limited nutrient availability can be resolved by reducing the loss of Zn added to the soil [60].
In our study, the resulting data indicate that the application of ZnO-NPs(2) improved all studied growth variables since Zn plays a key role in maintaining the plants' physiological health (Table 3). This result was obtained because ZnO-NPs(2) improved nutrient usability by plants to enhance photosynthetic pigments, the rate of photosynthesis that increases total carbohydrate accumulation, leaf dry matter production, and finally the outcome of plant growth parameters [59]. The Marschner [58] study reported an enhanced effect of ZnO-NPs on chlorophyll content, which was explained on the basis of micronutrients playing critical roles in the synthesis of chloroplast proteins and thus perhaps interfering with chlorophyll synthesis. It has been revealed that micronutrient deficiency inhibits chlorophyll formation through the inhibition of protein synthesis [58]. These findings can be explained in more detail by Mahmoud et al. [61], who indicated that Zn assimilation is more efficient when applied at a nano-metric size. Duhan et al. [62] showed increases in the growth of seedlings treated with ZnO-NPs as a foliar application, which is more effective than ZnO-B, as its release can be slow and gradual [63], but it has not been identified whether this effect is due to ZnO-NPs uptake or due to the dissolution of its products [64]. With results regarding relative chlorophyll content (SPAD values), Pullagurala et al. [65] attributed the increase in SPAD values to the fact that Zn plays an essential and vital role in plant metabolism, affecting the activation of important enzymes such as carbonic anhydrase containing a Zn atom that catalyzes the hydration of H 2 O and CO 2 , facilitating the diffusion of carbon dioxide to the carboxylation sites in plants [66]. These results are in good agreement with the findings of Atteya et al. [67] and Gheith et al. [68] regarding jojoba and maize plants, attributing the positive findings obtained to the fact that the foliar application of ZnO-NPs leads to a large acquisition of Zn by plants.
This study demonstrated that the role played by ZnO-NPs is not only to improve plant growth and physiological (membrane stability index and SPAD values) parameters, but also to enhance nutrient uptake and plant nutrient content (Tables 5 and 6). In this connection, the use of Zn in the form of ZnO-NPs is to overcome the adverse effects of Zn-deficient conditions and improve plant nutritional quality. However, the performance of nutrient nanoparticles depends on several properties such as particle size, chemical structure, service covering, and application dosages [69]. In this study, among all Zn treatments, ZnO-NPs applied at 40 mg L −1 (ZnO-NPs(2)) made carrot plants perform well under Zn-deficient soil conditions (Tables 3-8). This desirable output can be attributed to the fact that Zn plays an important role in increasing protein synthesis, membrane function, cell elongation, and encouraging plant roots to positively exchange cations, which helps plants absorb more nutrients. In addition, Zn modifies auxin influences by regulating the synthesis of the amino acid tryptophan, which is a precursor to the synthesis of indole-3acetic acid (IAA) auxin, which is very important for cell elongation and plant growth. It also acts as a cofactor for many enzymes such as superoxide and dehydrogenases [70,71].
This study revealed that the Cu content does not match that of Fe, Zn, and Mn, which can be attributed to the antagonistic effect between Cu and other nutrients, including Zn. This antagonism (between Zn and Cu) may lead to maintaining the Cu content at the appropriate level to prevent Cu toxicity, as Cu activates many enzymes related to photosynthesis and respiratory systems as an essential metal cofactor [72]. The general trend of Zn, Fe, and Mn contents was stable due to the synergistic effect, and the trend of the results obtained is in good agreement with those obtained by Mahmoud et al. [61]. As expected, outperforming the other forms of Zn, the application of ZnO-NPs to carrot plants noticeably increased the Zn content. Zn can be absorbed primarily in the leaf wax layer [11]. Size is probably a limiting factor in the diffusion of Zn in Zn-EDTA and ZnO-B across stomata (19.1 to 71.5 µm) [73], which only allows particles with smaller sizes to pass through [11]. In contrast, our data showed a decrease in calcium (Ca) uptake due to the application of ZnO-NPs, which can be attributed to the fact that Zn competes with Ca for plant uptake. The increase in the uptake of other nutrients may be attributed to the fact that ZnO-NPs enhance the cation exchange capacity (CEC) of the roots, which in turn enhances the absorption of nutrients, which is positively reflected in the improved plant growth and satisfactory productivity. Supporting plant growth and productivity, Zn also plays a key role in controlling the production of IAA hormone, as well as in carbohydrate and protein metabolism and synthesis [70,71].
