Agronomic Biofortification with Se, Zn, and Fe: An Effective Strategy to Enhance Crop Nutritional Quality and Stress Defense—A Review

Human micronutrient deficiencies are a widespread problem worldwide and mainly concern people whose diet (mainly of plant origin) consists of insufficient amounts of critical vitamins and minerals. Low levels of micronutrients in plants are linked to, i.e., their decreasing concentration in soils and/or low bioavailability and presence of abiotic stresses which disturb the proper growth and development of plants. Agronomic biofortification of crops is a very promising way to improve the concentration of micronutrients in edible parts of crops without compromising yield and is recognized as the cheapest strategy to alleviate hidden hunger worldwide. The review is focused on the factors influencing the effectiveness of biofortified crops (a type of application, form, and a dose of applied microelement, biofertilizers, and nanofertilizers). Also, the accumulation of zinc, selenium, and iron in edible parts of crops, their effects on metabolism, morphological and yield parameters, and an impact on plants’ defense mechanisms against abiotic stress like salt, high/low temperature, heavy metal, and drought was discussed. Finally, the directions of future agronomic biofortification studies are proposed.


Introduction
It is estimated that more than 2 billion people (one in three) globally suffer from micronutrient deficiencies, also known as a "hidden hunger" (Prom-u-thai et al. 2020). These deficiencies are usually prevalent in highly developed countries and are more common among growing and developing children, pregnant and lactating women, sportsmen, and manual labor workers. Among the micronutrients, those most associated with micronutrient malnutrition worldwide are zinc (Zn), selenium (Se), and iron (Fe).
Researchers around the world continue their attempts to develop Se-, Zn-, and Fe-enriched food products to minimize their related deficiency disorders. Proper nutrition is key to good human health and according to the World of Human Organization (WHO), it mainly depends on sustainable agriculture (Athar et al. 2020). Unfortunately, current agricultural systems are still mostly oriented toward achieving high crop yields rather than nutritional quality, thus enhancing the concentrations of mineral micronutrients has become a key task in agriculture production. However, it is challenging to simultaneously increase the production of food enriched with essential micronutrients which does not cause obvious negative symptoms for plants like, i.e., limiting growth and productivity.
The reduced level of micronutrients in crops may be a consequence of different constraints like low levels or low bioavailability of essential elements in the soil (Manojlović et al. 2019), sub-optimal abiotic conditions including extremely high or low temperature, pH, water deficit, or anaerobic conditions, and also the presence of other elements (micro and macroelements and heavy metals). It was estimated that about 50% of cereals cultivated soils are Zn deficient. The Fe deficiency mostly occurs in calcareous (Jalal et al. 2020). Micronutrient deficiencies are more common in humid temperate and tropical regions where the intense leaching associated with high precipitation is observed. Another cause is the use of plant species with a low ability to accumulate sufficient quantities of micronutrients in their edible parts.
Biofortification is one of the ways to provide an increased level of micronutrients in crops (Huang et al. 2020). It has been shown that biofortified crops increase micronutrient intake and have a significant positive effect on human health (Bouis and Saltzman 2017;Praharaj et al. 2021). There are three major approaches to biofortification: agronomic, conventional plant breeding, and plant breeding using genetic engineering (Garg et al. 2018). Among them, agronomic biofortification, which is aimed at supplying micronutrients that can be directly absorbed by the plant by application with mineral and/or foliar fertilizers and\or the improvement of the solubilization and mobilization of mineral elements in the soil, is recognized to be the simplest method used to enhance levels of microelements in crops. Agronomic biofortification is also recognized as one of the cheapest ways to reduce mineral deficiency in the human diet. Additionally, many reports evidence that biofortification, besides micronutrient enrichment of plants, also has a significant influence on the synthesis of other compounds that exhibit nutritional properties (Newman et al. 2021;Puccinelli et al. 2021bPuccinelli et al. , 2019aSkrypnik et al. 2019). For plants, application of Zn, Se, and Fe also effectively supports the fight with biotic stresses (Adrees et al. 2021;Noreen et al. 2020;Rizwan et al. 2019). The concentration range between the beneficial and toxic effects of Zn, Se, and Fe for crops is very narrow, thus a well-thought-out approach to choosing plant species for enrichment with microelements, strict selection concentrations and form in fertilizers, selection of appropriate type of fertilizer, and studies on the accumulation of these microelements by a specific species or even plant variety are necessary to obtain crops with high nutrition quality.
