Interactive effects of nitrogen and sulphur fertilization on grain protein quality and bread-making properties of spring wheat

Introduction

Not all high-protein wheat makes good bread. Two flours at 13% protein can produce loaves of different quality because what matters is not just how much protein the grain contains but what kind [1] . Gliadins give dough extensibility while glutenins provide strength. Good bread requires both in the right proportions [2] . Nitrogen fertilization increases total protein but tends to raise gliadin more than glutenin, weakening the glutenin-to-gliadin ratio [3] . Sulphur, by contrast, is needed to form the disulphide bonds that cross-link glutenin subunits into a strong polymeric network [4] . When sulphur is limiting, these bonds form poorly, and the resulting gluten is weak regardless of how much nitrogen is applied. Several European studies have shown N×S interactions on protein quality, but data from southern African wheat environments are scarce [5] . This research tested whether sulphur co-application could overcome the protein quality limitations of high nitrogen rates in a commercial South African spring wheat variety, measuring both compositional and functional quality parameters [6,7] .

Research Area description

The field experiment was conducted at the Hatfield Experimental Farm of the University of Pretoria (25.75°S, 28.24°E, 1,327 m elevation) during the 2020-2021 and 2021-2022 growing seasons. The site is in the summer rainfall region of Gauteng province, receiving 670 mm annually, with wheat grown under supplemental irrigation during the cooler months (May-October) [8] .

Soil characteristics

Soil was a Hutton form (Rhodic Ferralsol), sandy clay loam with pH 6.2, organic carbon 0.87%, available N 28 mg kg⁻¹, Bray-1 P 23.4 mg kg⁻¹, exchangeable K 186 mg kg⁻¹, and extractable S 8.3 mg kg⁻¹. The low sulphur level (below the 10 mg kg⁻¹ deficiency threshold) made this site suitable for testing S responses [9] .

Material and Methods Material

Spring wheat cv. SST 876 (a hard red spring type widely grown in South Africa) was sown at 80 kg ha⁻¹ in May of each year. Plot size was 5 m × 2 m. Four N rates (0, 60, 120, 180 kg ha⁻¹ as urea) and three S rates (0, 20, 40 kg ha⁻¹ as gypsum) were applied in a factorial combination. N was split: 50% at sowing, 50% at tillering. S was applied at sowing. P and K were applied uniformly at 40 and 30 kg ha⁻¹. Ethics: Protocol UP-EBIT/2020-058, approved 22 April 2020.

Methods

A factorial RCBD with three replications (12 treatments × 3 reps = 36 plots) was used. Grain was harvested at maturity, cleaned, and analyzed for total protein (Kjeldahl N × 5.7), protein fractions by sequential extraction (albumin, globulin, gliadin, glutenin) following the Osborne method [2] , SDS sedimentation volume (AACC Method 56-70), test weight, and thousandkernel weight. Experimental loaves were baked using a 100 g straight-dough method [1] . Data were pooled across seasons and analyzed with two-way ANOVA and Tukey's HSD in R [10] .

Results

Comprehensive interpretation

Nitrogen increased protein from 9.1 to 12.3% at the highest rate without sulphur, but adding 40 kg S pushed it to 14.6% (Table 1, Figure 1). More importantly, the glutenin-to-gliadin ratio dropped from 1.17 to 1.04 with nitrogen alone, but recovered to 1.19 with S at 40 kg ha⁻¹. SDS sedimentation rose 28.4% and loaf volume 19.3% at N180+S40 versus N180+S0. The N×S interaction was significant for all quality traits except TKW (Table 2). Protein fractions at the optimal combination showed glutenin dominant at 41.2% (Figure 2). The synergy flows through sulphur's role in disulphide bond formation, strengthening the gluten network (Figure 3).

Discussion

The decline in glutenin-to-gliadin ratio from 1.17 to 1.04 with increasing N alone confirms the widely reported 'nitrogen dilution' of protein quality [3] . At high N rates, gliadin synthesis responds more rapidly than glutenin, producing a softer, weaker gluten. Sulphur reverses this by supplying cysteine and methionine residues that glutenin subunits require for disulphide cross-links [4] . The 28.4% SDS sedimentation increase at N180 from S0 to S40 is consistent with European data showing 20-35% improvements under similar S responses [5,11] . The practical significance is considerable. Farmers in the Free State and Gauteng apply nitrogen routinely but rarely add sulphur. At current fertilizer prices, 40 kg S ha⁻¹ as gypsum costs approximately ZAR 480 ha⁻¹, while the protein and quality premium for bread wheat can exceed ZAR 2,000 ha⁻¹ at mills a strong economic case for balanced fertilization [12,13] .

Limitations

The trial used a single variety (SST 876) at one location. Genotype×nutrition interactions may mean that other varieties respond differently to N×S combinations. Baking quality was assessed with a 100 g micro-loaf method, which may not perfectly predict commercial-scale baking performance [14] .

Conclusion

Balanced nitrogen and sulphur fertilization significantly improved both grain protein content and bread-making quality of spring wheat. Nitrogen alone raised protein but worsened the glutenin-to-gliadin ratio. Sulphur co-application at 40 kg ha⁻¹ restored the ratio, increased SDS sedimentation by 28%, and raised loaf volume by 19%. The N×S interaction was significant for all major quality parameters. South African wheat growers applying high nitrogen rates should consider routine sulphur addition to capture both yield and quality benefits. Multi-variety, multi-location trials would strengthen these recommendations for regional fertilizer guidelines.

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