Biofortification, specialty fertilizers, nutritional security, nutrient uptake efficiency.

Good nutrition is a key to healthy life and it mainly depends on sustainable agriculture (Athar et al., 2020). In order to maintain good mental & physical health and lead fulfilling lives, humans need a balance of nutrients. Sodium, calcium, potassium, phosphorous, magnesium, sulfur, and chlorine are mineral elements that are categorized as essential nutrients that the human body needs in trace amounts. The remaining group of vital nutrients, known as micronutrients, are those that the body needs in minuscule amounts and involve zinc, iron, copper, manganese, selenium, cobalt, iodine, molybdenum, and nickel (Table 1) Prashanth et al.(2015). All of these nutrients are essential to human health and development, both physically and mentally (White et al. 2005). Even mild to moderate micronutrient deficiencies can lead to serious health problems for humans, including poorer immune function increased susceptibility to infections, growth failure, cognitive decline, and eventually lower productivity (Tulchinsky, 2010). Worldwide, micronutrient deficiencies impact approximately 38% of pregnant mothers and 43% of preschool-aged children. According to reports, over 30% of people on the planet are anemic and experiencing hidden hunger (Stevens et al., 2013). Compared to developed countries, developing countries have a higher prevalence of anemia.

According to the Times of India (2021), a report has been published based on a world wide survey of 220 healthcare practitioners (doctors & nutritionists), 70percent of the average Indian diet lacks appropriate nutrition. Additionally, the survey revealed that the two vitamins most commonly deficient in the average daily diet nationwide are vitamin B12 and D3, which are followed by the iron, zinc, folic acid, calcium, and vitamin C. Furthermore, it was suggested in the Indian Food Composition (Longvah et al., 2017) report by the National Institute for Nutrition (NIN), Hyderabad, that over the past thirty years or so, a large number of our food items have rapidly lost their nutritional value. For instance, the amount of carbohydrates in whole wheat has decreased by 9% and in bajra (Pearl Millet) by 8.5%. Similarly, the essential protein component of pulses is being lost; it has decreased by 10.4% in masoor (whole brown) and 6.12% in moong (whole green). On the other hand, potatoes have less zinc, magnesium, and thiamine (vitamin B1). In cabbage, micronutrients like iron, thiamine, magnesium, and zinc have decreased by as much as 41-56%. Iron, zinc, and thiamine have all decreased by 66-73 percent in red tomatoes. Apples and green tomatoes have lost 60% and 76.6 percent of their iron, respectively. We now prefer coarse grains because they are full of micronutrients, due in large part to our growing health consciousness and concerns about the gluten content of wheat and rice. Based on an analysis, pearl millet, barley, sorghum, and maize now have lower levels of thiamine, iron, and riboflavin.

A practical, affordable, and long-term method of increasing the amount of vital micronutrients, minerals, and vitamins (involving trace elements) in food is fortification. This enhances the food's nutritional value and lowers the likelihood of health issues for the general public. On the other hand, biofortification is the process of enhancing food crops' nutritional value through the use of contemporary biotechnology, conventional plant breeding, or agronomic techniques (World Health Organization, 2019). Unlike conventional fortification, which uses manual methods during crop processing, biofortification goals to raise levels of nutrients in crops during the growth of plants.

Moreover, from an economic perspective, industrial fortification of processed foods appears to be an expensive strategy with only negligible advantages for micronutrients like zinc & iron (Bromage et al., 2018). This problem could be solved by biofortification, but the best approach to encourage biofortification would be to include it in a wide range of sustainable, plant-based nutritional strategies (FAO, 2017). According to reports, biofortified crops significantly improve human health by increasing micronutrient intake (Praharaj et al., 2021).

The simplest way to increase the microelements levels in the crops has to be known to be agronomic biofortification, which aims to supply micronutrients which could be directly absorbed by plant by applying mineral and/or foliar fertilizers and/or improving the solubilization and mobilization of mineral elements in the soil. Furthermore, numerous reports demonstrate that biofortification not only enhances plants' micronutrient content but also significantly influences the creation of the any other compounds with nutritional qualities (Newman et al., 2021).

