Evaluation of Nitrogen Utilisation for Improved Nitrogen Use Efficiencies in Maize

Abstract

Ammonium (NH₄⁺) and nitrate (NO₃⁻) are the primary nitrogen (N) sources sustaining plant growth and metabolism. While both are efficiently absorbed, their integrated effects on maize carbon–nitrogen (C–N) interactions under deficiency, nitrogen form substitution (NFS; NO₃⁻→NH₄⁺/NH₄⁺→NO₃⁻), and low ammonium (LA) supply remain less understood. This study examined how these N forms influence growth, assimilate partitioning, and metabolic plasticity in maize inbred line TX‑40J. Under low nitrate (LN), maize exhibited stress‑adaptive responses, including enhanced root biomass and root‑to‑shoot allocation, but reduced shoot growth and protein synthesis. LN upregulated glutamine synthetase (GS) and glutamate synthase (GOGAT), while suppressing nitrate reductase (NR) and nitrite reductase (NiR), favouring NH₄⁺ assimilation. LN also induced accumulation of soluble sugars and starch, with transcriptional activation of carbohydrate metabolism genes. Carbon was concentrated in leaf tips, sheaths, brace roots, and lateral roots, reflecting inefficient sink utilization rather than impaired assimilation. NFS improved biomass, photosynthesis, and N assimilation efficiency. Coordinated NR, GS, and GOGAT activities supported dual assimilation of NO₃⁻ and NH₄⁺, while reduced sucrolytic activity enhanced sucrose utilization, optimized root‑to‑shoot allocation, and minimized leaf C accumulation, highlighting improved assimilate distribution, metabolic coordination, and resource efficiency.LA supply further stimulated photosynthesis, cob development, and growth via elevated GS‑GOGAT activity and carbohydrate metabolism. Collectively, maize demonstrates remarkable metabolic flexibility to diverse N forms, offering mechanistic insights into improving nitrogen use efficiency (NUE), crop resilience, adaptive capacity, and sustainable productivity under variable nutrient environments, while underscoring the importance of dynamic N–C partitioning for long‑term agricultural sustainability.