Soybean Growth and Development & Management Information for Replant Decisions

Soybean Growth and Development

An understanding of how a soybean plant develops can help you make important management decisions. This section discusses various seed and plant parts, explains how the soybean plant develops utilizing standardized vegetative and reproductive stage descriptions which are used by the National Crop Insurance Services (NCIS), and describes some of the important factors which affect growth and development of the soybean plant.

A soybean seed consists of a miniature plant attached to two nutrient storage reservoirs (cotyledons or seed halves) and surrounded by a protective outer wrapper or seed coat called a testa (Figure 1). The cotyledons contain about 40% protein, 20% oil, and about 35% carbohydrate. These materials provide nutrients to the developing embryo during the germination process. The embryo axis (miniature plant) consists of an epicotyl or plumule (first leaves and the growing point for the new plant), a hypocotyl and the radicle or root.

Figure 1. External and sectional view of a soy bean seed.

When planted in moist soil the seed absorbs moisture and begins to germinate. When about 50% moisture content is achieved certain enzymes are activated to digest the stored food in the cotyledons which is then transported into other parts of the embryo.

Figure 2. Early seedling development of soybean
(adapted from original drawings from R. Kent Crookston)

Figure 2 illustrates the early growth and development of a soybean plant. Growth begins with the emergence of the radicle from the seed coat within two to three days after planting. The radicle forms the primary root system of the seedling. This early root development firmly anchors the soybean plant in the soil. The hypocotyl also begins to elongate in a hook shape, raising the cotyledons and enclosed epicotyl above the soil surface. This process takes 5 to 15 days depending on moisture and temperature conditions. Once above the soil surface, the hypocotyl straightens. The epicotyl-plumule begins to grow and form the true stem and the true leaves of the plant. The cotyledons remain attached to the stem (at a point called the cotyledonary node) for a few days as their food reserves are utilized by the developing seedling. As stored food is removed the cotyledons turn yellow and shrivel before dropping off the plant. Loss of one or more of the cotyledons before the food reserves are fully utilized can slow early plant growth or result in death of the plant if photosynthetic leaf tissue is not formed quickly.

The first true leaves are a pair of single leaflets (unifoliolate or primary leaves) attached opposite each other on the stem at the primary node just above the attachment of the cotyledons. Loss or damage to these leaves can result in reduced growth but is probably less damaging to continued plant development than early cotyledon loss.

All remaining leaves produced by the growing point of the new seedling are trifoliolate (three parts) and are attached to alternate sides of the stem at points called nodes. A new three-part leaf is produced every 3 - 10 days depending upon growing conditions. These leaves are the light capturing, food manufacturing organs of the plant.

Branches can develop from axillary buds, which are undeveloped growing points, located on both sides of the stem at the attachment points of the cotyledons (cotyledon node) and the primary leaves (primary node). These branches develop if the main growing point is damaged or destroyed, or if the soybeans are growing at low populations in the field. Each developing branch then elongates and produces leaves. Table 1 presents the standardized system developed by the National Crop Insurance Services (NCIS) for describing the early vegetative growth, as well as the average number of days between each growth stage.

Before the vegetative growth period has been completed, flowering or reproduction begins in soybeans grown in northern areas of the United States. The growth habit of soybean varieties grown in the north is indeterminate because the last vegetative growth is completed after the reproductive phase has begun. Flowering is triggered by changing daylengths and temperatures. Varieties are assigned maturity designations designated Maturity Group (MG) 000 to MG X, with each designation indicating adaption to north-south zones about 100 miles wide. When a variety is planted south of its zone of adaptation it flowers and matures earlier because the shorter day lengths induce flowering. When a variety is planted north of its zone of adaptation, flowering and maturity will be delayed because the proper daylength occurs at a later calendar date.

Flowers are produced in clusters at each node and generally progress from the bottom to the top of the plant. Soybean flowers are self-pollinated, so pods can begin to develop from some of the flowers shortly after they are produced. The number of pods per plant has a major effect on harvestable yield. As many as 75% of the flowers and/or developing pods can be shed from the plant. Water stress, leaf loss and high temperatures affect this shedding. Pods enlarge for a period of time before actual seed filling begins, and there are generally 2 - 3 seeds in each pod. Stress induced shedding during podding can be partially compensated for by late developing flowers. Table 2 provides a standardized description developed by National Crop Insurance Services for the various reproductive stages of the soybean plant as well as the average number of days for each stage.

A calendar indicating the "typical" progress of soybean development in Minnesota from planting to harvest is shown in Figure 3 . The data for this calendar was provided by the Minnesota Agricultural Statistics Service (MASS) and shows the calendar dates at which various percentages of the soybean crop arrive at various growth and/or maturity stages.

Figure 3. Minnesota soybean development calendar (adapted from Soybean Plant Development in Minnesota , Crop News #47, D.R. Hicks).

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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