Introduction

Hidden hunger (deficiencies in micronutrients) is emerging as a major challenge for agricultural scientists. It is caused by lack of vitamin A, zinc, iodine, folate and iron (Jawaldeh et al. 2019). World population is increasing, and food consumption demand is augmenting. After 1960′s, we are focusing on increasing the productivity not the quality of food. Malnutrition of micronutrient is being effected 2 billion population especially in developing countries (Stein 2010; Cakmak et al. 2010). Metals deficiencies, especially Zn and Fe, are affecting over half of global population because they are depending cereal crops mainly wheat rice and maize for their daily diet (Kenzhebayeva et al. 2019). Zinc is structural part of many enzymes that are used in the metabolism of auxin and carbohydrates, in the synthesis of proteins and integrity of membrane (Cakmak 2000; Rehman et al. 2018). Additionally, it also takes part in pollen development, fertilization and chlorophyll synthesis (Pandey et al. 2006). Moreover, Fe is constituent of ETC (electron transport chain) and cytochrome, and activates many enzymes (Soetan et al. 2010). Malnutrition of micronutrients cause 5 Million. childhood death causalities every year (Anonymous 2007).

Wheat is the staple food in many micronutrient deficient regions, which fulfils 50% diet requirements (Cakmak et al. 2010). Cultivation of higher yielding crops with intensive use of fertilizers such as potassium, phosphorous and nitrogen resulted in micronutrient deficiency (Cakmak 2002). About 50% cereals cultivated soils are Zn deficient, and Fe deficiency is often present in calcareous and high pH arid regions. Moreover, irrigation of bicarbonate enriched water often resulted in Fe deficiency (Graham and Welch 1996). According to world scale estimation, Zn and Fe deficiency widespread is approximately 50% and 30% respectively (Cakmak 2002). Zn deficiency is more common in Pakistan, about 70% cultivated land in Zn deficient (Imtiaz et al. 2010). A series of experiments explored that Khyber Pakhtunkhwa soil is 37% Zn, 14% Fe and 60% B deficient, and Punjab soil is 57% Zn, 21% Fe and 50% B deficient (Memon et al. 2012).

Wheat is a staple food in Pakistan, and grown on 43% of cultivated area. About 60% Pakistani people diet is comprised on wheat and annual consumption per capita is 140 kg annum−1 (GOP 2017). Wheat production at 25.492 Million. tons during 2017–2018 was surplus than country prerequisite (GOP 2018). West and Central Asian countries, wheat provide about 50% daily calorie on average, probably it is more than 70% in the rural regions (Cakmak 2008). In South Asia, micronutrient deficiencies is highly reported in cropping system of Rice–wheat (Johnson et al. 2005). Rice and wheat are major crops that fills the food requirements all over the world. According to (Jackson 2009) Asian subtropics comprises on 24 Mha, with ~ 13.5 Mha cultivated land, and in South Asia it consisted on Indo-Gangetic floodplains to Himalayan foothills.

There are various possible techniques to overcome the hidden hunger including post-harvest food fortification, mineral supplementation, diversification in dietary habits and biofortification (Borrill et al. 2014). Agronomic biofortification is one of the major agricultural strategies to enhance the grain concentration of Fe and Zn (Cakmak et al. 2010). It is attained by applying micronutrients to soil or/and directly to the foliage of crop (De-Valença et al. 2017) as it is potentially more economical, easily implemented and more sustainable as compared to genetic engineering and other techniques (Cakmak 2008). Agronomic biofortification of crops is an evolving technique to overcome micronutrient malnutrition in developing countries (Ngozi 2013). Exogenous application of micronutrient can be helpful to improve the quality of grains with enhanced productivity of wheat crop. Keeping in view the above notations, current filed base study was performed to overcome the malnutrition of micronutrients by assessing these objectives i) to determine the sole or/and combined effect of Zn and Fe on growth attributes of wheat, ii) to compare the efficacy of foliar and soil supplementation to enhancing the grain yield.

Materials and Methods

Experimental Particulars

A two-year (2016–2017 and 2017–2018) filed study was designed to evaluate the efficacy of foliar and soil supplied Zinc (Zn) and Iron (Fe) on the all the growth and yield attributes of wheat. Wheat Research Institute Farm area, Faisalabad (altitude 184.3 m a.s.l.; 73.87E, 31.87 N) was used for conducting experiment. All the experimental soil is sandy clay loam soil, and the Physico-chemical properties of this soil are represented in Table 1. Randomized complete block design (RCBD) under factorial was used with three replications. The net-plot size was 5 m × 1.62 m.