The main disadvantages of biofortification via genetic engineering or conventional plant breeding are that the products are not approved in all nations and that it takes a long time to develop. In contrast, agronomic biofortification is the simplest, quickest, and most widely recognized approach to enriching food with micronutrients, vitamins, folic acids, and other essential elements while still reaching the poorest rural populations. For instance, compared to their non-AMF treatments, the integrated application of arbuscular mycorrhizal fungi (AMF), P, and irrigation regimes on okra increased the average total uptake of N, P, K, Ca, B, and Mo by 8, 24, 5, 14, 8, and 40 percent, correspondingly (Kumar, 2012). Alternatively, genetic biofortification-a crop types engineered to naturally absorb more mineral micronutrients through the soil-means that the grower does not necessarily need to apply the micronutrient in order for it to be fortified. However, agronomic biofortification would also be crucial in these circumstances. Crop varieties fortified with genetic engineering would scavenge soil micronutrients, which are also a finite and exhaustible resource. If not right away, agronomic biofortification would need to be added after a few years to sustain source-sink linkage in genetically enhanced crops. The review discussed the developments in cutting-edge methods that improve crop plants' ability to absorb micronutrients, which can increase the effectiveness of agronomic biofortification.

Zn AND Fe FROM SOIL TO PLANT

Plants primarily absorb nutrients through their roots, and the rhizosphere's local conditions can affect how bioavailable zinc and iron are to the plants. Numerous elements in the soil and plants influence how much zinc and iron plants take up. The ionic form and soil concentration of zinc and iron are the most significant factors influencing uptake. The concentration of competitive anions, soil mineralogy, pH, and clay content are other significant factors that affect how much zinc and iron plants accumulate.

Zn is mostly taken up by plants from the soil as the divalent cation Zn2+. Plants mainly take up zinc from the soil through their roots through a process called facilitated diffusion, which is aided by transporters and membrane potential (Marschner, 2012). However, plant roots can occasionally also absorb organic ligand-Zn complexes. The uptake involves two potential physiological mechanisms, which vary based on the ligand released by the roots of plants. Consequently, mechanism I entail the organic acids efflux, solvents, and H+ ions, which enhance Zn-complex absorption & liberate Zn2+ ions for absorption by the root epidermal cells. In contrast, mechanism II involves the phyto-metallophores outflow, that combines with zinc to form complex compounds that are subsequently taken up by root epidermal cells. However, according to Chatterjee et al. (2018), this absorption process (i.e., method II) is only available for cereal roots. These mechanisms necessitate differences in Zn content between root cell membranes and the involvement of molecules of water as the solvents.

H+ ATPase regulates the fast polarization of a root plasma membrane, which is the main factor causing the uptake of Zn 2+. The result of ATP hydrolysis is the strong pumping of H+ ions out of the cell by the H+ ATPase. By lowering the soil pH and hyperpolarizing the root plasma membrane, H+ production in the rhizosphere promotes cation absorption (Kamaral et al., 2022).With the aid of particular transporters found at the pericycle, xylem loading facilitates the root-to-shoot Zn2+ transport. Different strategies are used by plants to absorb different amounts of nutrients. For instance, the acquisition of Fe in dicot and non-graminaceous species "on the basis of Fe-limiting environment concept. In these circumstances, an active proton pump generates more Fe+2 & Fe transporter through ferric chelate reductase, increasing the solubility of Fe+3. In contrast, the tactic used by graminaceous monocots on the basis of phytosiderophores releases from the roots, which chelate with Fe+3 and then taken up by particular transporters" (Hell and Stephan 2003).

Transpirational forces draw the micronutrients up and deposit them in the plant's leaves after they have passed through the xylem. The leaves function as a sink for carbohydrates & nutrients up until this point. When a plant grows further, its leaves change from being a sink (a source of carbon and nutrients) to a source (a sink of carbon and nutrients) when the amount of carbon stored in photosynthesis exceeds the amount needed for the respiration & growth. The xylem carries nutrients through the roots to leaves, while the phloem facilitates additional translocation of the nutrients through leaves to the other sections (such as grain, fruit, etc.). When foliar fertilizer is applied (sprayed), micronutrients enter through the cuticle & stomata on surface of the leaf and then accumulate directly at the phloem. This application method can be more effective from a biofortification standpoint because the pathway is shorter than when roots are absorbed. For optimal biofortification, there must be a greater intake of micronutrients, better translocation within the plant, and raised accumulation at portions of edible.

