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Low light stress increases the chalkiness of rice; however, this effect has not been fully characterized. In this study, we demonstrated that low light resulted in markedly decreased activity of ADP-glucose pyrophosphorylase in the grains and those of sucrose synthase and soluble starch synthase in the early period of grain filling. Furthermore, low light also resulted in decreased activities of granule-bound starch synthase and starch branching enzyme in the late period of grain filling. Therefore, the maximum and mean grain filling rates were reduced but the time to reach the maximum grain filling rates and effective grain filling period were increased by low light. Thus, it significantly decreased the grain weight at the maximum grain filling rate and grain weight and retarded the endosperm growth and development, leading to a loose arrangement of the amyloplasts and an increase in the chalkiness of the rice grains. Compared to the grains at the top panicle part, low light led to a greater decrease in the grain weight at the maximum grain filling rate and time to reach the grain weight at the maximum grain filling rate at the bottom panicle part, which contributed to an increase in chalkiness by increasing the rates of different chalky types at the bottom panicle part. In conclusion, low light disturbed starch synthesis in grains, thereby impeding the grain filling progress and increasing chalkiness, particularly for grains at the bottom panicle part. [ABSTRACT FROM AUTHOR]

Ratoon rice (RR) is characterized by higher production combined with enhanced rice quality. However, limited information is available regarding the mechanism of quality enhancement of RR. In this study, three varieties were used to investigate differences in the components, the fine structural and physicochemical properties of starch, and the textural characteristics of cooked rice between the main crop and RR. Compared to the main crop, RR significantly increased the starch and amylose content, amylose to amylopectin ratio, weight average molecular weight of the starch granule, and fa (DP 6–12) and fb1 (DP 13–24) chain proportions of amylopectin. However, it decreased the amylopectin and protein content, mean diameter and number average molecular weight of the starch granule, and fb2 (DP25-36) and fb3 (DP > 37) chain proportions of amylopectin. This contributed to an increase in branching degree, polydispersity, ΔHgel, Tc-R, ΔHret, and retrogradation percentage, as well as a decrease in the average chain length and polymerization degree of amylopectin and crystallinity degree, and water solubility of RR. These differences might contribute to the increase in hardness and decrease in stickiness of cooked RR. Image 1 • Ratoon rice (RR) increased hardness but decreased stickiness of cooked rice. • RR has high starch and amylose contents, Mw, and amylopectin A and B1. • RR has low amylopectin and protein content, Mn, amylopectin B2 and B3. • RR increased Mw/Mn, ΔH but decreased crystallinity and water solubility. [ABSTRACT FROM AUTHOR]

• Chalkiness and the structural and thermal properties of rice starch were evaluated • Chalkiness is closely related to amylose content and crystallinity degree • Shading stress contributed to the increased starch molecular weight polydispersity • Chalky grain rate and chalkiness degree significantly increased under shade stress Chalkiness is a major concern in rice production and its acceptance and is increased by shade stress. However, the relationship between rice chalkiness and the structural and thermal properties of starch is unclear. Here, we investigated the effect of shade stress on rice starch properties. The chalky grain rate and chalkiness degree significantly decreased with the amylose content, Mn, and ΔH and increased with surface area- and volume-weighted mean diameters, branching degree, ratio of 1022/995 cm−1, and molecular weight polydispersity. Shade stress significantly increased the volume- and surface area-weighted mean diameters and Mw and decreased the amylose content, A chain proportion of amylopectin, Mn, and regularity of starch. These effects led to an increase in the molecular weight polydispersity and branching degree and a decrease in the crystallinity degree and 1045/1022 cm−1 ratio, thereby reducing starch ΔH and uniformity. These factors contributed to increased chalkiness of rice under shade stress. [ABSTRACT FROM AUTHOR]