Table 1 Physical and chemical analysis of soil of field trial site
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The following combination of treatment was applied for studding the above mentioned objectives;

  • Control

  • Foliar spray of 0.5% ZnSO4

  • Foliar spray of 1% FeSO4

  • Foliar spray of 0.5% ZnSO4 and 1% FeSO4

  • Soil supplementation of 10 kg of Zn ha−1

  • Soil supplementation of 12 kg of Fe ha−1

  • Soil supplementation of 10 kg of Zn ha−1 and 12 kg of Fe ha−1

Soil application of Zn and Fe was done at the time of sowing and foliar applications were applied thrice during the course of experimentation at tillering, booting and heading stages and.

Crop Husbandry

After harvesting of rice crop, a deep plough was used to break the hard pan and stubbles were incorporated in the soil. After that, seed bed was prepared by using cultivator twice with same number of planking. During both years wheat was sown in a well prepared fine seedbed on November 21, 2016 and November 25, 2017 help of Norwegian planter. Weather data are presented in Fig. 1. Seed rate was 100 kg ha−1. Moreover, K, N and P fertilization was applied at rates of 60 kg ha−1, 114 kg ha−1 and 120 kg ha−1, and respectively. Urea (46% N), diammonium phosphate (18% N and 46% P2O5), Zinc sulphate (ZnSO4), Iron sulphate (FeSO4) and muriate of potash (60% K2O) and were used as micronutrients. At the time of sowing, complete dose of K and P with 1/3rd of N was used as basal dose. Remaining N was supplied with first and second irrigation with equal split. Four irrigation was applied throughout the season of wheat crop. Wheat was harvested on 19 April and 28 April for the 2016–2017 and 2017–2018 seasons respectively.

Growth, Yield and Quality Attributes

Tillers (stem with ear) per unit area was measured accurately before harvest. Plant length and spike length was measured by using meter scale. No. of grains per spike and No. of spikelet was calculated manually. Moreover, weight biomass and 1000 grains weight was recorded by using weight machine. At the fully mature stage, 1m2 area was harvested for measuring the biological yield. To determine the grain weight, grains obtained from each pot was measured by using electrical balance. The harvest index was calculated by using the following formula (Fig. 1).

$${\text{Harvest index }} = {\text{ Grain yield/Biological yield }} \times { 1}00$$
Fig. 1
figure 1

Weather data of experimental station during the course of experimentation

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After harvesting, samples of grain were collected for minerals Zn, Fe, Cu, Mg and Ca analysis. Grain samples were dried in air circulated oven at 70 °C to get constant weight. Wet-digested method was followed using HNO3–HClO4 proposed by Rashid (1986). Supernatant liquid was decanted and was run on Atomic Absorption Spectrophotometer (Shemadzu 7000) to analyze Zn, Fe, Cu, Mg and Ca in the aliquots. Grain protein contents (%) were measured by using Microjeldahl method (protein % = N% × 6.25) proposed by Agrawal et al. (1980).

Statistical Analysis

The data were analyzed statistically using analysis of variance and the means were compared by F-test protected LSD values calculated for P, 0.05 (Steel et al. 1997).

Results

Impact of Zn and Fe on Growth and Yield Parameters of Wheat

Application of zinc sulphate and iron sulphate as a sole and in combination either foliar and soil applied significantly affected No. of tillers, plant height, spike length (Table 2), No. of spikelets per spike, No. of grains per spike, 1000-grain weight (Table 3), biological yield, grain yield and harvesting index (Table 4), calcium, magnesium, iron, zinc, copper and protein contents (Figs. 2, 3). There was a significant difference between the two years for only 1000-grain weight (Table 3). However, sowing season did not affect No. of tillers, plant height, spike length, No. of spikelets per spike, No. of grains per spike, biological yield, grain yield and harvesting index (Tables 2, 3, 4), calcium, magnesium, iron, zinc, copper and protein contents (Figs. 2, 3).

Table 2 Mean values of number of tillers, plant height and spike length as influenced by soil and foliar applied zinc and iron
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Table 3 Mean values of number of spikelets per spike, number grains per spike and 1000-grain weight as influenced by soil and foliar applied zinc and iron
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Table 4 Mean values of biological yield, grain yield and harvest index as influenced by soil and foliar applied zinc and iron
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Fig. 2
figure 2

Impact of soil and foliar applied zinc and iron on calcium, magnesium and copper contents of wheat grain

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Fig. 3
figure 3

Impact of soil and foliar applied zinc and iron on iron, zinc and protein contents of wheat grain

