Photosynthetic Rate of Soybean at Various Planting Dates.

Soybean [Glycine max (L.) Merr.] yield is typically maximized by early planting in the upper Midwest USA. Seasonal carbon dioxide exchange rate (CER) has not been quantified to explain the positive yield response to early planting. Five planting dates were established between 18-Apr and 22-May near Ames, IA in 2007. Following the appearance of three fully developed trifoliolates, photosynthetic rate was measured twice each week, through the R6 growth stage, between 1200 and 1400h CDT. Between V3 and growth stage R1, CER declined slightly for all planting dates reaching a minimum at R1. Between R1 and R6, CER increased at a linear rate for all planting dates reaching a peak of 32.2 µmol m^-2 s^-1 for 18-Apr planting and 30.0 µmol m^-2 s^-1 for 22-May planting. An important difference between the early and the late date was the duration of the linear increase period for CER. This period was 17 days longer for the 18-Apr date compared with the 22-May planting date. Carbon fixation occurred at an increasing rate for a longer period due to early planting and contributed to 22% more dry matter at R5.5 and 12.5% greater seed yield at harvest. This study will be repeated in 2009 and data from both years will be presented.