Light and Shade Effects on Abscission and C-Photoassimilate Partitioning among Reproductive Structures in Soybean

Field experiments were conducted in 1981 and 1982 to study the effects of low-irradiance supplemental light on soybean (Glycine max L.] Merr. cv Evans) flower and pod abscission. Cool-white and red fluorescent lights illuminated the lower part of the soybean canopy during daylight hours for 3 weeks late in flowering. At the same time, flowers and young pods on half the plants were shaded with aluminum foil. Flowers were tagged at anthesis and monitored through abscission or pod maturity.

Responses to red and white lights were similar. Supplemental light tended to reduce abscission and increase seed weight per node compared to natural light. Shading flowers and pods increased abscission and reduced seed weight per node. Number of flowers produced per node, individual seed weight, and seeds per pod were not affected by light or shade treatments.

Further studies examined the effects of shading reproductive structures on their capacity to accumulate ¹⁴C-photoassimilates. Individual leaves were pulse labeled with ¹⁴CO₂ 1, 2, and 4 weeks post anthesis. Flowers and pods in the axil of the labeled leaf were covered with aluminum foil 0, 24, 72, and 120 hours before pulsing.

Shading flowers and pods resulted in a 30% reduction in the relative amount of radiolabel accumulated from the source leaf. The reduction in ¹⁴C accumulation due to shading was evident regardless of the length of the shading period and was most pronounced when the shades were applied early in reproductive development. We conclude that light perceived by soybean flowers and young pods has a role in regulating both their abscission and their capacity to accumulate photoassimilates.