BACKGROUND High temperature during the grain‐filling stage is an important factor that can affect grain quality by altering the composition and structure of starch in rice. Therefore, it is important to study the regulatory mechanism of high temperature on rice starch biosynthesis. RESULTS Two japonica cultivars, the waxy rice Taihunuo and non‐waxy Nangeng 5055 were used to examine the effect of high temperature on the fine structure of starch during the grain‐filling stage. Analysis of starch chain length distribution indicated that exposure to a high temperature increased the content of starch with medium‐long chains and decreased the starch with short chains in both rice varieties. The differences of amylopectin synthesis responding to high temperature between waxy and non‐waxy rice can shed light on the interactions of amylose and amylopectin synthesis under high temperature conditions. In the non‐waxy variety, the amylose biosynthesis may affect the short and medium‐long amylopectin biosynthesis under high temperature. A mathematical fitting model was used to interpret the fine structure of amylopectin and a series of parameters with enzymatical significance (β and γ) were obtained. The fitting results showed that the waxy and non‐waxy rice had similar responses to high temperature. The variations of the parameter response to high temperature was more remarkable in Taihunuo. Activity analysis of starch synthesis‐related enzymes during the grain‐filling stage demonstrated the reliability of model fitting results. CONCLUSION The influences of high temperature on the fine structure of starch are similar between waxy and non‐waxy rice. Amylose biosynthesis may affect amylopectin biosynthesis under high temperature. © 2018 Society of Chemical Industry

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With the increasing greenhouse effect, high temperature has become the most unfavorable environmental factor for the rice grain filling process, affecting rice yield and quality mainly through changing the composition and structure of starch in rice grains. Research has focused on the rational management of water and fertilizer, and spraying of exogenous chemicals, which have become important measures to alleviate high temperature stress of rice. As a multifunctional molecule, melatonin has the potential to improve plant stress resistance by enhancing the scavenging efficiency of reactive oxygen species (ROS), thus protecting plants from the adverse effects of abiotic stress. The present study used a typical japonica rice variety Nipponbare (NPB) as the experimental material, which was treated with high temperature and melatonin during grain-filling stages. The effects of exogenous melatonin on the rice growth and quality traits, as well as starch synthesis, in response to high temperature were analyzed systematically. Exogenous melatonin significantly increased the rice leaf photosynthetic and heat-resistance properties. Melatonin could alleviate the effects of high temperature on the key physicochemical properties related to rice quality. Furthermore, milled rice from NPB plants treated with melatonin had better endosperm appearance under high temperature. Further study found that exogenous melatonin could stabilize the chain length distribution of starch in NPB (especially amylopectin), which implied that melatonin could be used in rice cultivation to alleviate the effect of high temperature on quality, optimization of amylopectin synthesis can also improve rice quality. The results of the present study provide a new idea and research direction to alleviate high temperature stress of rice in the context of global warming.

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Climate warming affects rice growth at different phenological stages, thereby increasing rice chalkiness and protein content and reducing eating and cooking quality (ECQ). The structural and physicochemical properties of rice starch played important roles in determining rice quality. However, differences in their response to high temperature during the reproductive stage have been rarely studied. In the present study, they were evaluated and compared between two contrasting natural temperature field conditions, namely, high seasonal temperature (HST) and low seasonal temperature (LST), during the reproductive stage of rice in 2017 and 2018. Compared with LST, HST significantly deteriorated rice quality, including increased grain chalkiness, setback, consistence, and pasting temperature and reduced taste values. HST considerably reduced the total starch and increased the protein content. Likewise, HST significantly reduced the short amylopectin chains [degree of polymerization (DP) <12] and increased the long amylopectin chains (DP > 12) and relative crystallinity. The starch structure, total starch content, and protein content explained 91.4%, 90.4%, and 89.2% of the total variations in pasting properties, taste value, and grain chalkiness degree, respectively. In conclusion, we suggested that rice quality variations were closely associated with the changes in chemical composition content (total starch and protein content) and starch structure in response to HST. These results indicated that we should improve the resistance of rice to high temperature during the reproductive stage to improve the fine structure of rice starch in further breeding and practice.