Japonica rice is produced mainly in northeast China, Japan, and the Korean Peninsula. Polyphenols and flavonoids are the main antioxidants in japonica rice. This study reported the polyphenol content and antioxidant activity of nine brown and white japonica rice cultivars. The total phenolic and flavonoid contents of brown rice were in the ranges of 241.98–296.76 GAE mg/100 g, and 225.30–276.80 RE mg/100 g, respectively. These values were significantly higher than that of white rice by 118.98–206.06% and 135.0–217%, respectively. The bound fraction from phenolics and flavonoids contributed 41.1–63.6% and 62.22–78.19% of the total phenolic and flavonoid content in brown rice, respectively, while these ranges were 55.5–73.5% and 46.07–66.83% in white rice, respectively. p-Hydroxybenzonic acid was the predominant phenolic acid in japonica rice. All four antioxidant capacities of brown rice (DPPH, ABTS, OH, FRAP) were higher by up to 1.68–2.85 times than those of white rice. The PZ21 (Yanfeng 47) japonica rice variety has outstanding antioxidant capacity based on the weights of each antioxidant index. According to the differences of functional substances among varieties, it can provide guidance for consumers and theoretical basis for the production of healthy food.

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Brown rice as a whole grain food is associated with various chronic diseases’ reduced risks. In this study, the effects of different doses of nitrogen fertilization (0, 160, 210, 260, 315, and 420 kg N/ 100 m2) on bioactive compounds and antioxidant activity of brown rice (yanfeng47) were investigated. At nitrogen level of 210–260 kg N/100 m2, the content of TFC (302.65 mg/100 g), β-sitosterol (1762.92 mg/100 g), stigmasterol (1358.735 mg/100 g), DPPH (74.57%), and OH free radical scavenging (74.19%) was the highest. The major phenolic acid was p-hydroxybenzoic acid. There were significant positive linear relationships between TFC (0.872, 0.843), β-sitosterol (0.896, 0.657), stigmasterol (0.543, 0.771), p-hydroxybenzoic acid (0.871, 0.875), and DPPH, OH antioxidant activity. These indicated that TFC and phytosterols were the most important components in brown rice that had strong antioxidant activity. Composite score of principal components indicated 210 Kg N/100 m2 exhibited a more ideal dose of nitrogen for nutritional composition and antioxidant activity of brown rice.

Nitrogen is a necessary element for plant growth; therefore, it is important to study the influence of N fertilisers on crop metabolites. In this study, we investigate the variability of endogenous metabolites in brown rice fertilised with different amounts of nitrogen. We identified 489 metabolites in brown rice. Compared to non-nitrogen fertilised groups, there were 59 differentially activated metabolic pathways in the nitrogen-fertilised groups. Additionally, there were significantly differential secondary metabolites, especially flavonoids, between groups treated with moderate (210 kg N/hm2) and excessive amounts of nitrogen (420 kg N/hm2). Nitrogen fertilisation upregulated linoleic acid metabolism and most steroids, steroid derivatives, and flavonoid compounds, which have antioxidant activity. The DPPH, ABTS, and hydroxyl radical scavenging rates were higher in fertilised groups than in the non-fertilised group. These findings provide a theoretical basis to enhance the health benefits of brown rice by improving fertilisation.