Variations in Dough Properties Resulting from High Temperature
The rheological properties of dough and bread loaf volume measured on the 240 samples revealed that SumT-JuJy had a strong impact on these important quality parameters. Indeed, the increase of the molecular mass of glutenin polymers in response to temperature was correlated with an increase in dough tenacity (Fig. 28.1b) and a decrease in both dough extensibility (Fig. 28.1c) and loaf volume (Fig. 28.1d). The standardized PLS coefficients attributed to SumT-JuJy were among the highest in explaining phenotypic variations in dough tenacity, dough swelling, and bread loaf volume. Dough tenacity was, as expected, positively influenced by grain hardness, Glu-B1 7-8, Glu-D1 5-10 and Glu-B3c, and negatively influenced by GluB1 6-8, Glu-B1 6.1-22, Glu-D1 2-12, Glu-B3b'(bp in Fig. 28.1) but surprisingly, also negatively influenced by Glu-A1 2* and Glu-B1 17-18. Dough extensibility increased with grain protein content (GPC) as is usually the case, but also increased with some glutenin alleles like Glu-A1 2*, Glu-B1 17-18. The glutenin subunits Glu-D1 5-10 were negatively correlated with dough extensibility and Glu-D1 2-12
Fig. 28.1 Partial least square regression performed to explain the molecular mass of gluten in polymers, Mw2 (a), dough tenacity (b) and extensibility (c), and bread loaf volume (d): the negative (left horizontal bar) and positive (right horizontal bar) standardized coefficients were computed for grain protein concentration (GPC), grain hardness (Hardness), HMW-GS and LMW-GS alleles, and the sum of mean daily temperature during the grain filling period (SumTJuJy: June and July)
was positively correlated with dough extensibility. In addition to the well-known positive effect of GPC and grain hardness on bread loaf volume, the standardized coefficients attributed to glutenin alleles clearly evidenced that the molecular mass of glutenin polymers had a negative impact on loaf volume. This result was supported by the positive correlation between loaf volume and glutenin alleles known to have a very negative effect on dough rheological properties (Glu-B1 6-8, GluA3ef, and Glu-B3j).
Some Proposals to Cope with Wheat Quality in a Changing Environment
High temperatures during grain filling have a more pronounced effect on the amount of starch than on the amount of protein accumulated per grain. All genetic factors associated with higher starch accumulation including light saturated photosynthesis, awned ears, and prolonged photosynthesis in grain peripheral layers, must be regarded as potentially favorable for maintaining higher amounts of starch per grain during HT events (reviewed by Cossani and Reynolds 2012). The influence of HT on the genetic regulation of major starch enzymes and particularly AGPase and β-amylase should now be investigated in wheat to identify potential markers for breeders. Whatever the potential benefit of using those markers, determination of the test weight, negatively impacted by HT during grain filling, is the easiest way to phenotype wheat progenies capable of completing grain filling even in a HT environment.
Only few individual SPs are influenced by HT or heat shock during the linear phase of grain filling. The ratio of gliadin to glutenin was not significantly affected by the HT treatments used here. However, higher temperatures (daily maximum >35 °C) have been reported to increase the gliadin to glutenin ratio (Blumenthal et al. 1995). Although individual SP may not be affected, we showed that daily maximum temperature lower than 35 °C can strongly increase the molecular mass of the glutenin polymers. The resulting effect is to increase dough tenacity and decrease extensibility. This may explain why, in the last decade, the ratio of tenacity to extensibility has often been reported to have increased in many countries where the alveograph test is used. To tackle such quality consequences, it is advisable not to discard alleles previously associated with dough extensibility like Glu-A1 2*, Glu-B1 17-18 and Glu-D1 2-12. The influence of SP diversity on the molecular mass of glutenin polymers and wheat properties also needs to be reassessed in relation with puroindoline alleles, as judiciously proposed by Ikeda et al. (2013). The molecular regulation of redox enzymes such as dehydroascorbate reductase, glutathione reductase, and glutathione transferase also need to be investigated to improve the stability of the molecular mass of glutenin polymers in HT environments.
Acknowledgments François Xavier Oury (INRA Clermont Ferrand) and breeders from the wheat breeding companies: Caussade Semences, Florimond Desprez, Lemaire Deffontaines, Limagrain, Momont et Fils, R2N, Saaten-Union Recherche, Secobra Recherches, Serasem, Syngenta and Unisigma, are gratefully acknowledged for their active participation in the multilocation field trials and in studies on tenacity/extensibility and bread loaf volume.