Season-long Maize Morpho-physiological Responses to Nitrogen Availability.

Due to genetic improvements for stress tolerance and input responsiveness, modern maize (Zea mays L.) hybrids are highly productive when grown at high plant densities with adequate nitrogen (N) availability.  However, when N is severely limiting at these population levels, intense intra-specific competition for soil N results and leads to increased barrenness and lower grain yield.  If further genetic improvements in both low N tolerance and high N responsiveness are to be made in the future, an in-depth, season-long understanding of maize morpho-physiological responses to limited and adequate N availability is required.  The objective of this 3-yr (2005-2007) study was to examine both per-plant and canopy-level morpho-physiological responses to N application (0, 165, and 330 kg N ha^-1) at multiple plant densities (54,000, 79,000, and 104,000 plants ha^-1) for two modern hybrids.  Morpho-physiological parameters examined at various stages of vegetative and reproductive development in 2005, 2006, and/or 2007 included leaf greenness (SPAD), leaf area index (LAI), stalk diameter, anthesis-silking interval (ASI), vegetative and total biomass, harvest index (HI), grain yield, total kernel number, vegetative and grain N content, and grain carbohydrate and protein content.  In general, a higher N rate and a reduced plant density facilitated (among other things) greater stalk diameter values during vegetative and reproductive development, reduced floral asynchrony, delayed leaf senescence and photo-assimilate translocation, and improved biomass partitioning to the ear.  Improving resource availability through either N application or lower plant populations also enhanced kernel number, grain N content, and total grain weight.  In this presentation, we will highlight additional key aspects of this season-long, morpho-physiological analysis.