The paper covers the newest findings under agronomic biofortification with Zn, Se, and Fe. The first part of the review is focused on the main factors that determine the effectiveness of micronutrient biofortification. The next section provides some examples of the use of fertilizers based on nanotechnology and supported by microorganisms. The following section describes the beneficial effect of biofortification on increasing microelement content in edible parts of plants and also synthesis many compounds show health benefits. The last part of the paper discusses the influence of Zn, Se, and Fe biofortification on the alleviation of symptoms of abiotic stresses. In the conclusion section, the directions of future agronomic biofortification studies are proposed. The most important findings and information about conditions/ type of experiments from collected research papers were presented in tables. Additionally, the enrichment factor (EF) was calculated. EF of the microelements was calculated as a ratio of results obtained from the most advantageous fertilization of the crops (C max ) in relation to the control group (C control ), according to the formula: Estimation of EF was performed based on the data contained in the articles (in a numerical or graphical form).

Factors Influencing the Effectiveness of Biofortification with Zn, Se, and Fe Edible Parts of Plants
Many factors influencing the effectiveness of biofortification with Zn, Se, and Fe, including plant species, genotypes, and phenotypes; soil characteristics; type of application; and dose/form of applied micronutrients and climatic conditions have been widely investigated in recent years (Ebrahimi et al.2019;El-ramady et al. 2021;Izydorczyk et al. 2021;Jones et al. 2017;Manojlović et al. 2019;Niyigaba et al. 2019;Ramzan et al. 2020;Smažíková et al. 2019;Sago et al. 2018). Most studies were performed under controlled conditions mainly on cereal, rice, grass, herbage, and corn (Ros et al. 2016). In this section, we discuss the types of applied fertilizers and the forms/doses of applied Zn, Fe, and Fe.

Type of Application
The application of micronutrients fertilizer to the soil is the most common practice and has been used for years; however, in addition to various limitations associated with soil properties, it should be mentioned that (i) applied fertilizers have a low recovery efficiency, (ii) this strategy requires regular application, and (iii) different granule size leads to uneven application of nutrients. Currently, much more attention is turned towards the application of foliar fertilization, where micronutrients are applied directly to plants leaves. It was found that the application of foliar Se fertilizer improved the durum wheat grain Se concentration twice in comparison with soil application at the same dose of Se (Galinha et al. 2014). On the other hand, Zn fertilization of wheat was the most effective for the application of combined soil and foliar fertilizers (Gomez-Coronado et al. 2016). In research on mungbean, the Zn grain concentration after application of 1.0% of solution of zinc sulfate was about 1.7 times higher (Haider et al. 2018a) than soil Zn application at a concentration of 10 mg kg −1 soil (Haider et al. 2018b). However, it is worth noting that successful foliar fertilization requires, i.e., higher leaf area for better adsorption of the applied micronutrient. Additionally, this type of fertilization can be limited by environmental conditions, particularly air temperature, wind speed and direction, rainfall, and relative humidity, and should be applied at the adequate stage of the growth and development of crops. For example, Wang et al. (2020b) suggested that the foliar application of Se in both forms (selenite or selenate) at the pre-filling stage has a greater effect on Se concentration in wheat grains in comparison to the application at the pre-flowering stage. Foliar spray of Zn and Fe was applied four times, one every 10 days in the flagging to grain filling stages in wheat (Jalal et al. 2020). Deng et al. (2017) proved that the concentration of Se in grains after application of both selenite and selenate at the full heading stage of rice was 2.9-3.5 times higher than at the late tillering stage. The best fertilizer effect for chickpea was obtained for application of zinc sulfate at sowing combined with foliar Zn application at flowering and pod formation stages (Pal et al. 2019). Soilless cultivation represents a promising opportunity for the agricultural section, especially in the regions characterized by soil degradation and limited water availability. For this reason, currently, more research is aimed at the enrichment of crops with micronutrients are performed under hydroponic conditions (Giordano et al. 2019;Puccinelli et al. 2019a;da Silva et al. 2020). Hydroponic cultivation has several advantages including, i.e., monitoring of nutrient concentration which in turn allows ensuring an optimal nutrient acquisition by plants without leading to nutritional disorders (Sambo et al. 2019). Skrypnik et al. (2019) noted that the application of Se to the nutrient solution had a significant effect on the essential oil content in basil leaves compared to foliar application.
