The chalkiness, starch fine structure, and physiochemical properties of rice starch were analyzed and their correlations were investigated under different nighttime temperatures during the early grain-filling stage. Compared to MT, medium temperature (MT) and low (LNT) and high (HNT) nighttime temperatures resulted in an increased chalky grain rate (CGR) and chalkiness degree (CD). LNT mainly affected the chalkiness by increasing peak1 (short branch chains of amylopectin), the branching degree, and the proportion of small starch granules but decreasing peak2 (long branch chains of amylopectin) and peak3 (amylose branches). This altered the pasting properties, such as by increasing the peak viscosity and final viscosity. However, HNT mainly affected the chalkiness by increasing peak2 and the crystallinity degree but decreasing peak1 and peak3. Regarding the thermal properties, HNT also elevated peak and conclusion temperatures. The CGR and CD were significantly and positively correlated with the proportions of small and medium starch granules, peak1, branching degree, gelatinization enthalpy, setback viscosity, and pasting time but markedly and negatively correlated with the proportion of large starch granules, amylose content, peak3, peak viscosity, and breakdown viscosity. These findings suggest that LNT and HNT disrupted the starch structure, resulting in increased chalkiness. However, their mechanisms of action differ.

Biochar exhibits a good adsorption ability for heavy metals in soil and has been widely used as a remediation material in Cd-contaminated soil. However, the status of Cd uptake by rice driven by soil physicochemical properties and rhizosphere microbial communities after years of biochar application is not well understood. In this study, the relationship between the rhizosphere microbial community and soil physicochemical properties and rice Cd accumulation were investigated during the main rice growth stages. The results showed that in comparison to the non-biochar treatment (control), a noticeable reduction in Cd content in rice stem sheaths, leaves, rice husks and milled rice with different growth stages were observed in the biochar treatment after four years, which decreased by 38.76–66.18%, 40.93–70.27%, 43.64–47.92% and 31.91–34.38%, respectively. Compared to non-biochar treatment (control), the properties of the soil in different growth stages by biochar treatment of the soil pH, soil organic matter (SOM), total nitrogen (TN) and available phosphorus (AP) were significantly increased, which increased by 10.5–16.13%, 8–25%, 75–130.13% and 132.95–191.43%, respectively. The content of available Cd (ACd) concentration in different stages by biochar treatment was significantly decreased, which decreased by 26.57–44.24%. Biochar application after four years changed the rhizosphere bacterial community structure composition (phyla level) in all stages. The relative abundance of Proteobacteria, Bacteroidetes and Nitrospirae was increased, while the relative abundance of Chloroflexi, Acidobacteria and Actinobacteria was decreased. Meanwhile, the biochar application enriched Rhodocyclaceae, Burkholderiaceae, Nitrosomonadaceae, Anaerolineaceae, Ignavibacteriales and Bacteroidales, which may contribute to the reduction of Cd uptake and accumulation in rice. These results suggest that biochar treatment after four years changed the rhizosphere microbial community structure and soil physicochemical properties and promoted the colonization of specific microbial populations in the rice rhizosphere to form a special protective system in the rice rhizosphere, which reduced Cd uptake by rice.

Water management during grain filling plays a critical role in determining rice quality, but its coordinated regulation of eating and aroma quality in aromatic rice remains unclear. In this study, two indica cultivars, Nongxiang 32 and Yuzhenxiang, were subjected to three irrigation regimes: continuous flooding (CF), alternate wetting and drying (AWD), and saturated soil irrigation (SSI). Our results showed that 2-acetyl-1-pyrroline (2AP) content exhibited the highest level under AWD and the lowest under SSI. Starch pasting properties were improved under AWD and SSI, as reflected by higher peak viscosity and breakdown and lower setback values. Further analysis of Nongxiang 32 showed that AWD enhanced amylopectin and total starch contents, elevated proline and putrescine levels, and differentially regulated enzyme activities, maintaining higher proline dehydrogenase activity at early filling stage and favorable Δ¹-pyrroline-5-carboxylate synthetase, ornithine δ-aminotransferase, and diamine oxidase activities at late filling. Meanwhile, AWD was associated with reduced γ-aminobutyric acid accumulation, consistent with greater precursor availability for 2AP biosynthesis. Antioxidant enzyme activities followed the order AWD > SSI > CF, whereas malondialdehyde and free fatty acids showed SSI > AWD > CF. Hormonal responses differed: gibberellic acid (SSI > AWD > CF), abscisic acid (AWD > SSI > CF), and indole-3-acetic acid (AWD > CF > SSI). Collectively, in Nongxiang 32, AWD optimally coordinates precursor metabolism, enzyme activities, antioxidant defense, and hormone regulation, thus improving both the aroma and starch quality of aromatic rice. This work provides a mechanistic understanding of water-driven quality formation in aromatic rice and offers a feasible irrigation strategy for high-quality aromatic rice cultivation.