Arrested development in the soybean field

Figure 4: Soybean raceme. Each ovule in an ovary requires a separate pollen tube for fertilization. If an ovary contains three ovules, at least three pollen tubes must enter the ovary if all three ovules are to be fertilized. For corn, the number of female flowers that become fertilized is an important determinant of seed number, and that number is highly influenced by weather. Fertilization of soybean flowers is nearly 100%. Reasons for high success rate are: many pollen grains are produced in the 10 stamens; no pollen grains are lost by wind, weather has little effect on maturity sync of stamens and pistil, pollen tubes must travel a short distance from stigma to ovary; and the flower petals cover the pistil, which reduces dehydration.

Figure 5: Soybean stem node exhibiting three racemes: primary (A), secondary (B), and tertiary (C). Soybean plants bear flowers on inflorescences called racemes. Racemes have multiple flowers attached by a short stalk (pedicel) to a central, unbranched axis (rachis). After flowers are fertilized, the rachis elongates and separates the developing pods (Figure 4). Flowers in a raceme are fertilized in a specific pattern starting with the flower nearest the plant and proceeding up the rachis to the last flower. It may take 4 to 10 days for all flowers to open on a single raceme. There are three buds at each leaf axil that can produce racemes, so flowers may continue to open at a node for two or more weeks (Figure 5).

Soybean plants produce two to three times more flowers than there will be pods at harvest. This excess capacity is part of a strategy to produce viable seeds even if stress causes flowers to abscise. Abscission is an important process that controls pod load (and seed number) on soybean plants. Poor growth conditions including drought stress, shading by weeds, leaf defoliation and even long periods of clouds increase the amount of abscission. But, even under excellent growing conditions, abscission of 50% or more of the flowers is normal.

click image to zoomFigure 6: Effect of position within soybean canopy on flower abscission probability. Node 1 is closest to soil surface. As stated before, nearly 100% of soybean flowers are fertilized. So, technically all of the reproductive structures that abscise are pods. But, many of the structures that abscise are very small pods that may have petals still attached. Not all flowers have an equal chance of remaining on the plant. Flowers produced on nodes near the bottom of the canopy are more likely to abscise than flowers located in the upper one-third (Figure 6).

Figure 7: Abscission probability of flowers/pods within a raceme. Picture is from Iowa State University. Abscission probability also varies among positions on a raceme. The ages of pods at position 1 (oldest) and position 5 (youngest) on the raceme in figure 7 differ by only 4 to 6 days. But, the sizes of the pods differ greatly. The pods at the first two positions are closer to the source of sugars and other nutrients. They are well nourished and dominate pods at other positions. Development of pods at position 4 and greater is often arrested so that they grow slowly or not at all. Under normal weather conditions, abscission probability is less than 5% for pods at positions 1 and 2 and 75% or greater for pods at position 4 and above (figure 7). Young pods located at upper positions on racemes develop into harvestable pods only if stresses damage pods at lower positions or caused them to abscise.