Factors Affecting Shedding of Flowers in Soybean (Glycine max(L.) Merrill)

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Abstract
Flower shedding in soybean, Glycine max (L.) Merrill, was studied using cultivar ‘Clark’, isoline E1t, which has relatively long racemes for convenient identification and observation of individual flowers. On each raceme studied, pod set was greatest at the proximal (basal) positions, whereas shedding was greatest at the most distal positions. When proximal flowers were removed as they reached anthesis, pod set increased at the more distal positions. Pod set was increased in some instances by application of water directly to the ovaries as a drop in the calyx cup. Peroxidase activity changed in parallel with ovary development, increasing rapidly in growing pods but not in shedding flowers. Increases in flower peroxidase was mainly in ovary walls. Flowers taken at or near anthesis from positions with high percent pod set could be grown in vitro with especially good ovary enlargement, whereas ovaries in flowers taken from positions of low pod set did not enlarge in culture. Unidentified substances were extracted from young pods which, when incorporated into lanolin and tested in an in situ bioassay, could mimic the effect of proximal flowers in inducing shedding of distal flowers. Indole-3-acetic acid resembled the extracted materials in inducing shedding, but differed by eliciting side-effects that extracts did not. The growth substances abscisic acid, gibberellic acid, and benzyladenine did not promote shedding in the in situ test. The evidence was taken to indicate that soybean flower shedding is induced in distal flowers by substances from the more proximal, fertilized ovaries, and that this is possibly due to interference with some of the intense metabolic changes that follow pollination and fertilization.