As depicted in Table 4, all treatments containing Zn nutrients in either nano or bulk forms caused an improvement in the cell membrane stability index (MSI) compared to the control, in which Zn was not applied. The enhancement in the cell MSI could be due to the inclusion of Zn in the defensive enzyme superoxide dismutase and peroxidases, which helps mitigate and minimize the harmful influences of oxidative stress and the accumu-lation of malondialdehyde and toxic ions such as Na + , which cause electrolyte leakage, and also helps to accelerate the synthesis of chlorophyll and the metabolic processes in the plant. In addition, it has been reported that ZnO-NPs notably help in the increase in soil organic carbon due to the release of root exudates from the roots of ZnO-NPs-treated plants, possibly increasing net photosynthetic rates because root exudates are considered to be part of the excess carbon from photosynthesis [74]. Therefore, Zn protects cell membranes from damage caused by Zn deficiency due to its rapid penetration into the plant cells when it is applied in the form of ZnO-NPs to fulfill its vital functions in the plant [61,75].
The positive effect of ZnO-NPs on enhancing the uptake of nutrients and their positive reflection on growth parameters and yield and its components ( Table 7) can be due to the role of Zn in controlling plant hormonal (IAA) content and its relationship to cell elongation and plant growth, and also due to its role in protein synthesis and carbohydrate metabolism in addition to its excellent role in gene expression related to abiotic stress. These results are confirmed by Sadeghzadeh and Rengel [76] and Quary et al. [77], who reported that ZnO-NPs manipulate all plant growth parameters, resulting in useful changes in productivity traits. The possible reason for each positive role of Zn is the enhancement in the activity of bio-substances and/or the activity of the photosynthetic system, or might be due to the activating role of Zn in the metabolic processes of plants and the photosynthetic rate, which are reflected in the improvement in the yield components. Compared with other forms of Zn (Zn-EDTA and ZnO-B), with the use of nano-fertilizers (1-100 nm), nutrient uptake is greatly improved due to their very small size, which in turn can improve the absorption and interaction of other micronutrients in plant tissues [78]. In addition, the greatest positive influence of Zn in the form of ZnO-NPs may be due to its most easy and rapid assimilation into plant tissues, which improves photosynthesis and leads to the efficient growth of carrot plants. It has been previously explained that sprayed ZnO-NPs are mostly retained in the leaf waxy layer [79], leading to an increase in Zn content in leafy tissues. This may be due to the fact that Zn binds to the hydrophilic, histidine-rich amino acid stretch in the middle of the ZAT1p protein, an uptake system found in cytoplasmic membranes [80]. Thus, ZAT1 expression leads to the accumulation of Zn 2+ in plant tissues. In addition, Zn transport does not require an energy source such as ATP or the proton motive force; thus, the Zn concentration gradient is sufficient to drive Zn transport by ZAT1p. It could be argued that the 25-amino-acid N-terminus of ZAT1p may be essential for energy coupling. Zn transport into cells by ZAT1p fits to a DY-driven uniport of Zn 2+ . Zn-transporting P-type ATPases seem to transport Zn 2+ cations bound to thiol groups, e.g., to glutathione. Even in the presence of high-activity efflux systems, ZAT1p functions as an uptake system for Zn 2+ , while CzcD and ZRC1p act as Zn-efflux systems. The COT1p protein is also an uptake system, but only at an outside concentration of 1 µM Zn 2+ [80].
