🌱Seed: Definition, Characteristics, and Germination Testing

Why Seed Quality Matters in Agriculture

A farmer’s entire season depends on the quality of seed sown. Poor germination means replanting costs; genetic impurity means reduced yield; seed-borne diseases mean crop losses. Seed testing is the scientific process that ensures farmers receive seeds that are pure, viable, healthy, and true to type. Every seed lot sold in India must meet minimum standards set by the Indian Minimum Seed Certification Standards (IMSCS).

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Seed

• Seed is defined either Agronomically or Botanically. Understanding both definitions is important because they serve different contexts in agriculture and plant science.

• Botanically: “Seed is a fertilized ovule consisting of intact embryo, stored food and seed coat which is viable and has got the capacity to germinate.” This definition focuses on the biological origin of the seed — it must arise from the process of fertilization and contain a living embryo capable of developing into a new plant.

• Agronomically: “Seed is any material used for planting & propagation whether it is in the form of seed (grain) of food, fodder, fiber or vegetable crop or seedlings, tubers, bulbs, rhizomes, roots, cuttings, grafts or other vegetative propagated material.” This broader definition recognizes that farmers use many types of planting material beyond true botanical seeds. From an agronomic standpoint, anything that can reproduce a crop counts as “seed.”

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Characteristics of the Seed

A good quality seed must meet several important criteria before it is considered suitable for planting:

• It must be true to its type, meaning it should accurately represent the variety it claims to be.
• The seeds must be healthy, pure and free from all inert materials and weed seeds. Any contamination reduces the quality of the seed lot.
• The seeds must be viable, meaning they must possess the ability to germinate and produce a healthy plant.
• The germination capacity is up to the standard and it has been tested recently. Only recently tested seed gives a reliable measure of its planting potential.
• The seeds must be uniform in its texture, structure and look. Uniformity ensures consistent crop performance in the field.
• The seeds should be truthfully labelled and produced under all due cares. Correct labelling protects the farmer from unknowingly purchasing substandard seed.
• The seed must not be affected by any seed-borne disease. Diseased seed can spread pathogens across the field and even to new regions.
• It should be dry & not mouldy and should contain 12-14 per cent moisture. Excess moisture promotes fungal growth and reduces seed longevity.
• Seed health refers that the seed is free from any kind of disease propagule (spore, mycelium, etc.), either active or latent. Even dormant pathogens can become active under favorable conditions, so seed health testing is essential.

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Difference between Seed and Grain

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Key Seed Organizations & Acts

• National Seed Corporation (NSC) was established in 1963. It is the primary agency responsible for foundation seed production in India.
• National Seed Act was passed in 1966.
• Maharashtra was the first State to establish an official seed certification Agency during 1970 as a part of the Department of Agriculture, whereas Karnataka was the first State to establish the seed certification Agency as an autonomous body during 1974.
• At present 22 states in the country have their own Seed Certification Agencies established under the Seed Act 1966.

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Types of Seeds by Number of Cotyledons

• Monocotyledonous seeds: These seeds contain embryo, endosperm and only one cotyledon. Endosperm provides food to embryo. Examples: Wheat, Bajra, Maize, Rice & Grasses.
• Dicotyledonous seeds: These seeds contain embryo and two cotyledons. Here cotyledons provide food to embryo. Examples: Mango, Gram, Pea, Pulses.

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Types of Seeds Based on Light Requirement (Photoblastic Seeds)

Seeds respond differently to light during germination. This classification is important for understanding germination ecology and is frequently asked in exams.

• Positive Photoblastic Seeds: Require light at the time of germination. E.g. Tobacco, Lettuce.
• Negative Photoblastic Seeds: Germinate in absence of light (darkness). E.g. Onion, Amaranthus.
• Non-photoblastic Seeds: Can germinate in light and dark or both. Most crop seeds are non-photoblastic.

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Seed testing

• First Seed Testing Laboratory was established at IARI in 1961. This marked the beginning of systematic seed quality evaluation in India and laid the foundation for ensuring that only quality seeds reach the farmer.

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Seed Germination

• Germination is the emergence and development of seedling from the seed-embryo which is able to produce a normal plant under favourable condition. In simple terms, it is the process by which a dormant seed “wakes up” and begins to grow.

• Agronomically germination means the capacity of seeds to give rise to normal sprouts within a definite period fixed for each crop under optimum field conditions. This practical definition focuses on field outcomes rather than just laboratory performance.

• Changes during Germination — The germination process involves a well-defined sequence of physiological events:

  • Swelling of seed due to imbibition of water by osmosis. Water uptake is the very first step, softening the seed coat and activating internal biochemistry.
  • Initiation of physiological activities such as respiration & secretion of enzyme. The seed shifts from a dormant metabolic state to an active one.
  • Digestion of stored food by enzymes. Reserve starches, proteins, and fats are broken down into simpler forms the embryo can use.
  • Translocation & assimilation of soluble food. These digested nutrients are transported to the growing points of the embryo (radicle and plumule).

• When seed is placed in soil gets favorable conditions, radical grows vigorously & comes out through micropyle & fixes seed in the soil. Then either hypocotyl or epicotyl begins to grow, depending on the type of germination.

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Types of Germination

Hypogeal germination

• The cotyledons remain under the soil. The epicotyl elongates and pushes the plumule upward while the cotyledons stay below ground, continuing to nourish the seedling. E.g. Cereals, Pulses, Gram, Arhar, Lentil.

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Epigeal germination

• The cotyledons pushed above the soil surface. Here the hypocotyl elongates rapidly, lifting the cotyledons above ground where they may even photosynthesize briefly before the true leaves take over. E.g. [Oil seeds] Mustard, Tamarind, Sunflower, Castor, Onion.

