Seed quality plays an important role in the production of quality plumeria and plumeria used for root stock. Characteristics such as germination, germination percentage, vigor, and appearance are important to growers planting plumeria and to homeowners establishing plumeria gardens. Achieving and maintaining high seed quality is the goal or should be the goal of every plumeria seed producer.

Seed Development and Structure

Seed Development

The process of seed development begins within the flower, the plant’s reproductive structure. The plumeria flower is both male and female. The female part is the pistil, and the male part is the stamen.

The pistil has a top portion (stigma), a middle portion (style), and a lower portion (ovary). The ovary may contain one or more ovules. The surface of the stigma produces a sweet, sticky solution as it becomes receptive to pollen fertilization.

Pollen is produced in the anthers at the ends of the stamen. Pollination occurs when pollen grains come into contact with the stigma. Wind and insects are largely responsible for the transfer of pollen from the anthers to the stigmas. 

If conditions are favorable, pollen grains begin to grow on the stigma surface and form pollen tubes. The pollen tube grows down the style and into the ovary, where it comes into contact with the ovule. Male gametes are transferred through the pollen tube into the ovule. Fertilization occurs when the male gametes unite with the female egg in the ovule. After pollination and fertilization, ovules develop into seeds.

• In self-pollinated plumeria, pollen produced within each flower pollinates the stigma of the same flower. 
• In cross-pollinated plants, the pollen grains pollinate flowers other than the one from which they originated.

Fertilization occurs shortly after pollination and begins the process of seed development. Early stages in the development of strong plants depend on favorable growing conditions. Seed quality, on the other hand, is largely dependent upon environmental conditions and the promptness with which the seed is harvested after it matures.

Seed Structures

Knowledge of seed structure can help in understanding how seeds respond during germination, and seedling emergence.

Seeds can be divided into two major classifications, monocots (monocotyledons) and dicots (dicotyledons), based on the number of cotyledons (seed leaves) in a seed. Monocots contain one cotyledon, whereas dicots have two. Plumeria are dicots.

Seeds are composed of three basic structures:

• (1) the seed covering (seed coat or testa);
• (2) the embryonic axis (embryonic root or radicle and shoot or plumule); and
• (3) supporting tissues (the cotyledons and endosperm).

If unbroken, the seed coat regulates water uptake by mature seeds. The seed covering characteristics often affect the quality of seed when exposed to adverse weather. Some seeds, such as plumeria, have a soft and semi-delicate seed covering. This covering can easily break or slip and expose the embryo, making it susceptible to injury, deterioration, and pathogen attack.

The embryonic axis normally includes the miniature plant, consisting of the root and shoot. Cotyledons and endosperm are usually considered supporting tissues. They are useful to the developing plant as a reserve food source through the course of germination and emergence until the plant can make its own food through photosynthesis. Supporting tissues in dicots are composed of mostly fats and oils.

The location of seed structures plays an important role in determining the seeds’ susceptibility to mechanical injuries and weather damage. The embryonic axis is often just below the seed covering. Impacts to the embryonic axis can cause severe damage, resulting in abnormal seedlings or death of the seed. Plumeria seeds can survive for years, but the germination rate declines over time.

Sizes and Shapes of Seeds

Plumeria Seeds come in many sizes. They range in size from the small to large. Shapes are similar all with a wing.

Dispersal Mechanisms

The dispersal mechanism of plumeria seeds is as simple as dropping of the seed from the parent plant onto the ground to the more exotic ways such as scattering by wind from the plant.

Chemical Composition of Seed

Like all living organisms, seeds are composed of many different types of chemicals, but seeds are unique in that they are a storehouse of chemicals that are used as food reserves for the next generation plant.

Seeds store three major classes of chemical compounds: carbohydrates (sugars), lipids (fats and oils), and proteins. The quantities of these compounds stored in seeds vary with the type of seed.

Proteins are made up of long chains of amino acids. Most seeds are not high in protein but are high in carbohydrates or lipids. The seeds of most plants store their food reserves mainly in the form of carbohydrates or lipids.