Some authors suggested that the combined application of Zn, Fe, and Se soil and foliar fertilizers (Gomez-Coronado et al. 2016;Rivera-Martin et al. 2020) and fertilization with amendments like, i.e., biochar or salicylic acid (Ramzani et al. 2016;Smoleń et al. 2019) influenced a better efficiency for microelements accumulation in crops compared to when used separately. For example, it was found that Zn fertilization improved Fe concentration in grains (Niyigaba et al. 2019). The addition of microelements is usually applied in combination with the appropriate macroelements fertilization (NPK). It is well-known that the plant's N status is an important factor influencing increased levels of Zn and Fe in vegetative tissue. A strong correlation between Zn and Fe grain concentration and urea application was found for wheat (Montoya et al. 2020) and chickpea (Pal et al. 2019). The foliar application of zinc sulfate in conjunction with urea significantly increased not only Zn uptake but also the total N uptake and the efficiency of urea N fertilization. Additionally, Gonzales et al. (2019) proved that application of Zn fertilizer allows for a possible reduction of N application while maintaining barley grain yield and nutrition quality.
The impact of combined fertilization and its effects on plants are presented in Table 1.

Form and Dose of Applied Micronutrient
Application of appropriate form and dose of micronutrients has a significant effect on the accumulation of micronutrients in crops. The increase of soil zinc sulfate application increased Zn grain accumulation in wheat in the field study ; however, Liu et al. (2019) noted that the percentage of Zn translocated from root to shoot decreased with increasing Zn application. In the study conducted by Gomez-Coronado et al. (2016), all of the tested wheat varieties (INIAV-1-10, Ardia, Nabao, Roxo) fertilized by soil Zn (applied as a zinc sulfate) at the concentration of the 50 kg h −1 accumulated in grains about two times less Zn in comparison to results presented for wheat by Liu et al. (2019). Fertilization with Zn, which is usually applied as zinc sulfate, is the most commonly used fertilizer. The application of zinc sulfate not only improves the Zn concentration in edible parts of crops but also is the source of sulfur for crops (Persson et al. 2016). The presence of S-rich proteins is important for Zn storage in the endosperm and suggests a synergy of Zn accumulation due to S application. However, Montoya et al. (2020) suggested that the application of Zn in an organic form enhanced grain yield more than inorganic form (i.e., zinc sulfate), especially with recommended N rate. Márquez-Quiroz et al. (2015) indicated that application of complexes of Fe (Fe-EDDTA) is a more effective way to enhance the level of Fe in cowpea bean seed compared to an inorganic form. Se is usually applied as a selenate which is recognized as having more bioavailability for the plant than selenite . It was found that at the same spraying stages, the grain Se concentration in rice was about two times higher for selenate than in selenite (Deng et al. 2017). However, there was no significant influence on grain yield and total biomass between the two forms of Se fertilizers. The soil fertilization with selenate caused the highest concentration of Se in radish in comparison to foliar fertilizer and selenite form application (da Silva et al. 2020). Contrary, Longchamp et al. (2015) noted that soil application of selenite could be attractive because selenite is less mobile than selenate and can enrich the soil in Se at each fertilization meaning that in the long term, plants grown on this soil will be enriched in selenium without the use of Se fertilizers.
An impact of the type of fertilization and trial, form, and doses of applied microelements is summarized in Table 2.

Biofertilizer
Combining Zn, Se, and Fe in interaction with the application of plant growth-promoting bacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) is beneficial for the development of the environmentally friendly biofertilizers used for the production of crops enriched in microelements. PGPR mobilizes the nutrients by various mechanisms including acidification, chelation, the release of organic acids, and exchange reactions (Triticum et al. 2015). Furthermore, the mechanism also strictly depends on applied PGPR and the chemical form of micronutrients, i.e., oxides, phosphates, or carbonates. Among plant growth-promoting bacteria (PGPR), Bacillus is the most popular for microelements biofortification. Bacillus aryabhattai and B. subtilis were used to enrich maize in Zn (Mumtaz et al. 2018). The presence of Bacillus was found to enrich solubilization of unavailable Zn, as the microbial strains favor the formation of organic acids available for plants. As a result, the uptake of N, P, K, and Fe can also be improved resulting in increased root length, dry weight of the plant, and even chlorophyll content. However, Padash et al. (2016) (Yasin et al. 2015a(Yasin et al. , 2015b. In a study conducted by Padash et al. (2016), the inoculation of fungi Piriformospora indica with Zn increased Zn level in lettuce. Fungus Rhizophagus intraradices can increase the root adsorption surface via hyphae. Thus, a significant increase in Se content in shallot and chickpea was obtained (Golubkina et al. 2019(Golubkina et al. , 2020, while Pantoea dispersa MPJ9 and Pseudomonas putida MPJ6 were used for mung bean biofortification with Fe, resulting in its content increase up to 100 mg dm −3 for MPJ9 after 60 days, when compared to control (30 ppm) (Patel et al. 2018). The positive effect on grain Zn concentration was observed for Zn inoculation with Rhizophagus irregularis above 150 mg Zn kg −1 of soil (Tran et al. 2019). Further details about applications of Zn, Se, and Fe with microbes and impacts on microelements accumulation and physiological parameters of crops are presented in Table 3.

