Light and Shade Effects on Abscission and 14C-Photoassimilate Partitioning among Reproductive Structures in Soybean

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Abstract
Field experiments were conducted in 1981 and 1982 to study the effects of low-irradiance supplemental light on soybean flower and pod abscission. Cool-white and red fluorescent lights illuminated the lower part of the soybean canopy during daylight hours for 3 wk late in flowering. At the same time, flowers and young pods on half the plants were shaded with Al 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 wk post-anthesis. Flowers and pods in the axil of the labeled leaf were recovered with Al foil 0, 24, 72 and 120 h 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. Light perceived by soybean flowers and young pods has a role in regulating both their abscission and their capacity to accumulate photoassimilates.