Plant Growth and Development of Peanuts

Germination and Seedling Development

The peanut seed consists of two cotyledons (also called seed leaves) and an embryo. The embryo comprises the plumule, hypocotyl and primary root. The plumule eventually becomes the stems and leaves of the plant, and the hypocotyl is the white, fleshy stem located between the cotyledons and the primary root. As the seed imbibes water, there is a resumption in metabolic activity, the seed begins to swell, and cell division and elongation occur. As the embryo grows, the testa (seed coat) ruptures and the seedling emerges. The minimum and maximum temperature requirements for peanut seed germination are not well defined. Research has shown that seed will germinate under a wide range of circumstances (consider volunteer peanuts); however, under field conditions the minimum average soil temperature should be 65 degrees F at the 4-inch depth, with a favorable weather forecast. This ensures rapid, uniform emergence and reduces the risk associated with stand loss from the seedling disease complex. The seedling uses food reserves from the cotyledons during the initial stages of growth. Under most situations, peanuts should reach the ground cracking stage 7 to 14 days after planting, depending upon soil temperature. The growth rate of the hypocotyl determines how quickly the shoot will emerge from the soil. Most current commercial varieties show little difference in emergence rates and/or seedling vigor. A final plant density of three to four plants per row foot is adequate.

Plant Development

As the plant grows, the root develops very rapidly in comparison to the shoot. By 10 days after planting, root growth can reach 12 inches. By 60 days, roots can extend 35 to 40 inches deep. Late season measurements have found peanut roots down to 6 to 7 feet. Roots grow at a rate of about 1 inch per day as long as soil moisture is adequate. The hypocotyl pushes the plumule upward causing "ground cracking." After emergence, the plumule is called a shoot and consists of a main stem and two cotyledonary lateral branches. At emergence the main stem has at least four immature leaves and the cotyledonary lateral branches have one or two leaves also. The seedling develops slowly showing as few as eight to 10 fully expanded leaves 3 to 4 weeks after planting. Leaves are attached to the main stem at nodes. There is a distinct pattern by which these leaves are attached. There are five leaves for every two rotations around the main stem, with the first and fifth leaves located one above the other. Leaves attached to the cotyledonary laterals and other lateral branches are two-ranked, so there is one leaf at each node, alternately occurring on opposite sides of the stem.

Peanut leaves have four leaflets per leaf, making them a tetrafoliate. The leaflets are elliptical in shape and have a prominent midvein. The main stem and cotyledonary laterals determine the basic branching pattern of the shoot. The main stem develops first and in runner type plants the cotyledonary laterals eventually become longer than the main stem. Additional branches arise from nodes on the main and lateral stems. The growth habit of peanut is described as bunch, decumbent or runner. Spanish and Valencia market types are classified as "bunch," with their upright growth habit and flowering on the main stem and lateral branches. Most Virginia and runner market types are considered to have a prostrate (flat) growth habit and do not flower on the main stem. Decumbent varieties have an intermediate growth habit between a runner and bunch. Several Virginia varieties are classified as decumbent. Peanuts are indeterminate in both vegetative and reproductive development (similar to cotton). This means that the plant is producing new leaves and stems at the same time that it is flowering, pegging and developing pods. Consequently, developing pods compete with vegetative components for carbohydrates and nutrients. Once a heavy pod-set has been established, the appearance of flowers is greatly reduced. Bloom About 30 days after emergence, peanut plants begin to produce flowers. Flower numbers will continue to increase until the plant reaches peak bloom at about 60 to 70 days after emergence, and then flower development will begin to decline. High temperature, moisture stress and low humidity can have a severe impact on the flowering response, limiting the number of flowers produced and reducing flower pollination. Ultimately, this can result in reduced yield and delayed pod set. However, the peanut plant can compensate to some extent by initiating a large flush of flowers when favorable environmental conditions return.

