Effect of Raceme‐Localized Supplemental Light on Soybean Reproductive Abscission1

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Abstract
The percentage of soybean [Glycine max (L.) Merr.] reproductive structures that abscise is a potentially important yield factor. To better understand the involvement of light in the abscission of reproductive structures, a series of in vitro raceme‐culture and growthchamber experiments were conducted. In the in vitro raceme‐culture experiments, racemes with four to six flowers at or past anthesis were excised from the soybean plant (genotype 1X93‐100), embedded in a complete nutrient, solid agar medium, and subjected to various light treatments. A series of three experiments indicated that the racemes contain a photoreceptor, possibly phytochrome, capable of regulating sucrose accumulation. In each of the growth chamber studies, supplemental light was supplied directly to individual soybean flowers via fiber optic light guides. The light source (a tungsten filament bulb) increased the photon flux to the flowers by 10‐fold. The first growth chamber experiment showed that flowers receiving supplemental light were more intense sinks for 14C‐sucrose than were controls (intensity value of 1.0 vs. 0.4 ✕ 10−7, intensity = [dps of flower/dps of raceme]/[kg dry wt of flower]). In a second study, 42% of flowers treated with supplemental light set pods, while only 26% of control flowers set pods. A third experiment showed that red supplemental light produced 55% fruit set, compared to 41% set for far‐red light, and 35% for controls (far‐red not statistically different than control, but all other comparisons are significantly different). These experiments indicate that both photoassimilate accumulation and abscission in young soybean reproductive structures may be regulated by light quality.