3.1 Effects of increased temperature on rice quality and accumulation of grain storage materials
The temperature increase of 1.6 ℃ to 3.1 ℃ during grain-filling stage induced various changes in the accumulation of storage materials during grain development, and further affected the rice yield and quality indicators. Results showed that tested rice yield in warming treatment was 21% lower than that of the normal temperature treatment (data not shown). Meanwhile, another significant change caused by warming is the reduction in rice quality. Results indicated the increase in temperature resulted in the increased grain thickness and decreased grain length. There was no significant change in grain width, and aspect ratio of grain shape. Compared with natural temperature, warming treatment significantly increased rice chalky rate, chalky area and chalkiness. Overall, the increase in temperature leads to overall deterioration in the appearance quality of rice. In addition, elevated temperature has a significant effect on milled rice rate and head rice rate. Compared with the natural temperature, the milled rice rate and head rice rate were decreased significantly. Increased temperature has led to the deterioration of the appearance quality of rice. The increase in temperature also caused significant changes in the rice cooking quality indicators. Gelatinization characteristics peak viscosity (PKV), hot-paste viscosity (HPV), and gelatinization temperature (GT) were significant increased and the final viscosity was decreased (Table 1).
Starch and storage proteins are the main substances that constitute the grain storage material. Under elevated temperature conditions, grain total starch was increased significantly, of which amylopectin was significant increased when responded to high temperature, while amylose decreased significantly compared with normal temperature treatment, and that further led to changes in the proportion of amylopectin /amylose. The response of grain storage protein components to high temperature appears as a significant increase in glutelin and a significant decrease in prolamin. There results suggested high temperature during grain-filling stage has a general negative impact on rice quality parameters.