APPLICATION METHODS IN AGRONOMIC BIOFORTIFICATION

Micronutrient deficiencies in plants and people can be addressed by applying micronutrients as fertilizers, which can also enhance the status of these nutrients in the soil. So far, when micronutrients are added to the soil, they get fixed right away and don't easily move to the parts of the plant that are consumed. It is then advised to apply micronutrients using alternative methods, like soluble form foliar sprays. Agronomic biofortification is a cheap and easy tool if certain factors are taken into consideration, like the type of fertilizer, how it is applied, and when it is applied. There are several methods for agronomic biofortification soil application of micronutrient fertilizer, foliar application utilizing nutripriming, diluted fertilizer sprays, and soilless cultivation have been the most popular method of biofortification.

A. Soil application

The most popular way to give crops micronutrients is through soil application. Maqsood et al. (2009) found that applying 6 mg/kg of soil zinc fertilizer increased grain zinc concentrations from 51.7% to 69.9%. Compared to broadcasting, the banding placement needs 3 times less micronutrient fertilizer (Sarwar et al., 2017). Although adding zinc to the soil can boost crop yield, it has a lower fertilizer utilization effectiveness and is not as effective at increasing the amount of zinc in grains (Chattha et al., 2017). It is affirmed that applying soil in addition to foliar application is a more efficient and superior method of raising grain yield than applying either substance by itself. Due to the excessive accumulation of wasted micronutrients, this method pollutes soil over time and has low micronutrient use efficiency and cost-effectiveness. Because plants have a strong tendency to concentrate zinc under conditions of water scarcity, applying zinc through the soil did not increase the concentration of zinc in grain under these circumstances (Gomez-Coronado et al., 2016). Despite being the most widely used technique for applying micronutrients to crops, soil application has primarily been investigated for increasing crop productivity as opposed to biofortification.

B. Foliar application

The most widely used method of micronutrient biofortification is foliar application because it is easy to implement, more fertilizer-efficient, needs fewer infrastructure, and doesn't need the technical know-how of methods like soil-less cultivation & nutripriming. Foliar application is preferable to soil application because there is very little micronutrient loss and the nutrients are absorbed by the tissue of a plant directly (Johnson et al., 2005). According to Zou et al. (2012), zinc foliar feeding has been the best method for raising the zinc content of grains. By analyzing metadata from published literature, Joy et al. (2015) concluded that foliar application is a more effective way to get zinc into rice, maize, and wheat grains than soil application.

When it comes to utilizing nutrients efficiently and quickly treating visual deficiency issues, foliar application of nutrients appears to be more beneficial than soil amendments (Fageria et al., 2009). For biofortification of a grain, foliar application later in the vegetative stages is preferable to foliar application early (Yilmaz et al., 1997). More than other prior applications, foliar zinc application after flowering and in the initial stages of dough & milk increased grain zinc content (Phattarakul et al., 2012). Without affecting biological yield, foliar Zn spraying increased test weight & grain protein content in the alkaline soils (Khattak et al., 2015). FeSO4 foliar sprayed at the time of anthesis increased the gluten and grain protein content in the durum wheat, particularly when seeded at a 125kg ha-1 rate (Melash et al., 2016).

Since there is less water available for the procedure of irrigation & the solubilization of soil-applied fertilizer in arid along with the semi-arid climates, foliar treatment is acknowledged as a crucial strategy for addressing micronutrient deficiencies in crops (Chapagain & Wiesman 2004). However, foliar application should only be regularly used when it has proven to be more beneficial than soil application. According to Narwal et al. (2012), 14 winter wheat varieties had higher levels of Zn and Fe after receiving foliar treatment. Given that applying soil has the drawback of causing micronutrients to fix in calcareous and alkaline soils (Alloway, 2008). In such cases, the foliar application creates more sense. According to Zhang et al. (2012), foliar zinc treatment has been an efficient way to provide people with more dietary zinc from products made from biofortified wheat because it has been much more successful than the soil zinc application at enriching wheat grain containing zinc. In addition, a study conducted on mungbean revealed that the concentration of zinc in the grain was approximately 1.7 times higher (Haider et al., 2018a) following the application of 1.0 percent zinc sulfate solution than when zinc was applied to the soil at a 10mg kg-1 concentration (Haider et al., 2008). It has been important to remember, though, that effective foliar fertilization necessitates greater leaf area in order to improve applied micronutrient adsorption.