Germination and Field Emergence

The following definitions will help in understanding seed germination and seedling emergence:

• Germination is the emergence from the seed and development of those essential structures that under favorable conditions produce a normal plant. Germination is more than just the protrusion of the root or shoot from the seed covering. It is important that all of the seedling structures necessary for continuation of the next generation be present and healthy.
• Field Emergence is the elongation of the seedling axis resulting in protrusion of the seedling shoot from the soil.
• Viability is the potential to germinate. A nonviable seed will not germinate under any conditions. Viable and a nonviable seeds may look exactly the same.
• Dormancy is the state of nongermination in viable seed. During this period, germination is blocked by conditions within the seed. Dormant seeds are often thought of as being in a resting state. Dormancy is broken when the plumeria seed is exposed to warmth and moisture. A seed may take up water and look fully able to germinate but, because other necessary conditions are not present, may fail to germinate. If a seed starts to germinate and is deprived of moisture, it may stop the germination process and die.

Germination

Requirements for initiation of germination include:

• a favorable moisture level in the seed;
• a favorable temperature in the environment around the seed;
• a favorable oxygen supply to the seed.

Note that favorable conditions must be present. Plumeria seeds do not require light conditions.

Germination occurs in several steps. The first is the absorption of water. Water begins certain biochemical processes within seed that accelerate cell activities. The minimum moisture level at which germination begins is known as the critical moisture level.

Critical moisture levels vary among crop seed. Most dicots will not begin germination until they have a moisture content of at least 50 percent.

Germination will not occur above or below the critical temperatures. Each species has a minimum, an optimum, and a maximum temperature for seed germination. Most species have a minimum germination temperature of approximately 55°F (13°C) and a maximum germination temperature of approximately 110°F (43°C). The optimum temperature is usually from 75 to 85°F (24 to 30°C).

Lack of oxygen is not usually a limiting factor for germination. However, wet or soggy soils may not contain enough oxygen for germination to begin. Seeds planted in such soils will absorb water quickly and have a tendency to decay.

Plumeria seeds germinate equally well with or without light. 

The internal conditions of seed, such as soundness and vigor, as well as the environment, affect the rate of germination. Seed vigor can be affected by maturity, age, mechanical injuries, disease infection, preharvest weather, and storage environment.

Adverse weather conditions after planting regularly influence the germination processes by affecting moisture, temperature, and oxygen levels. 

Once cell activity is initiated, the root is the first structure to emerge from the seed coat. Root growth is a result of both cell division and elongation. Hypocotyl growth is mainly a result of cell elongation. Shortly after root growth begins, the shoot meristem emerges from the seed coat and continues development by cell division and elongation.

Field Emergence

Different species exhibit different seedling emergence patterns. 

In plumeria, root growth occurs by cell division and enlargement of the root (Figure 3). As growth continues, the hypocotyl elongates by cell enlargement, and the midsection emerges from the soil (Figure 3). The seed coat usually remains below ground. As the bent hypocotyl elongates, it gradually pulls the cotyledons above ground (Figure 3). During the critical stage when the hypocotyl is still arched above ground and is pulling the cotyledons upward, damage to the hypocotyl can easily prevent seedling emergence.

Seed Vigor

Seed vigor is the property that gives seed the potential for rapid and uniform emergence and development of normal seedlings under a wide range of conditions. 

Many factors, such as maturity level at harvest, age of the seed, mechanical injuries, disease infection, and storage environment, can influence seed vigor. Genetic factors also influence vigor.

Seed Quality

Quality Characteristics

Seed quality describes the potential performance of a seed lot. The presence of inert matter, germination percentage, vigor, appearance, and freedom from disease are important aspects of seed quality. High-quality seed lots should meet minimum standards for each of these characteristics.

Trueness to variety indicates that the seeds in a bag are of the variety stated on the label. Trueness is usually determined by the trustworthiness of the seller and the grower.