Zn is taken up mainly as a divalent cation (Zn 2+ ion) by plant roots. However, in some cases, organic ligand-Zn complexes are also absorbed by plant roots. Depending upon the ligand secreted by plant roots, two physiological strategies are involved in the uptake of divalent cations such as Zn 2+ [81]. For a greater understanding of nutrient absorption, roots are not just static organs. Plant roots release various organic acids, amino acids, sugars, protons, and even some mineral ions, etc., in the rhizosphere that facilitate their adequate functioning and growth. Zn is absorbed as divalent metal ion Zn 2+ through mass flow and diffusion mechanisms by roots. In the case of the foliar application of Zn, as in the current study, passive Zn uptake by these mechanisms may be involved in the participation of water (solvent) molecules and the difference in Zn concentrations across the leaf cell plasma membrane. The main driving force in Zn 2+ uptake (cation uptake) may be the hyperpolarization of the leaf cell plasma membrane, which is mediated through the activity of the leaf cell plasma membrane H + -ATPase system. The leaf cell plasma membrane H + -ATPase system actively pumps H + ion extracellularly at the expense of ATP hydrolysis. The release of H + ions in the rhizosphere causes the hyper-polarization of the leaf cell plasma membrane on one hand, while it reduces the medium pH on the other hand, which results in an increased cation uptake rate. However, unlike water, charged Zn ions are not able to cross cell membranes freely, so these divalent cations are transported by specific transporter proteins. These proteins are not in close association with ATP breakdown, which confirms the passive uptake of Zn rather than active. Furthermore, Zn 2+ uptake also occurs by non-selective cation channels associated with the passive flux of diverse groups of cations. This additional driving force in the uptake of many metal cations is likely due to their very low cytoplasmic activity, which is a result of metal sequestration and their binding to intracellular sites (i.e., Zn finger proteins, organic acids, enzymes, etc.) [81].
Nanoparticles have increasing applications and the transportation and separation of nanoparticles have attracted many interesting investigations. Nanoscale manipulation refers to the controllable transportation, separation, and classification of nano-objects, which can be further used to design more complex nano-devices and nano-instruments with nano-elements, and should be very useful for development in nanosciences and nano-industries. Some of these technologies are still in the development stage and can only be performed at the laboratory level, which still has a long way to go for industrial applications. The precise manipulation of small objects or particles remains a challenge, not to mention the problem of classifying nanoparticles of different sizes [82].
Nanoparticles can start to move and the subsequent movement of the nanoparticles depends mainly on the competition between the driving force and the damping force, considering the effect of nanoparticle sizes and thicknesses, as well as the velocity of sliding blocks on transport behavior. The transportation of nanoparticles would be easier if the sliding block velocity was lower. Critical velocity is put forward as a key parameter to describe the transport behavior, under which nanoparticles can be made. In general, the critical velocity decreases with increasing nanoparticle size and thickness, and as a result, transport is easier for smaller and thinner nanoparticles. Moreover, transport would also be easier with a lower viscous damping force and a higher temperature. The initial compatibility of the interface has no obvious effect on the transport behavior, especially in the case of a pre-stretched substrate. Nanotechnology can be further applied with the design of suitable transporters from which nanoparticles of different sizes can be selected [82].

Conclusions
Our investigation displayed that foliar nourishment with different forms of zinc (ZnO-NPs, Zn-EDTA, and ZnO-B) positively affected carrot plant growth, physiology (including zinc content), and productivity to varying degrees based on the form of zinc applied, and more acceptable results were obtained with ZnO-NPs applied at 40 mg L −1 . A future investigation should be carried out to explore more about how and why ZnO-NPs are more successful than other forms of zinc (e.g., Zn-EDTA and ZnO-B) in influencing different aspects of plant physiology and biochemistry, which are reflected in plant growth and productivity. In addition, it should be explored how and why zinc in ZnO-NPs outperformed Zn in other forms in performing its functions in plants. It is clear, undoubtedly, that the growing population poses many challenges to all countries; therefore, the strategy of nano-size foliar fertilizers, including ZnO-NPs, at low concentrations can be applied to be one of the most important tools for achieving food security effectively and economically.