Additionally, Zinc sulfate applied at the sowing along with foliar Zn applied during flowering & stages of pod formation produced the best fertilizer effect for chickpeas (Pal et al., 2019). Zn-phytate interaction, when applied through various methods, is an important factor determining Zn's bioavailability. Consequently, adding enough zinc to the soil increases grain yield, but adding zinc as foliage during the booting stage increases zinc concentration & bioavailability in the wheat grain (Hussain et al., 2012). Additionally, some authors proposed that by applying the Zn and Fe together in soil & foliar fertilizers (Rivera-Martin et al., 2020) and fertilizing by the amendments like salicylic acid or biochar (Smoleń et al., 2019) enhanced the effectiveness of microelement accumulation in the crops while applying together rather than separately.

C. Nutripriming

Soaking seeds in a nutrient-rich solution prior to planting is known as nutripriming or seed-priming (Raj and Raj 2019). The pre-sowing hydration technique known as "seed priming" permits seeds to carry out their pre-germination processes without radical protrusion (Bradford, 1986). Compared to dry seeds, primed seeds have a higher potential for uniform stand establishment (Farooq et al., 2006). The main goals of seed priming have been to improve yield and facilitate germination, root system development, and seedling establishment (Farooq et al., 2019). On the other hand, nutripriming has also been linked to higher grain nutrient content, according to some researchers. Zinc-nutripriming chickpea with ZnSO4 (0.4 percent) increased grain Zn content by 29percent (Harris et al., 2008) and wheat by 12% to 15% (Praharaj et al., 2019). Because micronutrients have been added to the seeds prior to sowing, seed priming also has the advantage of being cost-effective for farmers to implement (Harris et al., 2008).

Micronutrient seed priming increases crop yield and micronutrient content while being economical and environmentally benign. Seldom has it been discovered that seed priming is ineffective (Farooq et al., 2012). After nutripriming with ZnSO4 & ZnCl2, correspondingly, at a rate of 1.25g Zn kg-1 seed, the grains Zn content has been raised from 21-35 percent, and production of grains increased by 33-55 percent Rehman and Farooq 2016). When weighed against soil application, seed priming was also more economical, with benefit-cost ratios from seed priming being 8 and soil application being 360, respectively (Harris et al., 2005). But priming chickpea in the diluted ZnSO4 (0.05percent) solution proved successful, yielding 10-122 percent more (average value of 48percent from 9 trials) than unprimed seeds, with a benefit: cost ratio of 1500 (Harris et al., 2005).

Choukriti et al. (2022) state that seed priming in all forms, including hydro-priming, encourages the germination of seed. Zinc priming was used to increase grain yield while enriching the seeds with this element. In particular, the yield and Zn content of seedlings treated with 0.5 percent Zn sulfate for 24 hours increased by 47 percent and 15 percent, respectively. Similarly, Zn treatment of the seeds significantly increased rice growth and grain yield, and this was more economical and practical than applying the zinc to the soil or not applying it at all (Slaton et al., 2001).

Many factors, including genotype, crop type, nutrient priming duration, priming solution osmotic potential, and environmental conditions, influence how effective seed priming is (Waqas et al., 2019). A South Asian experiment found that seed priming increases crop growth and productivity (Harris et al., 2007). According to Harris et al. (2018), seed priming with a 0.05 percent zinc solution raises the yield & concentration of zinc in the grain by 19 and 29%, respectively. In addition, farmers may not be as familiar with nutripriming techniques because they entail technical aspects of priming methodology. The seeds would need to be stored optimally or sown right away because priming could shorten their shelf life (Murphy, 2017).

