Soybean yield and biomass responses to increasing plant population among diverse maturity groups. II. Light interception and utilization

Previous experiments evaluating soybean [Glycine max (L.) Merr.] yield responses to increased plant population have primarily emphasized empirical relationships. We hypothesized that the response of soybean yield to increased plant population under well-watered conditions was governed by the cumulative amount of light intercepted from emergence until late reproductive development. Experiments evaluating maturity group (MG) 00 through VI soybean were sown at 10, 20, 40, 60, or 100 seeds m(-2) at Fayetteville, AR, in 2001, 2002, and 2003. Soybean yield and biomass had an asymptotic relationship with cumulative intercepted photosynthetically active radiation (CIPAR) from emergence to the full-seed (R6) developmental stage. To obtain 90% of asymptotic biomass required 1175 MJ m(-2) of CIPAR, whereas to obtain 90% of asymptotic yield required 605 MJ m(-2) of CIPAR. The lower CIPAR value necessary for yield as compared to that of biomass was a result of a linear decrease in harvest index (HI) as CIPAR increased. Cultivars that reached R6 in at least 80 d acquired sufficient CIPAR to obtain 90% of the asymptotic yield, but plant populations needed to reach these CIPAR levels were considerably greater than for cultivars that reached at R6 in 95 or more days. By knowing the duration of the period from emergence to R6 for a given MG, we could estimate a plant population that would result in yields that were 90 to 95% of the asymptotic yield. Overall, this research demonstrates that cumulative light interception under well-watered conditions explains soybean yield responses to plant population among MGs and environments.