Nanofertilizer
Zn, Fe, and Se nanoparticles (NPs) can be synthesized in several ways. For example, Subbaiah et al. (2016) synthesized ZnONPs with a size about of 25 nm and negative zeta potential of 39.6 mV by oxalate decomposition technique. The chitosan and sodium tripolyphosphate were used for the synthesis of positive charge (42 mV) of the zinc complexed chitosan NPs (Deshpande et al. 2017). In a study proposed by Hussein et al. (2019), the SeNPs with a size of about 10-30 nm were synthesized by mixed sodium selenite with ascorbic acid. SeNPs were also stabilized by polyvinylpyrrolidone (PVP) and ascorbic acid with a diameter of about 70 nm were studied by Siddiqui et al. (2021) The origin of Se formation at the nanoscale was characterized using spectrophotometer UV-VIS (peak at wavelength 400 nm described selenium formation at nano size). The application of Cu, Fe, and Zn NPs mixed with urea-modified hydroxyapatite (diameter about 38.21 nm) was studied by Tarafder et al. (2020). Uptake, translocation, and accumulation of NPs depend on plant species and characteristics of NPs like size, chemical configuration, stability, and concentration. Du et al. (2019) showed inhibited effect of ZnONPs on the germination rate of wheat. By contrast, the same dose on ZnONPs has no significant effect on corn and cucumber germination with a significant decrease of root elongation (Zhang et al. 2015). In the study performed by Subbaiah et al. (2016), the highest germination percentage of corn was observed at 1500 mg dm −3 of ZnONPs.
The idea of decreasing the particle size of applied fertilizer is to deliver the "right dose of nutrients" in the "right place" and at the "right time." Additionally, reducing the size of the particles leads to the increase in specific surface area of particles, thus the contact area of fertilizers with the plants will be increased resulting in the higher nutrient uptake by the plants in comparison to applied commercial fertilizers. The effect of Fe as nano and bulk Fe complex (Fe(III)-EDTA) applied at the same dose was studied under hydroponic conditions under Fe-deficient in tobacco cultivation (Bastani et al. 2018). It was found that the dry weight of the plant in 2 weeks after the application was about three times higher for nano Fe in comparison to bulk Fe. Elanchezhian et al. (2017) suggested that the application of FeNPs can significantly decrease the amount applied commercial Fe-fertilizers maintaining the proper growth and metabolism of crops. Du et al. (2019) compared the effects of foliar ZnONPs and zinc sulfate at the same concentrations on the growth of wheat (Triticum aestivum L.). The highest Zn accumulation in grain was recorded with 100 mg dm −3 of ZnONPs which was about 29% higher when compared to applied 2000 ppm of zinc sulfate. Selenite, selenate, and Se nanoparticles (SeNPs) at doses 0.01-50 mg dm −3 were investigated for assessment of the phytotoxicity, accumulation, and transformation in garlic under hydroponic conditions. The highest Se content in roots was observed after application of selenite; however, the lowest translocation index of Se was observed in the case of application of SeNPs. The same results were observed for rice seedlings (Wang et al. 2020a). Tarafde et al. (2020) investigated the synthesis Table 2 The effect of type of fertilizer, form, and dose of micronutrients application on biofortification with zinc, selenium, and iron. The enrichment factor (EF) was calculated as a ratio of results obtained from the most advantageous fertilization of the crops in relation to the control group Reference     The effect of NPs on biofortification with zinc, selenium, and iron is shown in Table 4.

