Responses of Japonica Rice Quality Indicators and Starch Properties to Low Temperature at Different Periods of the Grain-Filling Stage in Cold Regions

Low temperature during grain filling is a major constraint affecting rice quality in cold regions. This study investigated how low temperature influences rice quality and starch characteristics at different periods of the grain-filling stage using two Japonica rice cultivars, Kenjing 7 (KJ7, moderate stress tolerance) and Kenjing 8 (KJ8, strong stress tolerance). Low-temperature treatments (17/13 °C, day/night) were applied during the early (5–11 days after anthesis), middle (12–18 days), and late (19–25 days) grain-filling stages and milling, appearance, nutritional, eating and cooking qualities as well as starch physicochemical properties were evaluated. Responses differed markedly between cultivars and treatment periods. Under low-temperature conditions, brown rice and milled rice rates of KJ8 increased during the early and middle grain-filling stages, whereas those of KJ7 declined during the late stage. Low-temperature stress increased protein, total starch, and amylose contents, while reducing gel consistency and the taste value of KJ7. Grain chalkiness increased significantly during the late stage, whereas during the early and middle stages, grain chalkiness, peak viscosity, and breakdown decreased and setback increased. Low temperature increased starch granule size and the proportions of short and intermediate chains of amylopectin, reduced medium-long and long chain and relative crystallinity, without altering starch crystalline type, and produced uneven starch particle surfaces with small pores. These effects were most pronounced during the late grain-filling stage. Overall, low temperature altered starch content and structure, thereby modifying pasting properties and ultimately leading to differences in rice quality.