Physiological aspects of relative changes in nitrogen and plant density stress tolerances over a 38-year period of US maize hybrid introductions
Incremental gains in grain yield of maize hybrids over the decades are the consequence of genotype, environment and management interactions. Historically, genetic improvements in newer hybrids have included longer active grain filling periods (achieved by advancing silking and extending functional stay green in maize leaves); stronger source and sink during grain filling; enhanced tolerance to higher density; and canopy architecture changes. Newer hybrids were known to accumulate more dry matter and nitrogen in the post-silking period, but achieving a more comprehensive knowledge of pre-silking and post-silking dynamics required further understanding of dry matter and nitrogen partitioning in individual organs, as well as their associations with canopy traits. Genetic improvement studies would ideally involve side-by-side direct comparisons of hybrids from different eras with synchronized management strategies under a variety of environment conditions. The primary objectives of this field study were to: 1) quantify dry matter and nitrogen partitioning in organs (stem versus leaf) at silking and its impact on post-silking dry matter and nitrogen accumulation for different hybrid eras under a variety of management conditions; 2) evaluate historic improvement in source strength, including post-silking dry matter accumulation, and sink strength, including kernel number, kernel weight, and ear growth rates; 3) quantify historic changes in dry matter and nitrogen dynamics during pre-silking and post-silking periods, as well as nitrogen internal efficiency; and 4) quantify the correlations between canopy attributes with grain yield and its components.^ This study began in 2012 with a detailed consideration of 3 genotypes (two 2005 hybrids versus one 1975 hybrid); this study was then expanded to 8 genotypes in 2013 and 2014 (using a series of DeKalb hybrids from 1967 to 2005). Hybrids selected for this study represented some of the most widely grown US Corn Belt commercial germplasm from Monsanto between the mid 1960s and the mid to late 2000s. The six location-years of field research included three growing seasons (2012, 2013, and 2014) at two locations in Indiana.^ Leaf and stem dry matter and N dynamics were explored intensively in 2012 and 2013. Across treatments and environments, the net leaf source (i.e. between R1 and R6 stages) contributed 33% of the grain N content and 0% of the grain dry matter at maturity, while the net stem source contributed 22% of grain N content and 20% of grain dry matter at maturity. Both 2005 hybrids partitioned more dry matter to leaf than stem at silking, and maintained higher leaf dry mater and leaf N content at maturity, when compared with 1967 and 1975 hybrids, indicating more retention of leaf function during grain fill in newer hybrids.^ Both 2005 hybrids where more intensively contrasted with 1975 and 1967 hybrids in 2012 and 2013 to test canopy traits changes that were correlated with grain yield and its components. The two newer hybrids had higher leaf biomass and leaf N content at silking, which leads to higher leaf N content per unit leaf area at the onset of grain filling, and eventually resulted in a higher green leaf number during grain filling and kernel weight at maturity. Increasing N rate decreased the correlations between grain yield with secondary traits, including green leaf number during grain filling in both years, but with leaf biomass, leaf N content and leaf area index at silking only in the more favorable year (2013). In contrast, increased density enhanced correlations between grain yield and specific secondary traits, including leaf area index at the onset of and during grain filling, as well as with leaf biomass and leaf N content at silking, in both years.^ These findings from this study: 1) documented the primary role of leaves as a remobilized N source and the role of stems as both N and dry matter sources for kernels during the grain filling period; 2) confirmed breeding gains over time in retaining functional activities during the grain filling period in newer hybrids, and extended this finding to widely contrasting N rate and plant density conditions; 3) documented greater post-silking dry matter and nitrogen gains in more recent hybrids that suggested a newer opportunity for later-season N fertilizer applications; 4) highlighted the sometimes dominant pathway for achieving yield gain in newer hybrids by enhancing kernel weight instead of kernel number through a more robust source-driven grain filling period; and 5) found, via the non-significance of most hybrid by N rate or hybrid by plant density interactions, that the genetic gains in this series of DeKalb hybrids were predominantly independent of the management treatments under which all hybrids were compared. (Abstract shortened by ProQuest.)^
Tony J. Vyn, Purdue University.