Plant Biomass and Nitrogen Partitioning Changes Between Flowering and Maturity in Older Versus Newer Maize Hybrids.

Maize (Zea mays L.) yields have increased dramatically in recent decades, in part because newer hybrids are better adapted to maintaining sink strength at higher plant densities and to retaining functional leaf area during the grain fill period. In this 2-year study, we directly compared plant component dry matter (DM) and nitrogen (N) partitioning changes at R1 and R6 growth stages in older and newer hybrids at 3 plant densities and 2 N rates. Older hybrids released in 1967 and 1975 were compared to two newer hybrids released in 2005. At flowering, older hybrids (XL72AA in 2012 and average of XL72AA and XL45 in 2013) had 23% and 12% higher DM partitioning to stem instead of leaf (stem/leaf DM), as well as a 17% and 8% higher stem N to leaf N content ratio, than newer hybrids (DKC61-69 and DKC61-72) in 2012 and 2013, respectively. Drought stress in 2012 reduced the proportion of post-flowering N uptake (PostN) as a fraction of grain N content at maturity to an average of 42% in 2012 versus an average of 50% in 2013. However, newer hybrids accumulated substantially higher PostN in both years. Across all hybrid, N rate and plant density factors in 2013, depletion of leaf N between flowering and maturity (termed as remobilized N (RemN)) equaled 29% of grain N content at maturity. Similarly, depletion of stem N from flowering to maturity equaled 23% of grain N content at maturity. Higher stem/leaf DM ratios were negatively correlated with yield and grain N across hybrids, N rates and densities. The higher DM partitioning to leaf versus stem in newer hybrids at flowering benefited leaf activity which better satisfied grain N needs from both leaf RemN and PostN. The dynamics of DM and N distribution at flowering in newer hybrids can help inform future research in maize improvement from genetic and management perspectives.