Protein quality vs. quantity in South African commercial bread wheat cultivars

English: In field crops the measurement of any yield or quality parameter by a single factor that is inherently sensitive to the environment, may skew the result and have no application. Two classical examples exist in the wheat industry where firstly, grain yield alone does not fully reflect and determine the on-farm profitability of a wheat variety. The parameters that are part of the grading scale (hectolitre mass, grain protein content and falling number) together with grain yield and price per ton form the components responsible for the ultimate farm gate price. Secondly, grain protein content is mostly inadequate for explaining flour quality of wheat which is critical for determining sustainability of the milling and baking industry. Protein quality, referring to the concentrations and ratios of glutenin, gliadin and albumin protein fractions are proving to be more important than protein quantity. Wheat varieties from the irrigation region in the medium and low ranking group were a clear example thereof as these genotypes obtained Grade 2 (< 12% grain protein content) and Grade 3 (< 11% grain protein content) but produced loaf volumes well within the allowed 10% variation from the loaf volume of the quality standard. In irrigated wheat and rainfed wheat of the summer rainfall region and winter rainfall region, E had the largest impact on flour protein and grain protein content. E contributed most to the variation of soluble glutenin in total protein for irrigation whereas G x E interaction resulted in the highest variation for rainfed SRR and WRR. Soluble α/β, γ gliadin in total protein were primarily affected by E for irrigation and G for both rainfed in the SRR and in the WRR. E contributed most to the variation in soluble albumin/globulin in total protein for irrigation, rainfed SRR and rainfed WRR wheat. For the insoluble fractions, E largely determined the variations of insoluble glutenin in total protein for irrigation, rainfed SRR and rainfed WRR. Insoluble α/β, γ gliadin in total protein were primarily determined by E in irrigation and rainfed SRR whereas G determined variation in the rainfed WRR. Insoluble albumin/globulin in total protein were primarily determined by the E of irrigation and rainfed WRR whereas G x E determined variation in the rainfed SRR region. Glutenin concentrations were highest in wheat from the rainfed WRR, α/β, γ gliadin in rainfed SRR wheat and albumin/globulin in irrigated wheat. No clear and repeating trends regarding the concentrations and ratios of protein fractions could be established with correlations in any of the three production regions, which underlines the enormous role of the environment on protein quality. For example, large insoluble glutenin in total protein of irrigated and rainfed SRR wheat only correlated strongly and positively with high flour protein, whereas in rainfed wheat of the WRR large insoluble glutenin in total protein correlated positively and strongly with high flour protein, high grain protein and high loaf volume. The examples become more extreme with insoluble small glutenin proteins of irrigated wheat correlating strongly and positively with low loaf volume, whereas in rainfed SRR small soluble and insoluble glutenin correlated negatively with low loaf volume. In rainfed wheat from the WRR, small insoluble glutenin in total protein correlated negatively with high grain protein, but positively with low loaf volume. The climatic environment of the three production regions differs significantly in regard to annual rainfall and seasonal temperatures which wheat producers cannot control. Management of the environment by wheat farmers should focus on the production environment through selection of adapted wheat varieties, soil moisture conservation and optimal fertilising practises. These factors determine grain yield but also determine the protein composition that eventually affects wheat flour quality.