Soybean Reproductive Sink Size and Short-term Reductions in Photosynthesis during Flowering and Pod Set

The number of pods and seeds produced by soybean [Glycine max (L.) Merr.] is related to canopy photosynthesis during flowering. The effect of low photosynthesis during only a portion of flowering (growth stage R1 to R5), however, is not well defined. Two field experiments were conducted at Lexington, KY (38 degrees N), with seeds sown in mid-May in 0.76-m rows (20 seeds m(-1) of row) and all plots irrigated as needed. In 2005/2006, plants (cultivars Pennyrile and Ripley) were shaded (60% in 2005 and 80% in 2006) for 4- to 9-d periods just before or just after peak pod production. These treatments had almost no effect on seed number (significant reduction in only one of eight comparisons). In a second experiment (cv. Pennyrile, 2007/2008), shade cloth (60%) was placed over plants at growth stage R1.6 and removed at 7-d intervals. The first 7 d of shade did not affect seed number, but 14 d of shade ending at roughly growth stage R3.0 reduced seed number by 16% and longer periods caused proportionally larger reductions. When shade cloth (80%) was put in place at 7-d intervals (starting at R3.4) and left in place until maturity, seed number was reduced until the last treatment (put in place 4-7 d after the beginning of growth stage R6). Seed number was tolerant of short periods (4-9 d) of low assimilate supply during flowering, but could not recover from longer periods of shade (>= 14 d), even when they occurred relatively early in the flowering period.