1. Effects of Plant Density on Developmental Phenotypes in Maize Michael J. Stein 1 and Jode W. Edwards 2 1 Department of Agronomy, Iowa State University, Ames, IA and 2 USDA ARS, Ames, IA. Abstract The Iowa Stiff Stalk Synthetic population, BSSS, and the Iowa Corn Borer Synthetic #1 population, BSCB1, have undergone 17 cycles of reciprocal recurrent selection. Primary emphasis of selection has been for grain yield and agronomic performance. Recent studies have shown that selection for grain yield has greatly increased the adaptation of the population to high plant density. Advanced cycles achieve maximum grain yield at much higher densities than early cycles. However, less is known about the impact of high plant density of early plant development and morphology. The objective of this experiment was to examine the effect of plant density on morphological and female inflorescence development in BSSS and BSCB1 populations. Unselected base populations and the cycle 17 populations and population crosses were planted at four different densities in two replicates ranging from 3.8 to 12.4 plants/m 2 at the Iowa State University Agronomy Farm outside Ames, Iowa. High plant density reduced rate of morphological development and the rate of female ear development beginning early in development suggesting that understanding the effects of plant density on agronomic performance will require characterization of the effects of high plant density on early plant development. Introduction Since the 1930s, the average grain yield in the United States has increased from 1.5 to 8.5 Mg/ha at the end of the last century, with similar large increases being reported in nations around the world (Brekke et al., 2011). Associated with yield increases from the 1930s to the present has been an increase in adaptation to higher planting densities (Brekke et al., 2011). Increased plant densities have been shown to affect silking-anthesis intervals and total number of leaves in unimproved populations, and the final grain yield per plant in improved and unimproved populations (Duvick, 2005; Sangoi, 2000). Less information has been gathered concerning the effects that high density has on developmental phenotypes. Objective: To observe how different levels of plant density affect the developmental vegetative and female reproductive phenotypes in both improved and unimproved populations of maize. Methods Populations: Iowa Stiff Stalk Synthetic (BSSS)-Developed in 1934 from the intermating of 16 inbred lines. Iowa Corn Borer Synthetic #1 (BSCB1)-Developed in 1949 from the intermating of 12 inbred lines. BSSS(R)C17 & BSCB1(R)C17- The seventeenth cycle of reciprocal recurrent selection between the two populations Procedure: The four populations were planted at four different densities, 3.8, 6.7, 9.6, and 12.4 plants/m 2. The populations were planted in a split-plot arranged in a randomized complete block with two replications, with the populations as the whole plot and densities as subplots. Phenotypes On four dates in the growing season, 45, 51, 53, and 58 days after planting, the growing primary ear shoots were removed from BSSS plants, measured, and observed for abnormalities. On two dates in the growing season, 38 and 49 days after planting, the leaf stages of the plants were measured for each density in each population by counting the number of leaf collars. Mature ears from the BSSS and BSSS(R)C17 populations were harvested and their final grain yield characteristics were examined. Results Figure 1: V Stages Versus Plant Density at 38 Days After Planting Density by population interaction was significant at p<0.05. Figure 2: V Stage Versus Plant Density at 49 Days After Planting Density by population interaction was significant at p<0.0001. Figure 3: Ear shoot length versus Days after planting for BSSS Density by days after planting interaction was significant at p < 0.007. Figure 4: Ear Shoot Length Versus Leaf Stage for BSSS Density by V stage interaction for slope of the two lines was significant at p<0.04. Discussion We found that high plant density delayed vegetative development and female reproductive development. Selection reduced the developmental delay in BSSS (BSSS had a much greater delay than the advanced population, BSSS(R)C17) but not in BSCB1. In the BSSS population, high plant density reduced the rate of growth of the developing ear shoot relative to time, showing a delay in female reproductive development. High plant density also slowed the growth of the ear shoot relative to the development of the leaf stage suggesting that plant density also affects relative rates of vegetative and female reproductive development Future work is planned to study more vegetative and reproductive phenotypes to better characterize the differential impact of plant density in improved and unimproved populations. Acknowledgements We would like to thank the Paul Scott and Erik Vollbrecht Labs for allowing us to use their lab space and microscopes to dissect the ear shoots. References Brekke, B., Edwards, J., and 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 Sci. 51:2344-2351. Duvick, D. 2005. The contribution of breeding to yield advances in maize (Zea mays L.). Advances in Agronomy 86:83-145. Sangoi, L. 2000. Understanding plant density effects on maize growth and development: An important issue to maximize grain yield. Ciência Rural 31:159-168. V Stage Vegetative growth stage at 38 and 49 days after planting decreased with increasing plant density at both collection periods and in in all populations (Fig. 1,2). In the BSSS population, the most advanced cycle of selection BSSS(R)C17 had the largest decrease in V stage with increasing plant density whereas at 49 days after planting the unselected population, BSSS, had the largest decrease in V stage (Fig. 1,2). In the BSCB1 population, there the most advanced cycle of selection, BSCB1(R)C17, and the unselected base population, BSCB1, had the same response to increasing plant density (Fig. 1,2). Ear Shoot Length Ear shoots in BSSS increased in length much fast at low density than at high density (Fig 3). Ear shoot increased linearly with increasing V stage at both high and low plant density during the developmental stage studied here. Ear length increased faster, relative to vegetative growth stage, at low plant density than at high plant density (Fig. 4). Final Ear Length Mature ear length decreased at a rate of 4.1 mm for every 1.0 plant m -2 increase in plant density (data not shown). However, no differences were found between improved and unimproved populations in final ear length. Two hypotheses for the lack of difference in populations in final ear length are small experiment size (statistical power) and wind damage after pollinating that reduced effective plant density at the higher planting densities.