The Beneficial Effect of Se, Zn, and Fe on their Content and Nutritional Quality of Crops
Biofortification with Zn, Se, and Fe not only increases microelement content in the edible parts of crops, yield, and morphological parameters of crops but also have a beneficial impact on other nutritional parameters of crops like, i.e., increase of proteins, amino acids, phenolic acids, chlorophyll, carotenoids, and essential oil content. However, it is worth noting that success in crops enrichment by micronutrients can be achieved only when there are no negative symptoms on crops like, i.e., biomass reduction. Zn, Se, and Fe fertilizers have an impact on the increase of the contentment of antioxidant compounds. Understanding stress physiology in plant growth can allow for the controlled synthesis of antioxidant compounds which are very valuable food compounds. For example, phenolic compounds are believed to scavenge and/or inhibit the production of ROS in the human body, thus preventing a critical step at the onset of carcinogenesis. The increase of antioxidant properties in plants can be caused by (i) the synthesis of not only phenolic compounds but also by other secondary metabolites exhibit antioxidant properties, (ii) the effect of microelements on the redox metabolism of glutathione (GSH) and enzymes involved in GSH metabolism, and (iii) the direct antioxidant effect microelement and its organic metabolites (Skrypnik et al. 2019). It was found that application of Se to the nutrient solution at a dose of 12 mg dm −3 at the first and second cut has a significant effect on increased total phenol content in leaves basil. There were no significant differences in antioxidant capacity, rosmarinic acid content, total chlorophyll content, and leaf biomass between Se application and control (Puccinelli et al. 2020). The same results were obtained by Skrypnik et al. (2019) but for lower Se concentration (up to 0.78 mg dm −3 ). Contrary, Edelstein et al. (2016) noted that in order to avoid the reduction of the yield of basil, Se concentration in nutrient solution should be lower than 0.25 mg dm −3 . Differences in Se accumulation could be different in varieties of basil. Two varieties of basil were tested on essential oil and Se content after foliar Se fertilization. The "Red Rubin" variety distinguished higher fresh phytomass yield and about two times higher Se content in leaves compared to the "Dark Green" variety; however, the content of essential oil was higher in "Dark Green" variety (Mezeyova and Hegedusova 2016). In the study conducted by Barátová et al. (2015), the variety of "Red Opal" had greater polyphenol content at first and second cut than "Dark Green." Two varieties of lettuce were studied under hydroponic conditions with increasing Fe concentration. The accumulation of Fe of both varieties increased with increasing dose of Fe; however, the variety of "Red Salanova" performed the higher phenolic acids as well as phenolic content compared to "Green Salanova." Additionally, the significantly increased carotenoid content was observed only for the red lettuce variety. It is worth noting that only Fe applied at the concentrations at 0.5 mmol dm −3 did not decrease the fresh and dry biomass of lettuce per plant (Giordano et al. 2019). Contrary, the application of Zn at the concentration of 0.45 mmol dm −3 significantly decreased these parameters in the red variety of lettuce (Sago et al. 2018).
Out of all types of crops, those enriched with sprouted seeds and microgreens deserve particular attention as they are a natural source of cancer preventive compounds (Park et al. 2015;Turner et al. 2020). Their biofortification gives the possibility to self-produce nutrient-dense plants in a very short time with the use of simple soilless systems. Se in the form of selenate at concentration 4 and 8 mg dm −3 significantly improved germination index, Se content, antioxidant capacity, and dry/fresh weight of microgreens of basil (Puccinelli et al. 2019a, b). The concentration of Se in microgreens was about 2.2 times higher than in leaves of maturity basil. This suggests that microgreens have a higher nutritional value and greater health benefits compared to mature leafy vegetables. The increased antioxidant capacity detected in the Se-enriched microgreens is in agreement with the results obtained in P. oleracea (Puccinelli et al. 2021a) and Ocimum basilicum L. Coriandrum sativum L., and Allium fistulosum L. (Newman et al. 2021). The application of selenate and selenite at 100 μM dm −3 was tested on three varieties of broccoli. The results showed that the application of selenate improved anthocyanin and ascorbic acid content in tested broccoli varieties, whereas the selenite was more effective in the accumulation of flavonoid content. There were no significant differences between the form of applied Se for total phenolic content; however, in both cases, the total phenolic content was lower than control (Tian et al. 2016). In the study conducted by Vicas et al. (2019), application of SeNPs even higher concentrations did not affect changes in the total phenol content. On the other hand, it was found that both the soaking and spraying pea seeds with Zn significantly increased the total phenolic content up to Zn application at 40 mg dm −3 (Lingyun et al. 2016).
