Physiological Determinants of High Nitrogen-Use Efficiency in Waxy Sorghum: Integrated Analysis of Biomass Production, Nitrogen Assimilation, and Antioxidant Metabolism

Sorghum [Sorghum bicolor (L.) Moench] production faces persistent challenges from low nitrogen-use efficiency (NUE) under the intensive cultivation systems of Southwest China. To elucidate the physiological determinants of genotypic variation in NUE, a two-site field experiment was conducted in Guiyang and Anshun using a high-NUE waxy sorghum cultivar (HLF) and a low-NUE waxy sorghum cultivar (QV9) under two nitrogen levels: no nitrogen application (BSN, 0 kg N ha−1) and conventional nitrogen application (SN, 150 kg N ha−1). Results showed that grain yield (Yield) and NUtE responded significantly to genotype (G), nitrogen application rate (N), and G × N effects across both sites. For NUpE, the main effect of N was significant at Anshun but not at Guiyang, whereas genotypic differences were consistent across both sites. Under BSN treatment, HLF exhibited a modest 11.04% yield reduction relative to SN, whereas QV9 suffered a 24.31% decline. Across both nitrogen levels and sites, HLF consistently outperformed QV9 in NUpE and NUtE, with NUpE reaching 1.58–1.98 times that of QV9. Nitrogen application significantly enhanced leaf area index (LAI) and SPAD values, and HLF maintained higher canopy indices at post-anthesis compared to QV9. Total dry matter and nitrogen accumulation (TDMA and TNA), post-anthesis dry matter and nitrogen accumulation (DMAP and NAP), and proportion of post-anthesis dry matter and nitrogen accumulation (DMAPP and NAPP) of HLF were higher than QV9. Although the BSN-HLF produced higher dry matter translocation amounts than BSN-QV9, HLF exhibited lower dry matter and nitrogen translocation rates (DMTR and NTR) and contribution rates of dry matter and nitrogen to grains (DMTCR and NTCR). At the enzymatic level, HLF sustained significantly elevated activities of nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthase (GOGAT) and maintained elevated antioxidant enzyme activities (SOD and POD) at both anthesis and maturity stages under BSN treatment. Concurrently, HLF accumulated higher concentrations of osmolytes (proline and soluble sugars), while maintaining reduced malondialdehyde (MDA) content, indicative of strengthened cellular homeostasis under BSN treatment. Mantel tests further revealed that DMAPP, GS, GOGAT, POD, SSC, and MDA were commonly and significantly linked to both NUE and yield. Overall, HLF maintained higher NUE and yield stability under low nitrogen, which may be associated with its sustained nitrogen metabolic enzyme activities, optimized canopy performance, and enhanced antioxidant and osmotic adjustment capacities.