Genetic Variation for Nitrogen Utilization in Historical and Improved Maize Germplasm.

Increased nitrogen use efficiency (NUE) is an important target for future maize improvement.  Essential to the design of an effective breeding program to select maize hybrids with enhanced NUE is an understanding of past progress, variation among maize germplasm for NUE and its component traits, and identification of phenotyping approaches to optimize genetic gain.  We documented genetic variation for NUE and its component agronomic traits among a diverse collection of historical and recent elite maize inbreds and hybrids grown in field trials with different levels of soil N supply.  Many of the genotypes evaluated also represent important resources for maize functional genomics.  The results confirm previously reported trends for modern elite compared to historical hybrids, where grain yields have increased as a result of superior tolerance to higher plant densities, greater harvest index, and reductions in grain protein concentration.  In addition, we demonstrate that past breeding has likely optimized N uptake for high grain yields, but that significant opportunities exist to further improve how maize plants utilize acquired N.  We developed a phenotyping approach that estimates N utilization as the ratio of total biomass relative to total plant N, which effectively controls for the significant impacts of N-level, relative maturity, and heterosis on this trait.  Using this measure of total N utilization, we identify genotypes with promise as sources of enhanced N utilization with lower N inputs.