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All the foliar and soil supplementation improved the tillers per unit area but maximum improvement was recorded through soil applied Zn and Fe which was statistically at par with Zn and Fe combined foliar spray (Table 2) while least tillers per unit area were noted under control condition in both growing years. Results obtained in the current study regarding tillers, soil supplementation with of 12 kg of Fe ha−1 and 10 kg of Zn ha−1 produced maximum tillers, and showed statistical par with foliar spray of 0.5% ZnSO4 and 1% FeSO4. Plant height was significantly increased by sole and combine application of Fe and Zn through soil supplementation, which showed statistical par with other Zn and Fe treatments. Highest plant height (104.3 cm) was achieved in combined soil application of Zn and Fe in second year crop whereas lowest (98.8 cm) was observed in control in first year crop (Table 2). No. of spikelets per spike, spike length, No. of grains per spike were considerably higher through sole and combined application of iron and zinc (Tables 2, 3). Largest spike length (11.1 cm) was measured in combined foliar application of Fe and Zn in year one (Table 2). Significantly maximum No. of spikelets per spike (19.83) were observed in combined foliar spray of Zn and Fe in year one while minimum (18.0) was counted in control of year two (Table 3). Highest No. of grains per spike (49.3) and lowest (42) were observed in combined foliar supplementation of Fe and Zn (year one) and control (year two) respectively (Table 3). All these parameters were not differed during two season and did not have interaction affect (Tables 2, 3). 1000 grain weight was higher in combined and sole supplementation of Fe and Zn. Highest 1000 grain weight (41.8 g) was observed in combined Fe and Zn soil supplementation in year one that showed statistical par with combined Zn and Fe foliar application (41.3 g) year two whereas lowest (32.5 g) was noted in control of crop year two (Table 3). 1000 grain weight was significantly increased in both cropping years (Table 3). Biological yield, grain yield, harvesting index were significantly increased through sole and combined application of iron and zinc (Table 4). While higher biological yield (12.1 t ha−1) was obtained in combined soil application of Zn and Fe (year one) while minimum (10.8 t ha−1) was observed in control (year two) (Table 4). Higher grain yield (4.15 t ha−1) was observed in foliar application of Zn and Fe (year one) whereas minimum yield was observed (3.20 t ha−1) in control of year one (Table 4). The highest harvest index (35.5%) was recorded from combine foliar spray Zn and Fe in year one whereas lowest harvest index (29.4%) was observed in control in year one. There was no difference in harvest index, grain yield and biological yield across the years (Table 4).

Impact of Zn and Fe on Mineral Elements in Wheat Grain

Sole and combined application of iron and zinc significantly affected the Ca, Mg, Fe, Zn, Cu and protein contents in grains (Figs. 2, 3). There was a no significant difference between the growing years for protein, Ca, Cu, Mg, Fe and Zn contents. Maximum Ca content (0.0115 g kg−1) was found in foliar application of Zn (year two) that was statically at par soil application of Zn (11.4 mg kg−1) and minimum (8.63 mg kg−1) was recorded in control (year one) respectively (Fig. 2a). Highest Mg content (17 mg kg−1) and lowest (13.3 mg kg−1) were observed in foliar application of Zn and Fe (year one) and control (year two) (Fig. 2b). Highest Cu contents (12.6 mg kg−1) and lowest (8.3 mg kg−1) from soil application of Zn and Fe (year two) and control (year two) respectively (Fig. 2c). The maximum Fe contents (78.33 mg kg−1) was recorded from foliar application of Fe in year one whereas minimum Fe contents (43.33 mg kg−1) was observed in control of year one (Fig. 3a). Highest Zn content (61.33 mg kg−1) and lowest Zn content (41.08 mg kg−1) were recorded in foliar application of Zn (year two) and control (year one) respectively (Fig. 3b). Protein contents was also affected by treatments application, maximum protein contents (14.37%) were found in control of year one while minimum protein contents (12.63%) were recorded in combined application of Zn and Fe through soil application method of year one (Fig. 3c).

Discussion

Yield and Its Attributes

Micronutrient might be easily available to plant for metabolism applied via foliar spray. The improvement in components of yield of wheat crop might be due to involvement of Zn and Fe and their critical role in biochemical process including photosynthesis. Findings of current study was in agreement with Niyigaba et al. (2019), they reported that foliar treatment of Zn and Fe significantly improved the yield and quality parameters of wheat crop including thousand kernel weight, crude proteins, spike length, kernels per spike, grain yield, Zn and Fe contents in grains. Sultana et al. (2016) also stated that foliar treatment of Zn played a significant role in enhancing grain yield and other yield components of wheat. They recorded maximum plant height, grain yield, 1000-grain weight, No. of grains per spike and spike length by foliar application Zn @ 0.04%.

Pahlavan-Rad and Pessarakli (2009) reported that highest number of grains were obtained by combined application of Zn and Fe in wheat crop. Hassan et al. (2019) stated that treatment of Zn in wheat crop produced more number of seedlings, biological yield, grain yield and also others yield related parameters. Findings of current research (Table 2) are also supported by Narimani et al (2010) who stated that combined application of Zn, Fe and Cu through foliar spray significantly enhanced plant height, number of fertile florets and number of kernel per spike when compared with control. They also stated that all fertilizers when applied as foliar spray were suggested better to enhance the productivity of durum wheat because of maximum utilization of applied nutrients (Fe + Cu + Zn). This improvement in plant growth is due to the specific role of Zn in enhancing cellular functioning such as carbon metabolism, auxin metabolism, gene expression and functional and maintain structural integrity of cellular membranes. (Marschener 1995).

