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

Abstract

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 ofshading reproductive structures on their capacity to accumulate '4C-photoassimilates. Individual leaves were pulse labeled with '4CO2 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 '4C 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.Soybeans flower profusely, but only a small fraction of those flowers develop into mature pods with seeds. Combined flower and pod abscission from 32 to 63% for indeterminate (7,23) and from 57 to 82% for determinate genotypes (23, 26) has been reported. Although the cause is unknown, this extensive reproductive abscission does not appear to be due to inviable pollen (23) or lack of fertilization (1, 3). Abscission is not restricted to flowers, as a significant proportion of young pods also drop, usually before the onset of rapid seed growth (24,26). Percent

Correlation between Yielding Ability and Dry Matter Productivity during Initial Seed Filling Stage in Various Soybean Genotypes

“…They found the greatest reduction in yield by shading the canopy shortly after R5. Although they also found that light enrichment by using a refl ector before R5 had a greater effect on yield than that after R5, the light enrichment inside the canopy using a refl ector might not only increase assimilate supply, but also have a photomorphogenic effect on pod development (Heindl and Brun, 1983). Yoshida et al (1983) observed the dimensional growth of reproductive organs in fi eld-grown determinate and indeterminate cultivars, and showed that pods reached maximal length and width within a few weeks after R5.…”

Light Regulation of Sink Metabolism in Tomato Fruit

“…In addition, grain set was not affected when photosynthesis was completely inhibited by treatment with DCMU, indicating that the reduction of set was not mediated by photosynthesis. It also has been reported that shading flowers and pods increased abscission and reduced accumulation of photoassimilate in soybean (9).…”

Source/Sink Relations of Abscising and Nonabscising Soybean Flowers

“…Some studies have implied that reproductive abscission in legumes is hormonally mediated (6,8,12,14), while others have implied that it is related to> the availability of photosynthetic assimilates (4, 10, 11). However, since reproductive abscission occurs at a time when plant photosynthetic rate is relatively high and reproductive growth rate is relatively low, it appears not to be limited by the contemporary supply of photoassimilate.…”