Moderate Drought Stress Enhances Grain Quality in Upland Rice by Optimizing Nitrogen Metabolism and Endosperm Structure

Water scarcity is a major constraint to upland rice production, and optimizing drought management to balance yield and quality is critical for sustainable agriculture. This study investigated the effects of three soil water potential (SWP) levels—0 kPa (control), −20 kPa (moderate drought), and −40 kPa (severe drought)—on grain quality, nitrogen metabolism, and endosperm structure in two upland rice varieties (Brazilian upland rice and Zhonghan 3). Compared with the control, moderate drought significantly improved grain quality: whole milled rice recovery increased by 5.3–7.8%, chalky grain rate decreased by 16.1–29.6%, amylose content declined by 8.65–12.19%, and glutelin content rose by 9.3–12.9%. Under moderate drought, nitrogen metabolism appeared to be upregulated, as indicated by increased activities of glutamine synthetase (GS, +18.6%) and glutamate dehydrogenase (GDH, +14.2%) and higher glutamate content (+21.4%) in Zhonghan 3, with similar but slightly attenuated responses in Brazilian upland rice. Moderate drought was associated with elevated glutelin accumulation and a more compact endosperm microstructure, suggesting a potential link between nitrogen metabolism and grain development. In contrast, severe drought impaired both grain quality and yield. Correlation analysis (n = 12) revealed that the GS/GDH ratio and glutelin content were significantly correlated with improved grain quality—positively with milled rice recovery (r = 0.58* to 0.82**, p < 0.05 or 0.01) and negatively with chalkiness, amylose content, and setback viscosity (r = −0.58* to −0.93**, p < 0.05 or 0.01). These findings indicate that maintaining SWP at −20 kPa represents a feasible strategy to enhance upland rice grain quality, offering a theoretical basis for water-saving, quality-oriented production systems.