Soybean Reproductive Sink Size and Short-term Reductions in Photosynthesis during Flowering and Pod Set
2010
Egli, D. B.
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°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.
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