Abstract: Nitrogen-efficient cultivars provide a way to reduce nitrogen fertilizer input while maintaining high grain yield.Much work has been done to understand the physiological and genetic basis for nitrogen use efficiency,such as isolation and functional analysis of nitrate and ammonium transporters,root growth as regulated by nitrogen availability,gene expression profile under nitrogen deficiency stresses,as well as QTL mapping for nitrogen use efficiency related traits.Most of these works are focused on vegetative organs.On the contrary,the yield formation organs,i.e.earshoot and grains,are neglected.Taking maize as an example,the effect of low-N stress on earshoot and grain development is reviewed.The physiological mechanism for the regulation of nitrogen availability on earshoot and kernel development is discussed,especially from a viewpoint of nitrogen metabolism enzymes and plant hormone cytokinins.A theoretical model is suggested to explain how nitrogen deficiency affects earshoot and grain development.