Plant biomass and nitrogen partitioning changes between silking and maturity in newer versus older maize hybrids

Characterization of the pre- and post-silking period differences in dry matter (DM) accumulation and nitrogen (N) uptake and partitioning between older and newer maize (Zea mays L) hybrids is useful in the context of providing possible mechanisms of yield and N efficiency gains over the decades of genetic improvement. However, there is substantial uncertainty about the mechanisms by which DM and N partitioning into distinct plant organs at silking (R1) affect their respective post-silking dynamics in modern versus older maize hybrids. Clarity is also lacking about management impacts on how source (leaf and stem) strength and sink (grain) strength drive post-silking DM (PostDM) and post-silking N (PostN) dynamics in genotypes of different eras. In this two-year and two-location study, we compared two newer hybrids (commercialized in 2005) to one older hybrid (commercialized in 1975) in 2012 and to two older hybrids (the same 1975 hybrid, and one commercialized in 1967) in 2013. All hybrids were compared under two N fertilizer rates (55 kg N ha(-1), 220 kg N ha(-1)) and three densities (54,000 pl ha(-1), 79,000 pl ha(-1), 104,000 pl ha(-1)). Although both moderate and high plant densities increased leaf N contents at silking and remobilized N from leaves during grain fill, density x hybrid interactions were not significant for these or almost all parameters measured. Older hybrids consistently partitioned more of their total DM at silking to stem than leaf relative to both newer hybrids. Both newer and older hybrids increased PostDM (an average increase from 8.3 to 10.1 Mg ha(-1)) and PostN (an average increase from 36.3 to 63.6 kg N ha(-1)) in response to the higher N rate over the 2-year period. Newer hybrids accumulated 2.1-2.3 Mg ha more grain DM than the single older hybrid in 2012, and newer hybrids accumulated 1.3 and 3.1 Mg ha(-1) more grain DM than the 1975 and 1967 hybrids in 2013 when overall PostDM gains were much higher than in 2012. In 2013, more of the Grain N content (GrainN) was derived from post-silking N uptake in newer hybrids versus older hybrids. Plant component DM and N changes between silking and maturity stages in 2013 suggested 33% of final grain N originated from leaves (with no net DM depletion), and 22% of grain N originated from stems (accompanied by a net 20% DM depletion), during grain filling in a rather consistent manner for all four hybrids. However, newer hybrids maintained a higher leaf DM and leaf N content at maturity (despite a lower leaf N concentration and higher grain N harvest index) compared to older hybrids. These results indicated that retaining leaf function by enhancing leaf biomass and N content and, consequently, PostN accumulation during the grain filling, benefited from a higher DM partitioning to leaves at silking in newer hybrids. (C) 2015 Elsevier B.V. All rights reserved.