Introduction: Among environmental stresses, drought stress is the most critical ecological constraint that has could affect yield and product quality. It is important better to understand the effects of high temperatures and drought stress because, according to the forecasts made and seasonal temperature changes, we are facing a drought. Almost half of all human food needs, especially in Asia, are met directly from cereals. Wheat is one of the most adaptable cereals in different climatic conditions. Starch and proteins are the main components in wheat endosperm that affect the quality of the final product. Starch composition in wheat has a decisive effect on seed quality, flour processing, and yield. Physicochemical properties and final application of wheat starch are closely related to the structure of starch granules and the distribution of amylose and amylopectin. The internal structure of A and B-granules are somewhat different in composition, size, gelatinization temperature, and physicochemical and functional properties that lead to changes in swelling strength, gelatinization, and adhesion properties. Protein and starch, not only directly but also indirectly through the creation of bonds and effects on various reactions, can have a significant impact on the quality of the bakery. For these reasons, its recognition and investigation during drought stress as the most critical biological structure of wheat from the point of view of agricultural economics is of particular importance.
Materials and Methods: Two bread wheat cultivars named Navid, with medium to poor baking quality, and Pishtaz, with high baking quality, were used in this study. Two wheat cultivars were cultivated in three replications in the research farm of Shahed University of Tehran in November 2019. Irrigation once a week was used as a normal treatment and stopped irrigation at the bolting stage for drought treatment (The criterion of drought stress is 30% of the maximum field capacity). Each replication consisted of two rows of cultivation with a length of 2.5 m and distances of 25 cm. At 35 days post-anthesis, the seeds of each plant were harvested from the middle two third of the spike. Traits such as total starch, wet gluten, gluten index, grain protein percentage, Zeleny sedimentation volume, bread volume, flour moisture, hardness index, and water absorption in flour were measured. Also, total starch and granules are separated was analyzed by by FT-IR. Statistical analyses were performed with SAS software (version 9.4) and Average Comparison with t-tests at the level of 0.05. and 0.01.
Results and Discussion: In both wheat cultivars, drought stress showed a significant decrease in total starch content. Also, between the two cultivars under normal and drought stress conditions, the amount of starch in the Navid cultivar decreased significantly. Under drought stress, loss of AGPase activity leads to premature cessation of starch accumulation. In both cultivars, drought stress caused a significant increase in wet gluten compared to the normal. The results showed that the shorter the time between flowering and seed ripening, the smaller the seeds and the higher the gluten content. Drought had no effect on the Pishtaz cultivar but was influential on the Navid cultivar and caused a significant decrease in gluten index. Also, in both normal and drought stress, the gluten index was significantly higher in the Pishtaz cultivar. some researchers have shown that high-quality wheat cultivars have a higher gluten index. Drought stress in both cultivars significantly increased seed protein. It has been proven that reducing starch under stress conditions disturbs the protein ratio to starch and increases the amount of protein per unit volume. In both cultivars, we saw an increase in the percentage of Zeleny sedimentation volume. The higher the gluten content and the better the gluten quality, the slower the deposition and the increased the Zeleny sedimentation volume. in measuring the percentage of bread volume, the drought did not have a significant effect in the Pishtaz cultivar. But, in the Navid cultivar, drought stress caused a significant decrease. Drought stress in the Pishtaz cultivar had no significant effect on wheat seed moisture. However, in the Navid cultivar, drought stress with a decrease significantly reduced the grain moisture. Low or high humidity can increase crop damage during harvesting and storage, increasing the cost of post-harvest processes to some extent. The Percentage of hardness index in the Pishtaz cultivar in drought stress conditions with a significant difference increased. But in the Navid cultivar, drought stress had no significant effect on the hardness index. A very strong correlation has been observed between the hardness index and the rheological properties of the dough, and the quality of gluten. Both wheat cultivars, drought stress significantly increased the water uptake percentage of flour. Higher Glutenin to Gliadin ratio under drought stress conditions may make gluten and dough more robust, and This is accompanied by an increase in water absorption by flour. Drought stress in the Pishtaz cultivar did not cause molecular changes, and the molecular structure remained constant. In the Navid cultivar, for total starch, A-granules, and B-granules in both normal and drought stress in the positions of the peaks did not change. However, the intensity of one of the peaks in the range of O-H bond tension showed a decrease in the condition of drought stress.
Conclusion: Investigating the mechanisms that enable plants to adapt to drought stress and maintain their growth in these conditions ultimately helps select stress-resistant plants for cultivation in dry and semi-arid soil. In general, drought stress reduced the yield and baking quality of both wheat cultivars; of course, it was more in the Navid cultivar with medium to poor baking quality, which indicated that this cultivar is more vulnerable to drought stress than the Pishtaz cultivar. The results of this research can provide insight into the impact of potential mechanisms through drought on the ultrastructures and physicochemical properties of wheat starch and its relationship with baking quality.