Divergent selection for grain protein affects nitrogen use in maize hybrids

The Illinois high (IHP), low (ILP), and corresponding reverse (IRHP, and IRLP) protein-strains of maize represent genetic extremes for differences in grain protein concentration. The objective of this study was to determine how divergent selection for grain protein affects N use in hybrid plants. Inbreds derived from the protein-strains were crossed as males to a common tester and the resultant hybrids evaluated at eight N rates in the field over 3 years. A more than two-fold difference in grain protein concentration was observed among the strain-hybrids with ILP averaging 65 g kg^-1, IRHP 89 g kg^-1, IRLP 111 g kg^-1, and IHP 148 g kg^-1 of grain protein. Except for IHP at the lowest N levels, the strain-hybrids were similar in their whole shoot biomass production both pre- and post-flowering. Conversely, the strain-hybrids differed markedly in their uptake and accumulation of plant N, and these differences were already evident at flowering before a grain sink was present. Although all hybrids had the same overall N use efficiency at maturity (approximately 24 kg kg^-1 N), they differed in their N use components with IHP and IRLP exhibiting a higher uptake efficiency, and ILP and IRHP exhibiting high utilization efficiency. The remobilization of leaf N was also more extensive for IHP and IRLP. Changes in grain protein concentration from divergent selection were directly related to changes in uptake and use of N by the plant.