Drought is typical in the Midsouth during flowering and seed filling stages for soybean. Ultra-early maturity group (MG) soybean cultivars can be used to substantially decrease irrigation, but they require high population densities for full light interception. We hypothesized that 7- and 5-leaflet soybean genotypes would accumulate more light than 3-leaflet near isogenic genotypes, and therefore, reduce the population densities required for full light interception. The objectives of this research were to (1) compare cumulative intercepted photosynthetically active radiation (CIPAR) and yield between near-isogenic 3- and 5-leaflet and 3- and 7-leaflet genotypes and (2) map the gene conferring the 7-leaflet trait using molecular markers. At Fayetteville, AR, plots for experimental genotypes from MG 00 to II were drill-seeded at populations ranging from 4 to 80 plants m-2 in 19-cm rows. The fraction of light intercepted (FLI) by near-isoline pairs was measured using digital imagery. The FLI values were used to estimate daily light interception, and these values were cumulated to determine CIPAR. Covariate analysis was used to determine the effect of leaflet number on CIPAR and yield. Generally, 7- and 5-leaflet isolines had similar or less CIPAR and yield than 3-leaflet isolines at similar populations. These results were contrary to the hypothesis of this research. Backcrossed 3- and 7-leaflet near-isogenic lines were used to map the 7-leaflet (If2) gene. There were 115 out of 482 SSR markers that exhibited differences (polymorphisms) between near-isogenic 3- and 7-leaflet plants. One SSR marker from LG B1, Sat_272, was linked (LOD > 4.0) to the If2 gene with map distances ranging from 2.77 to 11.2 cM. This research was successful in mapping the If2 gene to LG B1 of the soybean molecular map. Now, Sat_272 can possibly be used to merge a part of the classical soybean genetic map with the soybean molecular map as well as map other genes linked to the If2 gene to the molecular map.
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