Germination potential and vigor are at their highest potential when the seed reaches physiological maturity. It is important that seed be harvested as soon as the seed pod matures and opens. After maturity, germination potential and vigor begin to deteriorate.

The rate of deterioration depends on the weather at maturaty, harvesting, conditioning, and storage practices. Seed can deteriorate rapidly if excessive damage occurs during harvesting and conditioning. 

If seed moisture levels are higher than those recommended for safe storage, the seeds should be allowed to dry out before storage. Seed storage conditions, which can also greatly affect deterioration rates, is discussed in more detail in a later section.

The quality of a seed lot can be improved by conditioning. Conditioning is used mainly to eliminate or reduce undesirable contaminants such as excess moisture, fungus, broken or split seeds.

Chemical Seed Treatment

Pathogens (organisms that cause disease) are often present on or in the seed. These organisms can cause diseases that destroy the seed or seedling. Chemicals applied to the seed can prevent or reduce the harmful attacks of many pathogens. These treatments include fungicides, or insecticides.

Fungicides are chemicals that are normally used to combat seed rots, surface molds, and seedling blights caused by seed- and soil-borne organisms. The ideal fungicide should be highly effective in controlling a specific disease organism. Insecticides are used to protect the seed from soil-borne insects and storage insects. Some treatments combine a fungicide with an insecticide to protect the seed against soil insects and disease organisms. The chemicals used should be harmless to seed, stable for long periods of time, easy to use, and low in toxicity to people and animals.

Systemic treatments are absorbed by the seed or developing seedling. These treatments protect both the seed and the young plants against diseases. For example, Baytan, a seed treatment that became commercially available in 1990, protects plants from many foliar diseases (including powdery mildew and rust) for up to eight weeks after seedling emergence.

Treatment chemicals are formulated as dusts, wettable powders, or liquids. A critical problem is obtaining uniform and adequate chemical coverage on each seed. Undertreatment is ineffective and overtreatment may injure the seed; therefore, accurate metering devices are necessary to dispense the proper amount of chemicals to a given quantity of seed.

Storage

Plumeria seeds are stored for some period of time. During this time, seed may deteriorate considerably. Good storage conditions can slow the rate of deterioration, but seed germination and vigor cannot be improved, regardless of the storage facilities.

Many costly storage problems actually begin during field exposure, harvesting, and conditioning of the seed. Excessive harvesting delays, mechanical injuries, and improper drying techniques, followed by poor storage conditions, can lead to rapid deterioration of seed germination and vigor.

Longevity of Seed in Storage

Different species of seed have different storage lives. The storage life of plumeria seeds ranges from several weeks (as in some tropical tree species, whose seed loses viability when dried) to several years (as in the case of the plumeria rubra and obtusa).

Typically, the storage life of plumeria seeds ranges from 1 to 5 years under ideal storage conditions. For example, plumeria seeds have been stored in ideal conditions for as long as 10 years. Plumeria species seeds are not normally carried over from one planting season to the next, while Plumeria rubra may be stored for several years if stored in the proper environment.

Seed Deterioration

The general pattern of seed deterioration is approximately 10% per year. Germination remains unchanged for a period of time during storage and then declines rapidly.

As seeds age, both germination and vigor decline, slowly at first and then more rapidly as the end of the seed’s useful life is reached. Vigor declines faster than germination.

Storage Conditions

Deterioration is slowed when seeds are stored under cool, dry conditions. Temperature and relative humidity should therefore be considered in planning for safe storage. A rough rule of thumb for safe seed storage is that the sum of the temperature (°F) and relative humidity should not exceed 100 units. For example, if the temperature is 60°F, the relative humidity should not be over 40 percent.

Seed moisture is the most important factor affecting seed longevity. Seeds adjust their moisture content until they are in equilibrium with the surrounding storage environment. Temperature and relative humidity affect the amount of moisture in the air and will influence the equilibrium moisture content of seed. A general rule is that each 1 percent decrease in moisture content of the seed nearly doubles the safe storage period. Thus it is important to put dry seed into storage and keep it dry.

