Role of Assimilate and Carbon-14 Photosynthate Partitioning in Soybean Reproductive Abortion

The yield production of soybean [Glycine max (L.) Merr.] is partly determined by the flower and pod abortion process. Since the causes of abortion are not well understood, field experiments were conducted over 2 yr to study the relationships of assimilate concentrations and photosynthate partitioning to flower and immature pod abortion. Soybean (cv. Kent) was grown in the field during two seasons using conventional cultural practices. Flowers that opened at R1 (early flowering) and R3 (late flowering) were marked and the abortion levels were determined. In the second year immature pods present just after early and late flowering were also marked and abortion determined. Flower and immature pad samples were taken on the day of marking for chemical analysis. Early flower abortion averaged 40%, and late flower abortion averaged 80%. Early immature pod abortion was 15Yo but late immature pod abortion was 57%. In both years the concentrations of ethanol-soluble carbohydrates, starch, ethanol-soluble N, ethanol-insoluble N, NO⁻₃, Mg²⁺, K⁺, and Ca²⁺ in the fully open flowers or in immature pods was not related to the abortion percentages exhibited by the flowers or pods. The ethanolsoluble carbohydrate concentration ranged from 71 to 107 g kg⁻¹ in flowers and from 40 to 45 g kg⁻¹ in immature pods. Starch concentration ranged from 24 to 29 g kg⁻¹ in flowers and 18 to 30 g kg⁻¹ in immature pods. The results did not support the hypothesis that low assimilate concentrations are the major cause of abortion in soybean. During both early and late flowering, a single leaf located in the middle of the main stem that subtended fully open flowers or immature pods was labeled with ¹⁴CO, at both 0 and 3 days after flower opening. Flowers exhibiting low abortion percentages contained a greater percentage of the total ¹⁴C recovered (0.049 to 0.095%) as well as a greater specific activity of ¹⁴C (0.018 to 0.027Yo mg⁻¹ flower) than those exhibiting high abortion percentages (0.014 to 0.024% and 0.005 to 0.009% mg⁻¹ flower, respectively). Immature pods exhibiting low abortion also contained greater percentages of ¹⁴C recovered (0.15 to 0.20%) than those exhibiting higher abortion (0.012%). Although it is difficult to distinguish between a cause and a symptom of abortion, abortion seems to be at least related to the lack of photosynthate partitioning to flowers and immature pods.