Agronomic Traits and Maize Modifications: Nitrogen Use Efficiency

Abstract

Developing nitrogen (N) efficient cultivars is a major challenge of modern plant breeding. Globally, soil-N deficiency belongs to the most serious constraints of maize production. Improvement of N-use efficiency (NUE) would therefore significantly contribute to securing world food and feed production and raise agricultural incomes. In this chapter I will use the term NUE synonymous for ‘adaptedness to low N supply’. NUE is a highly complex, polygenically inherited trait. Significant genetic variance for NUE and NUE-related traits has been found in various genetic materials from temperate as well as tropical growing regions. The genetic correlation between grain yields under low and high N supply (LN and HN, respectively) decreases from strong via moderate to weak as the stress caused by N deficiency increases. Mapping studies in different populations revealed consistent QTL for NUE on most chromosomes. The most important QTL regions seem to be located on chr. 2, 4, 5, and 8. Experimental evidence about putative candidate genes for NUE is still very meager. The only consistent results were obtained for gene gln4 on chr. 5 encoding a cytosolic glutamine syn-thetase (GS). Divergent selection for grain yield under LN and HN leads to germ-plasm being specifically adapted to one or the other of these two growing conditions. However, in large-scale selection programs, breeders have also found genotypes being well adapted to both N-supply levels. The relative importance of N-uptake and N-utilization efficiency, the two constitutive components of NUE, largely depends on the genetic material. Genotypes with low grain protein concentration were found to excel in N-utilization and those with high concentration were superior in N-uptake efficiency. Several stress-indicator traits such as anthesis-silking interval, leaf chlorophyll concentration, ears per plant, and kernels per ear may be strongly correlated with NUE. Selection based on an index composed of grain yield and these indicator traits was shown to improve breeding progress considerably.