As I wrote in Part 1, the two primary yield components for grain crops are seed number and seed size. And, seed number is the more important of the two yield components. Understanding how soybean plants regulate seed number and how this yield component responds to stresses and crop management are helpful in understanding soybean yield production.
Soybean seed number is determined by the number of flowers produced, the number of pods retained on the plant, and the number of seeds per pod. In Part 1, I briefly discussed flower number, and focused on the number of pods retained. In Part 2, I will discuss adjustments in seed number per pod.
Because soybean is a member of the legume family, its fruits are called pods. Pods are mature ovaries and the seeds inside are mature ovules. Soybean ovaries contain two to four ovules before fertilization. There can be no more seeds in a pod than there were ovules. It is highly unlikely that pods in Missouri soybean fields will contain more than four seeds.
Almost immediately after fertilization of the ovules in the ovary, the pod wall begins to expand. Pod expansion in nearly complete before seed filling begins (Figure 1). When pod wall growth is finished, developing seeds have obtained only about 5% of their final dry weight. In a normal pod with normally developing seeds, the seeds at stage R6 will almost completely fill the pod cavity and cause the pod wall to bulge outward at each seed position (Figure 2)
Soybean pods at harvest contain one to four seeds (Figure 3). The number of seeds in a pod is determined by number of ovules in the ovary, number of those ovules fertilized, and number of seeds that continue development until maturity. Somewhat surprising, the ovule near the tip of the ovary (furthest from raceme rachis or position 1 (Figure 2) is fertilized first and its seed begins development one or two days before the other seeds.
Successful fertilization of an ovule does not mean that the resulting seed will continue development through maturity. Arrested development, often called abortion, may occur to any of the growing seeds in the pod. From 10 to 20% of fertilized seeds abort. Positions within a pod differ for abortion probability. Again somewhat surprising, the seed in position 1 aborts nearly twice as often as seeds further from the raceme rachis.
Seed abortion can occur at any stage of development, but more than 90% of the abortion incidences occur before 30 days after fertilization. The 3-seeded pod in figure 4 possesses two seeds with arrested development. Abortion at position 1 probably occurred 7 to 12 days after fertilization. Abortion at position 2 occurred later, maybe 20 to 24 days after fertilization. Abortion that happens late in seed-filling often results in a seed with a wrinkled appearance (Figure 5).
The most common seed number per pod in Missouri soybean fields is three. Figure 6 illustrates that seed abortion can occur at any position, and that more than one seed may abort.
Sometimes pods are “flat” at harvest and appear to contain no seeds (Figure 7). Flowers that produce flat pods were fertilized because pod growth does not happen unless the flower is fertilized. So, at some time during development, all of the fertilized ovules underwent arrested development. Before maturity, flat pods may appear “normal” with normal pod length and clearly visible chambers where seeds should be (Figure 8). These “flat pods” might contain partially developed seeds (Figure 9).
The causes of seed abortion are similar to the causes of pod abscission. To continue development, seeds require a steady flow of water, carbohydrates, and mineral nutrients. Stresses that reduce any of these requirements may increase seed abortion. Because developing seeds are most vulnerable to abortion early in their development, stress during growth stage R4 is more likely to reduce seed number per pod than stresses that occur earlier or later in the growing season.
REVISED: October 1, 2015