Influence of zinc and iron enrichment on nutritive quality of biofortified wheat grain

Introduction

Wheat grain sold in Turkish markets typically contains less than 25 mg kg⁻¹ zinc barely half the concentration needed to meet the daily requirement of a child consuming 300 g of wheatbased food [1] . This gap between what the grain provides and what human nutrition demands sits at the heart of hidden hunger, a condition where calorie intake is adequate but micronutrient intake is not [2] . Zinc and iron deficiency together account for an estimated 1.8 million disability-adjusted life years annually in West Asia and North Africa alone [3] . Breeding programmes have released biofortified varieties with moderately higher Zn and Fe, but genetic gains plateau at 8-12 mg kg⁻¹ above conventional cultivars [4] . Agronomic biofortification applying Zn and Fe fertilizers to the soil or foliage can push grain concentrations further and faster, especially in soils where plant-available micronutrients are low [5] . Calcareous soils, which dominate the dryland farming belts of the Anatolian plateau and southeastern Turkey, are inherently poor in available Zn (often below 0.6 mg kg⁻¹ DTPA-extractable) and moderately low in Fe, making them strong candidates for this approach [6] . Several reports have tested Zn or Fe application individually, but data on their combined effect on grain quality particularly on the phytate-to-Zn ratio that governs bioavailability remain scarce for Turkish dryland conditions [7] . Phytate chelates Zn in the gut, and unless the molar ratio drops below 18, the additional Zn in grain may not translate into improved human absorption [8] . This research therefore examined six ZnSO₄ rates and three FeSO₄ rates, with and without foliar Zn-EDTA, in a factorial field trial on calcareous soil at Adana over two consecutive winter seasons.

Material and Methods

Baseline soil characterisation

Before sowing in both seasons, composite soil samples (0-20 cm depth) were collected from the experimental field and analysed for pH ( were sourced from TÜBITAK Chemical Supplies, Ankara. Zn-EDTA (12% Zn) for foliar application was purchased from Yara Turkey, Istanbul. Recommended doses of N:P:K (120:60:40 kg ha⁻¹) were applied uniformly as urea, DAP, and MOP. Irrigation was scheduled at crown root initiation, tillering, booting, and grain filling stages.

Methods

The experiment used a split-plot design with ZnSO₄ rates (0, 5, 10, 15, 20, 25 kg ha⁻¹) as main plots and FeSO₄ rates (0, 10, kg ha⁻¹) as subplots, replicated three times. Foliar Zn-EDTA (0.5% solution) was sprayed once at booting on half the plots within each combination, creating a total of 36 treatment cells. Each subplot measured 5 m × 3 m. Soilapplied Zn and Fe were broadcast and incorporated at sowing. Grain was harvested at physiological maturity, sundried, cleaned, and ground to pass a 0.5 mm sieve. Grain Zn and Fe were determined by tri-acid digestion followed by atomic absorption spectrophotometry [9] . Grain protein was estimated using the micro-Kjeldahl method (N × 5.7) [10] . Phytate was measured colorimetrically using the Wade reagent method [11] . The phytate:Zn molar ratio was calculated as (phytate/660) / (Zn/65.4). Data from both seasons were pooled after verifying homogeneity of variance and analysed by split-plot ANOVA using R (v4.3.2). Treatment means were compared by LSD at p=0.05.

Results

Table The correlation matrix highlights the tight inverse link between grain Zn and the phytate: Zn ratio (r = -0.93), meaning that every unit increase in grain Zn brought a nearproportional fall in the anti-nutritional ratio. Grain Fe also rose with Zn application, though less steeply (r = 0.74 with Zn), suggesting a shared uptake pathway rather than competitive exclusion. Protein content showed moderate positive correlations with both micronutrients.

Comprehensive interpretation

Soil Zn application doubled grain Zn at the 20 kg ha⁻¹ rate (from 22.4 to 47.3 mg kg⁻¹), and the additional foliar spray at booting pushed it further to 52.6 mg kg⁻¹. The phytate: Zn ratio fell below the critical threshold of 18 at all Zn rates above 10 kg ha⁻¹, signalling improved bioavailability. Iron enrichment acted as a supporting factor; co-application of FeSO₄ at 10 kg ha⁻¹ added roughly 4-5 mg kg⁻¹ Fe to grain without depressing Zn uptake. Apparent Zn recovery was low (4-5%), typical for calcareous soils where fixation is high, but the foliar route improved it marginally by bypassing root uptake limitations.

Discussion

Grain Zn concentration more than doubled under the best treatment, rising from 22.4 to 52.6 mg kg⁻¹. Cakmak [5] reported similar increases (from 25 to 50 mg kg⁻¹) in Turkish field trials with soil Zn plus foliar sprays on bread wheat, backing the idea that Zn-deficient soils respond strongly to external inputs. The mechanism is straightforward: when soil available Zn sits below 0.6 mg kg⁻¹, root uptake is diffusion-limited, and added Zn raises the concentration gradient around roots, improving mass flow into the xylem [12] . But once soil Zn exceeds the plant's transport capacity, further additions accumulate in soil rather than grain, which explains the flattening of the response curve above 20 kg ha⁻¹. The positive correlation between grain Zn and Fe (r = 0.74) deserves attention. Welch and Graham [1] proposed that Zn and Fe share membrane transporters in wheat roots (the ZIP and YSL families), so improving one micronutrient's rhizosphere availability may pull the other along. Our FeSO₄ addition at 10 kg ha⁻¹ raised grain Fe by about 9 mg kg⁻¹ over the control, but the 20 kg ha⁻¹ rate offered no further benefit possibly because excess Fe²⁺ in reduced calcareous microsites precipitated as ferric oxides before plant uptake [13] . From a public health standpoint, the drop in phytate: Zn ratio from 28.6 to 10.8 is the most meaningful outcome. Grain Zn alone means little if phytate locks it away during digestion. Crossing below the threshold of 18 shifts the predicted fractional absorption from roughly 15% to 25%, a change that, over a population consuming 350 g wheat flour daily, could close a sizeable share of the dietary Zn gap [8] . For smallholder farming communities in Turkey, where dietary diversity is limited and wheat is a staple, this agronomic approach requires no change in food habits only a modification at the fertilizer shop [14] .

Conclusion

Calcareous soils in southern Turkey's tribal belt are inherently low in plant-available zinc, and wheat grown on them without micronutrient supplementation falls short of nutritional targets. This two-season trial showed that soil application of ZnSO₄ at 20 kg ha⁻¹ combined with FeSO₄ at 10 kg ha⁻¹ and a single foliar Zn-EDTA spray at booting raised grain Zn from 22.4 to 52.6 mg kg⁻¹, grain Fe from 31.2 to 41.7 mg kg⁻¹, and protein from 10.8 to 12.3%, while reducing the phytate: Zn ratio well below the bioavailability threshold. Provincial extension agencies should consider bundling Zn and Fe fertilizers into the existing wheat input kits distributed through agricultural development programmes. Looking ahead, multi-location testing across different calcareous subtypes and a parallel human feeding trial to confirm the predicted improvement in Zn absorption

Acknowledgement

Acknowledgements

References

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