Physiological and Molecular Mechanisms of Nitrogen Regulation on Grain Quality in Cereal Crops at Later Stages

Enhancing cereal grain quality while maintaining yield stability represents a pressing global challenge for sustainable agricultural development. Optimizing grain quality in cereal crops, which account for more than 60% of global dietary energy, relies heavily on managing nitrogen dynamics during the heading and grain-filling stages. Late-stage nitrogen application (from heading to early grain-filling stages) optimizes the temporal dynamics of nitrogen supply and exhibits substantial regulatory potential in mediating the yield-quality trade-off. Nitrogen availability can profoundly influence source-sink dynamics, carbon-nitrogen metabolic coordination, and the biosynthesis of storage reserves. This systematic review consolidates current understanding of the molecular and physiological mechanisms by which late-stage nitrogen application affects grain development and final quality in cereals, with a particular focus on major cereal crops including wheat, rice, and malting barley, which represent contrasting quality objectives and nitrogen management requirements. At the physiological level, late-stage nitrogen application delays functional leaf senescence, sustains photosynthetic carbon assimilation capacity, facilitates assimilate transport and partition to developing grains, and optimizes the accumulation dynamics and compositional profiles of starch and protein. At the molecular level, this review elucidates the sequential regulatory cascades governing nitrogen signal perception and transduction, the coordinated transcriptional networks underlying carbon-nitrogen metabolic crosstalk, and the expression dynamics of genes encoding starch biosynthetic enzymes and storage proteins. Integrating those recent research advances, this review also highlights several critical challenges currently facing the field. To address these challenges, we delineate promising avenues for future research including constructing time-series multi-omics frameworks, employing genome-editing technologies to functionally validate key regulatory genes and integrating artificial intelligence and big data analytics. The goal of this review is to establish a theoretical basis for precision nitrogen management strategies designed to optimize cereal crop production, targeting high yield, superior quality, and improved nitrogen use efficiency concurrently.