By analyzing genetic variation for post-silking N-uptake in a set of maize recombinant inbred lines, it has been found that allele of three clusters of QTLs correspond to a deep and thin root system, which can be efficient for N uptake in deep soil layers. Other three clusters of QTLs correspond to a positive association between root diameter and superficial root development, which can be efficient for N uptake before silking between the thick ones (diameter >0.5 mm) and N uptake rate (Sullivan et al., 2000). In a field experiment with 10 maize cultivars, nitrate depletion and root length densities in the subsoil layer are closely correlated