The effect of biofortification with Zn, Se, and Fe on plant metabolism is provided in Table 5.

The Influence of Biofortification with Zn, Se, and Fe on the Defense of Plants Against Abiotic Stress
Abiotic stresses such as salt, high/low temperature, heavy metal, and drought cause, i.e., overproduction of reactive oxygen species (ROS) and inducement of oxidative stress in plants. It was evidenced that biofortification with Zn, Se, and Fe using different types and forms of fertilizer can reduce the damage caused by oxidative stress by an increase of ROS-scavenging enzymes like (superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione peroxidase (GPX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione S-transferase (GST), and peroxiredoxin (PRX) content in different sites of plant cells (Amira et al. 2015;Noreen et al. 2020). The application of microelements also enhanced the content of nonenzymatic antioxidants such as GSH, ASA, carotenoids, and proline which are also crucial for the maintenance of ROS homeostasis in the plant. In addition, to maintain the balance of ROS in plants, the application of micronutrients significantly decreased the level of heavy metals in plant tissues, and improved morphological growth parameters and Zn, Se, and Fe concentration in edible parts of crops. Of all toxic heavy metals, the most frequently studied was the alleviation of the Cd stress which is recognized as one of the most hazardous environmental contaminants. Rizwan et al. (2019) found that soaking wheat seeds with ZnNPs and FeNPs significantly decreased wheat grain Cd concentration (about 82%). Similar results were obtained for soil and foliar application of FeONPs . Wheat seeds, with different concentrations of intrinsic Zn, were planted in artificially Cd contaminated soil. The lowest grain Cd concentration was observed for crops that grow from seeds with high intrinsic Zn cultivated on soil enrichment with Zn and biochar.
It has been estimated that the one billion hectares of arid and semi-arid areas of the world remain barren due to salinity or water scarcity (Sahab et al. 2021). The application of Se can significantly mitigate the toxic influence of salt stress. For example, Se application at a low concentration of 5-10 mg dm −3 under salinity conditions significantly increased the antioxidant enzymes activities, the total phenol and flavonoid content, and the enhancement of the K/Na ratio in grapes (Karimi et al. 2020). The application of SeNPs at the concentration of 10 mg dm −3 also increased about 1.85 times enzymatic antioxidant content (APX, GPX, CAT, and SOD) in tomato fruits cultivated on Table 4 The effect of nanofertilizers on biofortification with zinc, selenium, and iron. The enrichment factor (EF) was calculated as a ratio of results obtained from the most advantageous fertilization of the crops in relation to the control group Reference   (Noreen et al. 2020). It is worth noting that numerous studies on the impact of micronutrients on alleviation of negative effects of abiotic stress have been performed under laboratory conditions, and only a few were conducted in natural, field conditions . Table 6 summarizes the influence of biofortification with Zn, Se, and Fe on the adaptation of plants under stress conditions.

Future Perspectives and Strategies
Agronomic biofortification is the most promising way to successfully alleviate micronutrient malnutrition by increasing the mineral content in the crops and simultaneously enhancing their bioavailability by reducing antinutritional compounds and/or enhancing the concentration of mineral absorption promoters. Zn, Se, and Fe biofortification efforts should consider the concentration and species of micronutrients accumulated in the plant in relation to the effects that such micronutrients enrichment could exert on the production of health-beneficial and/or stress-defense compounds. Additionally, the balance between the production and nutrient requirements of Zn, Fe, and Se should be included in considerations of sustainable intensification.
