## 4 Discussion  
Drought-Induced Improved Baking Quality Is Related to Changes in Protein Composition Rather Than Total Protein Content  
This study was conducted based on the hypothesis that severe drought during the grain filling phase of wheat increases grain protein content, alters the protein composition and thus affects the bread baking quality. Classification of wheat into quality classes is largely based on the total grain protein content. Similar to other studies [18; 26], no significant effect of drought stress on total grain proteins was detected. Nevertheless, a slight increase in total grain protein content was observed under severe stress in five out of six genotypes, which might be related to the production of fewer, but larger grains under stress, in line with the observed slight increase in thousand grain weight under severe stress, as well as with other studies [15; 18]. Larger grains may represent a benefit for successful germination of the next generation [27]. In addition, it was also suggested that a drought-induced enhanced ABA production was negatively correlated with grain starch concentration, and may favour the accumulation of grain storage proteins [28; 29]. Despite the lack of a significant effect on total grain protein under severe drought, strong changes in the protein composition and an improved baking quality were observed in all six wheat genotypes. Specifically, increases in the sub-fractions \(\omega\)-gliadins as well as decreases of \(\alpha/\beta\)-gliadins and LMW-C glutenins were observed across all genotypes, while HMW glutenins increased only in the German genotypes. It is already known that baking quality is not only determined by the total amount of wheat proteins, but also by the protein composition of the various protein fractions [3; 8], and it was proposed that \(\omega\)-gliadins and HMW-glutenins in particular have a strong influence [21; 30]. Both \(\omega\)-gliadins and HMW glutenins belong to the protein sub-fractions with low sulfur content [21] and therefore have only few disulfide bonds. Their stress-induced increase may result in a reduced degree of polymerization of the gluten protein network. In addition, \(\omega\)-gliadins also have a high glutamine content (45% to 53%), which is involved in the formation of glue proteins by the formation of temporary hydrogen bonds and hydrophobic interactions [31; 32; 33]. In the present study, \(\omega\)-gliadins were strongly positively correlated with the specific bread volume (Figure 6), and they were the only protein sub-fraction consistently and significantly increased in all six genotypes, while HMW glutenins increased only in the four German genotypes. In addition, the specific bread volume was significantly higher in the two Iranian genotypes compared to the German genotypes Rumor and Discus. This indicates that the \(\omega\)-gliadin sub-fraction may have been the most relevant protein fraction with respect to baking quality in the present study. This is somewhat surprising since usually glutenins (especially HMW and LMW-C) are considered very important. However, neither of these two glutenin sub-fractions was affected by drought in the Iranian genotypes. On the other hand, total glutenins were less abundant in the Iranian compared to the German genotypes, possibly related to the fact that Iranian genotypes are selected for the production of \"flat bread\" rather than \"loafy bread\", and that a high glutenin content is less important for this feature. Previous studies suggest that an increase in the HMW/LMW glutenin ratio is correlated with a higher baking volume [8]. This effect was confirmed in the present study, but only for the German wheat genotypes under severe drought stress, and the concomitant significant reduction of LMW-C glutenins in the German varieties under drought may have additionally contributed to the shift of this ratio. However, this effect cannot explain the improved specific bread volume observed in the Iranian genotypes.