D. Soilless cultivation

Soilless cultivation is the approach of growing plants without soil as a rooting medium. In soil-less crops, the fertilizers which are to be supplied to the crops are dissolved in the appropriate concentration in the irrigation water and that resultant solution is named as nutrient solution. Soil-less cultivation includes Hydroponics, Aquaponics, and Aeroponics. The ability to regulate pH, water availability, and nutrient concentrations in the root zone (Gruda, 2009)-as well as the efficiency with which root contact with nutrient supply can be maximized-are among the special advantages of soilless culture techniques (Treftz and Omaye 2016). Moreover, soilless cultivation disregards soil constraints like fertility of soil and transmission of disease in favor of extending the cultivation cycle and enabling year-round production (Rouphael and Kyriacou 2018). Tomasi et al. (2015) list the absence of weeds, low labor requirements, ease of processing & harvesting, and an automated system for the maintenance of plant as additional advantages of soilless agriculture. According to research, soilless farming can effectively raise the zinc and selenium content of "lettuce (Lactuca sativa L.) (Sahin, 2020) and the zinc content of white cabbage (Brassica oleracea L.) (Barrameda-Medina et al., 2017)". Fe deficiency in cherry tomatoes can be effectively treated and fruit quality improved through biofortification (Buturi et al., 2022). Even though soilless cultivation is extremely effective and environmentally friendly, it requires infrastructure growth which is out of reach for several regions.

E. Chelated fertilizers

Chelated fertilizers have been those types of fertilizers where the nutrient ion is shielded from oxidation, precipitation, and immobilization by a large organic molecule called a Ligand/Chelator. It has been demonstrated that chelated fertilizers offer superior nutrient protection against soil conditions (pH, moisture, etc.) that can lead to nutrient immobilization or loss through leaching, precipitation, or oxidation. As a result, plants absorb chelated nutrients more readily and lose less of them. Chelated micronutrients are superior to organic and biofertilizers due to their high nutrient content and increased efficiency over inorganic micronutrient fertilizers. Chelated nutrients have lessened loss to the environment. In a greenhouse study, Zhao et al. (2019) discovered that Zn-EDTA fertilization, even with a lower treatment volume, achieved greater Zn biofortification as compared to ZnSO4.7H2O fertilization.

Content of zinc in wheat has been raised by the foliar application of Zn-consisting salts & Zn chelates (e.g., ZnSO4, Zinc-EDTA, generally @ 0.5-0.7kg/ha) (Zhao et al., 2014). As per the findings of Shivay et al. (2016a), the application of Zn-EDTA three times on leaves during the booting, tillering, and grain-filling stages led to notable improvements in growth, yield attributes, concentration, and uptake of Zn compared to ZnSO4.H2O. According to Márquez-Quiroz et al. (2015), applying Fe-EDDTA complexes is a more efficient method of raising the amount of Fe in the cowpea bean seed than using an inorganic form.

F. Biofertilizers

According to Bhardwaj et al. (2014), biofertilizers have been "microbial inoculant preparations made of microorganisms that aid in enhancing the productivity & development of the host plant. It is advantageous to combine Zn and Fe with the application of AMF and PGPR (Plant Growth-Promoting Bacteria)". Through a number of processes, such as exchange reactions, chelation, acidification, and the release of organic acids, PGPR mobilizes the nutrients (Triticum et al., 2015). Additionally, the mechanism is solely dependent on the micronutrients chemical form, such as phosphates, oxides, or carbonates, and the application of PGPR. Bacillus is the most widely used plant growth-promoting bacterium (PGPR) for microelement biofortification. Maize was enriched in zinc using Bacillus aryabhattai and B. subtilis (Mumtaz et al., 2018).

The Zn level in lettuce was raised by inoculating the fungus Piriformospora indica with zinc, as investigated by Padash et al. (2016). When Zn was inoculated with Rhizophagus irregularis above 150mg Znkg-1 of soil, a positive impact on the  grain Zn concentration has been noted (Tran et al., 2019). The Anabaena sp. & cyanobacteria Azotobacter sp., as well as wheat (Ramesh et al., 2014), Bacillus aryabhattai (Prasanna et al., 2015), and the soybeans (Glycine max L.) (Ramesh et al., 2014), facilitate zinc biofortification in corn grains. It has been discovered that AMF improves root development and guarantees N, P, Cu, Zn, Fe, and Mn uptake. Rhizophagus irregularis raises the concentrations of primary metabolites & minerals sucas  Mn, Fe, Zn, and Cu in the medicinal plants like Petroselinum hortense & Mentha pulegium (Gashgari et al., 2020). Sharma & Johri reported that Pseudomonas chlororaphis & Pseudomonas spp. isolated from maize enhanced Fe uptake, plant growth, germination, and output of crop (2003). Plants cannot absorb ferric forms of iron (Fe3+) due to their extremely low solubility; however, microorganisms secrete siderophores, which are Fe-chelating compounds that help microelements be absorbed at varying pH levels.