Based on the collected data there is still a lot of missing information that should be considered when planning future research: 1 most of the research into biofortification with micronutrients was conducted in laboratory conditions under strictly controlled environmental factors and it is important to verify those results under a wide variety of environmental conditions; 2 there is a lack of information in the collected data about the form of application of fertilizer on the transfer of microelements, e.g., granules vs. liquid form vs. encapsulated fertilizers. What is also missing is information around the influence of soil properties on the release of micronutrients during long-term application; 3 in many studies, there is also a lack of basic characteristics of soils, like pH, the content of organic matter, salinity, and moisture which are very important factors regulating the form and concentration of micronutrients in the soil; 4 the total micronutrient content in plants is not always an appropriate indicator of its useful nutritional quality as the human body can absorb only a particular form and dose of micronutrients and only a few studies covered the determination of the form of compounds accumulated micronutrients; 5 further studies are required with more species and/or cultivars of the same species under variable growing conditions to define the best practice for Zn, Se, and Fe agronomic biofortification; 6 further research is required to estimate Zn, Se, and Fe bioavailability in biofortified microgreens, sprouts, and baby greens; 7 some papers proved that fertilization with more than one micronutrient gives better effects than applied alone. However, it is worth mentioning that is needed to reach fine-tune doses to obtain an adequate accumulation of micronutrients in edible parts of crops without limiting growth and quality parameters. More studies are required to gain an understanding of the antagonistic and competitive effects of nutrient elements on plant uptake of Se, Zn, and Fe; 8 to achieve sustainable agricultural productivity, growers should switch from a high input-based production system to the cultivation of soil-plant-microbiom interaction-based systems. More work should be focused on: -studying biological inoculants in the agriculture field under changing climatic conditions and competition by indigenous microorganisms -better understanding the effect of plant beneficial rhizobacteria on the effective utilization of these microbes in mitigating various abiotic stresses -studying promotion of plant growth with a combination of microorganism with a mix of micronutrients at different concentrations; 9) the increasing commercial use of NPs may result in unintended exposure to flora and fauna of the environment. Key aspects that influence toxicity in plants are the following: the concentration of NPs, particle size, surface area, stability, physicochemical properties, plant species, plant age/phenological stage, the medium of exposure, and dilution agent. The future research should be conducted utilizing the processes of translocation and accumulation of micronutrient NPs in crops. This area of research has not been studied adequately and much more studies have been performed up to the germination stage, providing only limited information. Due to the finite term of existence of NPs, there is a need to perform research under the stability of NPs in time. Additionally, from the environmental protection point of view, strict dosage and distribution control of NPs is a very important issue for ensuring that the application of NPs poses no potential risk for plants, animals, and humans. The application of NPs is a very promising way in crops  fertilization; however, the focus should be on the greener approach for the synthesis of metal oxide nanoparticles, which would, to some extent, help in limiting toxicity towards the environment; 10) it is quite easy to evaluate the responses of single species or a few plant species to Se, Zn, or Fe under the effect of one single stress in the laboratory. There are many factors in field conditions that are responsible for plant stress, the reaction of plants can differ in comparison to results obtained on a laboratory scale, and it is important to perform more experiments in field conditions. One of the main future challenges is to better recognize Se-, Zn-, and Fe-plant interactions under abiotic stress to reveal the beneficial role of these micronutrients. Furthermore, studies with full life cycles of plants are also needed, especially to better understand the impacts of NPs on heavy metal accumulation by plants grown in realistic contaminated soils.

Conclusion
Agronomic biofortification of staple and non-staple crops with Zn, Se, and Fe using mineral and organic fertilizers has an exceptional potential for a fight with hidden hunger worldwide. The review is focused on the state-of-art application of Zn, Se, and Fe fertilizers including the selection of the type of fertilizers (including nanofertilizers and biofertilizers), type and dose of applied micronutrients, and their accumulation by selected crops. Besides an insight into the application of Zn, Se, and Fe in terms of increasing the nutritional value of crops, the review also presents positive influence of micronutrients on alleviating the damage caused by abiotic stress. The success of agronomic biofortification depends on many important factors and, although numerous papers have been published regarding Zn, Se, and Fe fertilization and the effect of many factors on the effectiveness of agronomic biofortification, the understanding of this topic is still unclear. Based on collected literature, it could be concluded that more research should be performed concerning: field experiments with variable conditions and full life cycles of plants, the impact of soil properties on the release of micronutrients during long-term application, the determination of the form of compounds accumulated micronutrients in edible parts of crops, the reaction of species and/or cultivars of the same species on Zn, Se, and Fe fertilization and different ways of application fertilizer, an antagonistic and competitive effects of different factors on plant uptake of Se, Zn, and Fe, impact of biofertilizers and nanofertilizers on environment and accumulation in edible parts of crops, the impact of Zn, Se, and Fe on heavy metal FeNPs.
There were no significant differences between the types of fertilization. The FeNP fertilization increased superoxide dismutase and peroxidase activities, dry weight of roots, shoots, grain, and husks, chlorophyll and carotenoid content, and photosynthetic and transpiration rate 2.85 (grain)