Moreover, Zn is also involved in seed germination coleoptile development, and early stages of radical (Ozturk et al. 2006). Seed enriched with higher content of Zn showed higher seed germination, and early seeding growth (Cakmak 2008). Zn also play crucial role by being part of many physiological process including respiration and photosynthesis that ensured higher grain yield and early flowering (Zeidan et al. 2010). Results of present experimentation are also supported by Hasina et al. (2011) they concluded that exogenous application of Zn enhanced the number of productive tillers, it may be due the availability of more nutrients for developing tillers. Findings of Soleimani (2006) are also in line with current research (Table 3) that Zn application also increased the number of grains per spike in wheat crop. It may be due to Zn contribution in the cellular enlargement, elongation and cell division process (Cakmak 2000). Moreover, a close relationship was obtained between spike length and number of spikelets i.e. grain numbers increased with increased spike length. The outcomes of this research was also in line with finding of Habib (2009), who deduced that Zn and Fe foliar supplement increased seed yield and other yield components.

Quality Attributes

Findings of our study are supported by Cakmak et al. (2010) who stated that soil and foliar application method of Zn is very effective which increase the Zn concentration three times in the grain. They also stated that application time and application method of Zn treatment is so crucial to improve Zn contents in grains. According to Pahlavan-Rad and Pessarakli (2009), combined application of Zn and Fe was significant regarding Zn concentration in grains. They stated that by application of 1% FeSO4 and 0.5% ZnSO4, Fe and Zn contents were also significantly increased. Xu et al. (2012) reported the results from their studies which against the finding of our study. They stated that there is additive effect for Zn, Fe and protein concentrations in wheat grain but in our study, antagonistic relation is observed. By the increase of Fe and/or Zn contents, protein contents are decreased. Regarding the concentration of Cu in grains, our findings are in line with outcomes of Imtiaz et al. (2003) who concluded that application of Zn showed an adverse impact on Cu contents in wheat plant. Our results are also supported by Zeidan et al. (2010), they stated that foliar application of micro elements including Zn and Fe significantly improved Fe and Zn contents in the grains of wheat as compared to control treatments. Zhao et al. (2014) investigated that, Zn application as foliar is more beneficial in enhancing Zn content in plant tissues as compared to soil supplementation. Same results were found in current study, Zn application proved more effective as compared other supplements. Habib (2009) reported that Zn and Fe application at tillering stage increased plant Zn and Fe content. Zn application as foliar at reproductive stage significantly increased the straw yield and grain productivity, because at these stages it ensures the plant nutrition that is more required at the time of seed filing (Khan et al. 2010). Contrarily, Ozturk et al. (2006) deduced that maximum Zn content was observed in grains by foliar application of Zn at milk stage of wheat crop. Sharma et al. (2008) reported higher protein content with foliar supplied Zn plants, because Cu and Zn are involved in higher protein content. Sole application of Zn increased grain Zn content. Interestingly, grain Zn content is antagonistic to Fe content, when Zn content increased Fe content decreased, vice versa. The combine application of Zn and Fe in different formulation and concentration affect differently the growth and yield parameters. Zuchi et al. (2015) stated that uptake and translocation of Fe to shoot might be prevented by an inadequate supply of other nutrients and Fe deficiency may lead to a high decrease in total nutrients. The interaction among the plant nutrients can be either synergistic, antagonistic, zero-interactive and/or Liebig-synergistic. Above mentioned interactions clarify that the supply of nutrient can affect the function of another nutrient that ultimately influence the crop growth and yield (Rietra et al. 2017). Application of micronutrients and their interactions affect the physiological and biochemical processes of plants, which significantly influence the quality and grain yield (Wang et al. 2015). A thinkable reason for the improvement in quality and yield of wheat crop is that foliar applied Zn may be phloem mobile and can easily translocate into developing grains of wheat (Zuchi et al. 2012). Antagonism is also reported for Zn and Fe when Zn is applied in high concentration and vice versa by Alam et al. (2001). Similar findings are also recorded by Li et al. (2016) in their study that foliar application of Zn and Fe are responsible to improve the concentration of Zn and Fe in wheat flour.

Conclusion

Zinc and iron combination via foliar spray (0.5% ZnSO4 and 1% FeSO4) increased the yield traits of wheat crop as well as quality parameters of grains. Foliar application method is more appropriate for availability of nutrients to plants for optimum growth as compared to soil application method. Combined application of Zn and Fe (0.5% ZnSO4 and 1% FeSO4) through foliar spray is recommended to enhance the productivity of wheat crop with good quality of grains.