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Title:   Author  item
                                            BREKKE, BRENT - Iowa State Universityitem
                                            Edwards, Jodeitem
                                            KNAPP, ALLEN - Iowa State University
  Submitted to: Crop Science
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 11/13/2010
                                    Publication  Date: 11/1/2011
                                    Citation: Brekke, B.H., Edwards, J.W., Knapp, A. 2011. Selection and adaptation to high plant density in the Iowa Stiff Stalk synthetic maize (Zea mays L.) population: II. Plant morphology. Crop Science. 51(6):2344-2351.Interpretive Summary: Maintaining or increasing present rates of genetic improvement in corn hybrid performance will require corn breeders to better understand the basis for past gains and to expand the diversity of currently available corn lines and populations. However, nearly all diverse sources of corn breeding populations are very poorly adapted to the high plant desnities that farmers currently plant. We evaluated the improvement in adaptation to high plant density in populations that have been selected over 70 years of corn breeding in order to identify specifically how these corn populations have become adapted to high plant densities. We found that long term selection has produced popluations that silk earlier with respect to pollen shed, have few tassel branches, and more upright leaves. Furthermore, silking was two days later at high plant density than low plant density in an unselected population, but was not affected by density in highly selected popluations. These results demonstrate that selection has changed specific traits needed for high grain yield in modern plant densities. The results further suggest that expanding diversity of populations used in corn breeding will require adapting populations with no breeding history to high plant densities as a first step. Utilization of available diversity of corn populations for agricultural purposes has been extremely difficult, and thus new approaches are needed.
                                Technical Abstract: The plant density at which Zea mays L. hybrids achieve maximum grain yield has increased throughout the hybrid era while grain yield on a per plant basis has increased little. Changes in plant characteristics including flag leaf angle, anthesis-silking interval (ASI), plant height, tassel branch number, and total number of leaves have been characterized in comparisons of commercial hybrids representing different eras of hybrid maize production, but have yet to be examined in a recurrent selection program. The objective of this experiment was to determine if direct selection for grain yield and agronomic performance in the Iowa Stiff Stalk synthetic population has indirectly improved adaptation to high plant density.  Material from an unselected base population, Iowa Stiff Stalk Synthetic (BSSS), was compared to the advanced cycles of selection from two different recurrent selection programs at four Iowa locations in 2008 and 2009. The advanced cycles and base population were compared at densities of 38,300, 57,400, 77,500, and 95,700 plants ha-1. Treatments were replicated twice per location and arranged in split plot design with plant density as whole plot and breeding population as sub-plot treatment. Advanced populations had reduced ASI. Plant density did not affect flag leaf angle which became more vertical in advanced populations. Increasing plant density in advanced populations increased plant height while not effecting ASI or tassel branch number; supporting our hypothesis of increased adaptation to high plant density.


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