NITROGEN USE EFFICIENCY IN VEGETABLE CROPS: FROM NITROGEN ACQUISITION AND METABOLISM TO MOLECULAR REGULATION AND BREEDING

Abstract

Nitrogen (N) use efficiency (NUE) is a major determinant of sustainable vegetable production because high N demand, intensive fertilization and low recovery of applied N can increase production costs and environmental losses. In vegetable crops, NUE is a complex trait governed by coordinated processes operating from the soil–root interface to whole-plant N allocation and utilization. This review integrates current physiological, biochemical and molecular understanding of NUE in vegetable crops, with particular emphasis on the mechanisms that connect N acquisition with productive N utilization. The review first examines the conceptual components of NUE, including N uptake efficiency and N utilization efficiency and then synthesizes the roles of root architecture, nitrate and ammonium transporters, nitrate reduction, the GS/GOGAT cycle, amino-acid metabolism and carbon–nitrogen coordination. Particular attention is given to long-distance N transport, storage and remobilization, followed by emerging insights into nitrate sensing, NRT1.1-mediated signalling and NLP dependent transcriptional regulation. Evidence from tomato, eggplant, cucumber, spinach, potato and other vegetable systems demonstrates that genotypic differences in NUE arise from coordinated variation in root traits, N transport, assimilation, photosynthetic capacity, metabolic plasticity and regulatory networks rather than from individual genes or enzymes. Transcriptomic, metabolomic and co-expression approaches are increasingly revealing candidate genes and regulatory modules associated with NUE, providing opportunities for molecular marker development, marker-assisted selection and genome editing. However, translation of molecular discoveries into field-level improvement remains constrained by the complexity of NUE, genotype × environment interactions and the need for robust phenotyping under contrasting N regimes. Future progress will depend on integrating physiological phenotyping, multi-omics, molecular breeding and precision N management to develop vegetable cultivars that maintain yield and quality with reduced N inputs.