G. Nanofertilizers

According to Bhardwaj et al. (2022), nano fertilizers have been fertilizer forms in which the active ingredients have been adsorbed, dispersed, entrapped, or encapsulated at host material. These particles, micelles, or pockets range in size from 1 to 100 nm. A well-liked substitute technique for enhancing nutrition is wheat crop nano-biofortification (Khan et al., 2021). Tailored fertilizers known as nanofertilizers have the potential to completely transform the way we currently farm (Rai et al., 2018). Nanofertilizers have been sophisticated delivery systems which are simple to use, safe, and target-bound. Most polymeric-type fertilizers have a higher area of surface to volume ratio, which makes nano-formulations more effective, slow-releasing, and efficient sources of nutrients for crops (Fig. 1). As a result, nano-fertilization provides a platform for an innovative and sustainable nutrient delivery system which could investigate a plant's nanoporous surface.

Plant species & chemical characteristics of the nanomaterials, like size, composition, and concentration, are key determinants of the effectiveness of nanofertilizers (Thakur et al., 2018). Since nano-fertilizers have been designed to potentially compensate for a specific nutrient deficit, biofortification with nano-nutrients appears to be a promising avenue to pursue. The plant will grow and store these nutrients in its edible parts with the help of these fertilizers (Li et al., 2016).

According to "Dapkekar et al. (2018), even though zinc complexed chitosan nanoparticles were used at a 10-fold lower concentration, grain Zn enrichment has been seen to rise by approximately 36percent with Zn-CNP nano-carrier & approximately 50percenthaving ZnSO4. The 2 durum wheat varieties (MACS 3125 & UC 1114) have been treated with Zn-CNP (40mg L-1) & conventionally applied ZnSO4 (0.2 percent; 400mg L-1 zinc).A foliar treatment of 100mg L-1 of the ZnOnano fertilizer caused Zn concentrations of 100-150mg kg-1 dry weight in shoot & root tissues & 45mg kg-1 in wheat grain", according to Hussain et al. (2021). This rise in the content of Zn in the plant has several parts. Hussain et al. (2019) found that grain Fe content was higher (110mg kg-1) with foliar application of Fe nanofertilizer than with soil application (90mg kg-1). However, shoot Fe concentrations were higher with soil application of Fe nanofertilizer than with foliar spray.

When citrate-coated nano Fe2O3& Fe2O3 (conventional form) have been applied to the plants of soybean, Alidoust and Isoda (2013) discovered that no phytotoxic effects were seen. Compared to conventional Fe2O3, nano-Fe2O3 had a more stimulating impact on the growth of the root. Nanoparticle properties such as size, chemical configuration, stability, and concentration affect the plant species that absorb, translocate, and accumulate the particles. Reducing the fertilizer particles size applied is intended to provide the "appropriate amount of nutrients" at the "appropriate location" and "appropriate time." Furthermore, smaller particles have a higher specific surface area, which increases the area in which fertilizers come into contact with plants. This increases plant nutrient uptake relative to when commercial fertilizers are applied.

Table 1: Essential macro and micro nutrients required for good human health.

Source: Prashanth et al. 2015

Fig. 1. Nano fertilizers benefits the soil as well as plant.

Prerna Dogra, Dalpat Lal, Omprakash, Kamini Parashar, Aabha Parashar, Aparna, S.R. Rundala, Ramesh Asiwal and Lakshay Choudhary (2024). Traditional to Modern Approaches for Agronomic Biofortification of Food Crops with Zn and Fe: A Review. Biological Forum – An International Journal, 16(1): 174-182.