Nitrogen (N) fertilization is critical for high yield production of maize. However, over-application of N often has negative impacts on the environment such as nitrate contamination in groundwater, acid rain and soil acidification, and air pollution.  Field and greenhouse studies were performed to explore the physiological mechanisms of maize ear and ear leaf during the silking stage under different N regimes.  Both field and greenhouse experiments revealed that 150 kg N ha^-1 at planting was insufficient to maximize grain yield.  Ear length, biomass and grain number at silking of high-N treatments (225 kg ha^-1, 300 kg ha^-1 and 350 kg ha^-1) were greater than in the 0 and 150 kg N treatments.  In the greenhouse, N applications equivalent to 300 kg N ha^-1 did not significantly increase the leaf area at each node, but it delayed leaf senescence particularly of the six uppermost leaves compared to 0 and 150 kg N treatments.  High-N treatments also increased chlorophyll content and photosynthesis rates during grain filling, and altered light response and A/Ci curves.  Nitrogen application increased the soluble protein concentration in the ear leaf, while glucose and sucrose concentrations were not influenced.  Interestingly, ear leaf starch concentration decreased with increasing N fertilizer application.  These results suggest that maize kernel yield is limited by sink size and not photosynthate availability.  A lack of yield increase in response to sucrose stem infusions in a greenhouse experiment, substantiates such a conclusion.