Nitrogen application rates mediate rice cooking quality by interfering with root anatomical and senescence physiological traits

Nitrogen fertiliser is a key determinant of rice yield and grain quality; however, the synergistic mechanisms through which nitrogen regulates root anatomical structure, physiological traits, and cooking quality in rice varieties with different eating properties remain unclear. In this study, a pot experiment was conducted using two moderate-eating-quality cultivars (Xudao 3 and Huageng 9) and two superior-eating-quality cultivars (Zhengdao C42 and Nangeng 9308) under four nitrogen levels (0, 0.59, 1.18, and 1.76 g/pot, designated as N0, N1, N2, and N3, respectively). Cooking quality was assessed by amylose content, gel consistency, and alkali spreading value. The results demonstrated that, with increasing nitrogen application, amylose content, alkali spreading value, malondialdehyde (MDA) content, root aerenchyma area, and aerenchyma proportion decreased initially, then increased, reaching their lowest values at the N2 level. In contrast, gel consistency, root antioxidant enzyme activities (SOD, POD, CAT), photosynthetic rate and cortical living cell proportion increased first and then decreased, peaking at N2 treatment. Compared with moderate-eating-quality varieties, superior-eating-quality varieties exhibited significantly lower amylose content, alkali spreading value, MDA content, and aerenchyma proportion, but higher gel consistency, living cell proportion, stele-to-root diameter ratio, antioxidant enzyme activities, and photosynthetic rate. Correlation analysis revealed that root antioxidant enzyme activities, stele diameter and living cell proportion were negatively correlated with amylose content, but positively correlated with gel consistency. Conversely, MDA content, aerenchyma area and aerenchyma proportion showed opposite correlation patterns. These findings indicate that an appropriate nitrogen application rate (1.18 g/pot) enhances root physiological activity, optimises root anatomical structure, and ensures sufficient source supply to the grain sink, thereby synergistically improving cooking quality - an effect particularly pronounced in high-eating-quality rice varieties.