The soybean, Glycine max, is a major global crop. The precise timing of its flowering is the single most important factor determining its final grain yield and maturity date. The transition from vegetative growth, when the plant is just producing leaves and stems, to the reproductive stages, which produce flowers and pods, is a highly regulated biological switch. Understanding when this switch occurs is necessary for planting the correct variety in a given location and ensuring a successful harvest.
The Role of Day Length in Flowering
The primary mechanism governing when a soybean plant flowers is a process called photoperiodism. Soybeans are classified as quantitative short-day plants, meaning they initiate flowering when the period of light exposure, or day length, falls below a specific maximum value, known as the critical photoperiod.
This biological sensitivity to day length forces growers to select varieties adapted to their latitude. Soybeans are categorized into Maturity Groups (MGs), which range from 000 in the northern regions to X in the deep south. Planting a variety too far north for its MG will cause a significant delay in flowering and maturity due to the long summer days. Conversely, planting a variety too far south will cause it to flower very early, shortening the vegetative growth phase and resulting in a smaller plant and lower yield.
Defining the Reproductive Stages
The exact time a soybean flowers is identified by the onset of the reproductive stage R1, known as Beginning Bloom. This stage is officially marked when the plant has at least one open flower on any node of the main stem. Flowering usually starts around six to eight weeks after the seedling emerges, typically beginning on the third to sixth node and progressing upward and downward along the stem.
R2: Full Bloom
The progression continues quickly to the next phase, R2, or Full Bloom, which is typically reached between one to seven days after R1. R2 is defined by the presence of an open flower at one of the two uppermost nodes on the main stem that has a fully developed leaf. During this phase, the plant has generally accumulated about a quarter of its total dry weight and reached approximately half of its final height.
R3: Beginning Pod
The next stage, R3, or Beginning Pod, is reached on average between five to fifteen days after R2. A plant is officially at R3 when a pod measuring at least 3/16 inch (0.5 cm) long is found in one of the four uppermost nodes on the main stem. This stage signifies the end of the initial flowering period and the beginning of rapid pod development.
How Environmental Stress Impacts Timing
While the photoperiod acts as the initial trigger for flowering, external environmental pressures can significantly modify the timeline and duration of the reproductive stages. Water stress, particularly drought, and excessive heat are two major factors that impact the plant’s progression. Soybeans can shorten the flowering period as a survival mechanism when faced with severe moisture stress.
In response to sustained drought, the plant may accelerate its development through the reproductive stages, which can lead to earlier maturity. This shortening of the period available for pod and seed fill often results in reduced final yield. High temperatures above 95°F can also negatively affect the process by significantly decreasing the number of pods that successfully set.
The plant attempts to compensate for these issues by shedding flowers and small pods, a process called abscission, which can range from 20% to 80% of flowers under normal conditions. If the environmental stress is relieved, the soybean plant has the ability to produce new flowers and pods, allowing it to compensate for the earlier losses. However, once the plant reaches the seed-filling stages, particularly R4 through R7, stress has a more permanent impact, often leading to the abortion of developing seeds.