Choosing Seeds

Seeds usually represents a relatively small part of the monetary cost of adding to your plumeria collection. Generally, the investment is less than 2 percent of a plumeria plant. Keep in mind that plumeria seeds do not produce true to its parents.

What factors should be considered when choosing seeds? Each quality characteristic is important. When you are buying seeds, you should establish an acceptable quality level for each characteristic.

The first step in choosing seeds is to select seeds from a known trustworthy seller. The importance of this decision cannot be overemphasized. The seller choice and the quality of the seed set an upper limit on yield and quality.

Seed dealers can use many aids to help customers select cultivars most likely of producing desired results in the seedlins. In addition, seed dealers should have a record of the parents and may have information on the lineage and seedling produced by the parents.

Varietal purity cannot be determined by laboratory tests. Purity can best be determined by keeping accurate records of production and relying on customer feed back. Your best assurance of varietal purity is through the use of seeds from named parents.

Seed parentage information can reveals some of the important characteristics to consider in choosing seeds. It is difficult, however, to establish definite standards for these characteristics. The general rule is to choose seeds from parents that have a history of producing quality seedlings and have a high germination rate (above 70 percent).

Germination and other soundness characteristics may vary from season to season, In many cases, seed growers have to accept the germination and soundness quality available. It takes careful searching to find the best seeds.

The reputation of a seed wholesaler or local producer is rated in relation to the quality of seed sold. Retail seed dealers should patronize those wholesalers who most consistently supply high-quality seed.

Dealing with companies that specialize in “bargain” seed often proves embarrassing and costly to the grower. The initial price of bargain seed may appear low, but it frequently becomes very costly by the time your seedling blooms. 

It is never a good practice to purchase or plant seeds if at least one parent’s information is not available. 

Planting Good Seed

Damage done to seed while planting is often overlooked. Seed size varies considerably within and between cultivars and even between varieties, so it is a good practice to check each seed for viability.

Soil preparation is also a part of good planting practices. A smooth, firm seedbed will permit uniform seeding at proper depth. Uniform planting depth is important for a uniform emergence pattern. Depth settings are especially critical with multiple-row planting equipment.

Analyzing Germination and Keep Records

Problems should be analyzed in three general areas:

• (1) the seed;
• (2) planting procedures, including such things as planting equipment and methods, use of chemicals, and fertilizer placement; and
• (3) weather and soil conditions at planting and shortly thereafter. In analyzing seedling establishment problems, all possibilities should be considered.

It is important that the facts in each case be gathered as soon as possible. It is difficult to obtain reliable information about seed problems after the plumeria died or has been sold.

Adverse weather and soil conditions can cause seed failures. Widely fluctuating temperatures or moisture levels during the seed germination period can affect seedling emergence. Under these conditions seed failures may be variable if the growing containers contains several soil types.

Soil insects and insecticides create germination problems in some crops. It is always a good practice to read the labels of any pesticide applied to the soil or young seedlings to prevent harmful side effects.

It is critical that information about all chemicals and rates be examined. Some soil fumigants require a waiting period before crop seed can be planted safely. If these intervals are not observed, slow germination and spotty emergence may result. Previously used soils may have herbicides used on previous crops may cause carryover injury. Always read pesticide labels carefully, and follow all directions.

Make it a practice to use quality seed tags for all seeds planted.

Summary

Seeds are complex, living organisms that play a very important role in producing new plumeria cultivars. High-quality seed lots of adapted varieties are the result of breeding, research, and careful management during all phases of seed production.

Consider seed selection as the first step in production. Cultivar selection and seed quality set the maximum performance level for all plumeria.

Anyone purchasing seed should:

• choose cultivars that are well suited your desired plumeria characteristics
• read the seed parents history carefully, avoiding seed with no parents specified and those with low germination
• plant the seeds when soil conditions are favorable, thus promoting rapid germination and seedling growth.