The peanut flowers are borne in leaf axils on primary and secondary branches. Several flowers can originate from each node, however, only about 15 to 20 percent will produce a harvestable pod. The peanut flower is a perfect flower (male and female structures present in the same flower) and is self-pollinated. It has a showy yellow bloom and when it first emerges, the petals are folded together. The early morning of the following day the petals unfold and pollen is shed. Fertilization takes place in 3 to 6 hours.

The fertilized ovary begins to elongate and grows downward from the node to the soil. This specialized structure, called a peg, becomes visible about 7 days after fertilization. The sharp-pointed peg enters the soil about 10 to 14 days after pollination. The developing pod is located in the tip of the peg. Once in the soil, it begins to enlarge and forms the pod and kernels. It is interesting to note that the pod will not begin growth until the peg is in the presence of darkness. Because several flowers can develop from each node, several pegs and pods can be found originating from a single node. The indeterminate fruiting habit of the peanut means the plant will have pods of varying maturity. Consequently, peanut harvest determinations are based on the presence of 70 to 80 percent mature pods. Pod and Kernel Development During the early stages of pod development, the tissue is soft and watery. As the pod develops, the hull and kernels.

Peg growth and development, begin to differentiate. The cell layer just below the outer cell layer of the pod changes from white to yellow to orange to brown to black as it matures, providing a color indication of optimum harvest date. The inner pod tissue separates from the seed and darkens as the seed grows and presses against the hard layer of the hull. This is indicated by the dark brown to black veination on the inside of the hull. Pods attain full size about 3 to 4 weeks after the peg enters the soil. Although the pod has reached full size, kernel development has barely begun. Mature, harvestable pods require 60 to 80 days of development. In Texas, a mature crop can be produced in 130 to 140 days in south Texas, 140 to 150 days in central Texas, and 150 to 170 days in west Texas. Temperature (both day and nighttime) interacts with variety, planting date, seasonal moisture, etc., in controlling development of the crop. However, the controlling factor in all plant development is temperature.

Maturity and Harvest Determination as pods mature, the inside portions become brown to black, while immature pods retain a fresh, white appearance. The cellular layer just below the outer layer of the pod undergoes several color changes during the maturation phase. This cellular layer is called the mesocarp. It changes in color from white to yellow to orange to brown and finally black as the pod matures. This color distinction can be used to estimate crop maturity with the "hull scrape" method. Hold the pod with the beak pointing down and away from you, and with a pocket knife scrape away the outer hull in the area from the middle of the pod to the peg attachment point. This region of the pod is known as the saddle. Pods should be moist when the color determinations are made. To get an accurate representation of the field, collect three adjacent plants (about 1 foot of row) from three to five locations in the field. As with all field assessments (soil and plant tissue testing, insect and disease scouting, etc.), the results are only as good as the collection procedure, so collect an adequate sample.

Determining the optimum digging time is a crucial decision for a grower! Using the calendar to predict digging dates is a good way to lose yield, grade and money. There is no substitute for scouting fields and observing pod development, especially late in the season. The optimum time to dig a peanut crop is when it has reached its peak yield and grade. If dug too early or late, yield and crop quality will be sacrificed. Because of the indeterminate fruiting habit of the peanut, each plant will have pods of varying maturity. Consequently, the risk of losing early-set mature pods versus later-set immature pods must be considered, and a compromise must be achieved. Runner types should be dug at 70 to 80 percent maturity, Virginia types at 60 to 70 percent and Spanish and Valencia at 75 to 80 percent maturity. Peanuts may gain from 300 to 500 pounds per acre in yield and one to two grade points during the 10- to 14-day period preceding optimum digging time. Conversely, similar yield and grade losses can occur if digging time is delayed 1 to 2 weeks. Overmature and diseased plants (pod rot complex, leaf spot, southern blight, sclerotinia blight, rust, etc.) have weakened peg attachments, resulting in significant pod loss during digging and combining.