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👉🏻 Four essential factors for germination of seeds:

• Capacity of seed to germinate — the seed must be alive and vigorous.
• Moisture — water triggers imbibition and activates enzymes.
• Temperature — each species has an optimum range for germination.
• Oxygen-supply (Air) — aerobic respiration is necessary for the energy demands of the growing embryo.

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👉🏻 Factors affecting the emergence of seedlings are:

These are field-level constraints that often cause germination under laboratory conditions to differ from germination in the field:

• Deep sowing or depth of soil cover over the seed — excessively deep planting makes it harder for the seedling to reach the surface.
• Inadequate or excess soil moisture — too little water prevents imbibition; too much can cause waterlogging and oxygen deprivation.
• Poor aeration — compacted or waterlogged soils restrict oxygen supply to the germinating seed.
• Higher soil compaction or impermeable layer of soil or outer material — physical barriers reduce seedling emergence.
• Low temperature — slows metabolic processes and delays germination.
• Rapid desiccation of soil — if the topsoil dries out quickly, the germinating seed may lose moisture and die.
• Longer time period — seeds left too long before favorable conditions arrive may lose viability.
• Injurious level of salt content — high salinity creates osmotic stress, making it difficult for seeds to absorb water.
• Poor seed capacity — inherently weak or aged seeds have lower vigor.
• Detrimental physical, chemical and biological soil conditions — these include soil pathogens, toxins, and unfavorable pH.

• In general, the mortality of seedlings is more in kharif than rabi. So, germination of Rabi crops is more. This is because kharif season brings higher temperatures, excess moisture, and greater pest and disease pressure, all of which reduce seedling survival.

• Germination tests are always carried out with the seeds counted at random from the pure fraction of seeds. In accordance with the I.S.T.A. rules, a minimum number of 400 seeds are required for germination test. Using 400 seeds (typically in four replicates of 100) provides statistical reliability.

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Xenia and Metaxenia

These are special effects of pollen on seed/fruit characteristics that appear in the same generation (not in the next generation as in normal inheritance):

• Xenia: The direct/visible effect of pollen on the endosperm and related tissues in the formation of a seed color. Since endosperm is a product of triple fusion (involving one sperm nucleus), the pollen parent can directly influence endosperm traits like colour and composition.

• Metaxenia: The effect of pollen on the maternal tissues of the fruit (pericarp). This is a rarer phenomenon where the pollen influences tissues that are entirely maternal in origin.

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Methods for Testing Germination

Several standardized methods are used to evaluate the germination potential of seeds. The choice of method depends on the size of the seed and the facilities available.

1. Petridish Method

• Two blotters or filter papers are placed on the bottom of the petridish and they are soaked with water. A convenient number of seeds, ranging from 10-20, are placed on the surface of water-soaked blotters in the petridish. The kind of seed, date and time of seed soaking are written on the glass-cover of petridish with the help of a glass marking pencil.
• Usually, the germination percentage is calculated in two counts and reported on the basis of the results of germination of about 100 to 200 seeds. This method is suitable for small seeds viz. tobacco, tomato, radish, knol-khol, cabbage, cauliflower, mustard, lettuce, brinjal, chillies, etc.

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2. Rolled Towel Method

• Two wet towels are placed on a smooth table top. The appropriate number of seeds are placed on the upper surface of the towels and are covered by two wet towels. A fold is made of the bottom of the towel to prevent the seeds from falling out. The towels are then rolled from right to left.
• The full informations regarding the test i.e. the kind of seed, lot number, date and time of seed soaking are noted on the roll with the help of an indelible pencil. This method is suitable for comparatively large sized seeds viz. maize, wheat, pea and gram.

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3. Folder Paper Towel Method

• Two wet paper towels (big bamboo sheets), specially made for germination test, are placed on the working table-top. The surface of paper-towel is marked into two halves. The right half of the towel surface is planted with seeds and the left side half is folded to cover the right half and informations regarding the seed sample, date and time of seed soaking are written on the fold with an indelible pencil.
• Water is frequently sprinkled on these towels and observations on germinating seedlings and their numbers are taken periodically. This method of germination test is convenient for relatively large sized seeds.

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4. Sand Method

• Seeds are planted in uniform layer of moist sand and then covered to depth of 1-2 cm with sand. Substrates for germination are moistened with 0.2 per cent KNO3 solution. The use of potassium nitrate helps break any residual dormancy and promotes uniform germination.

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5. Rags or Gunny Sacs Method

• The seeds are simply wrapped in a moistened rag or gunny sac, which is then rolled and tied loosely in the form of bundle. This is now kept at a proper temperature on a rack or convenient place for germination.
• The rolls are opened after a fixed period of time and the number of germinated seeds is counted. This is a simple, low-cost method especially useful in rural settings where laboratory equipment may not be available.

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6. Mechanical Method

• In this method, apparatus like Germinators are used. Cabinets of the incubator type with thermostatically controlled temperature may be used for the purpose. These provide the most controlled and reproducible conditions.
• In these cabinets the seeds may be placed evenly on moist filter paper in petri dishes or between filter papers kept moist by folds of moistened flannel or large seeds be sown in dishes containing sand or fine soil. Adequate water is applied needed by the help of a wash-bottle or sprayer.

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Seed Purity

• The purity of seed denotes the real percentage of desirable seed from a lot of with several impurities i.e. seeds of other varieties, other crop seed, defective seeds, weeds seeds, inert matter, sand straw, stones, soil and iron particles, pebbles etc. A high purity percentage means the seed lot is clean and predominantly composed of the intended crop seed.

• Dockage is the impurity percentage of seed. It represents everything in the seed lot that is not the desired pure seed.

• Genetic purity is tested by Field Plot test (also called grow-out test). In this test, seeds are grown out in the field and the resulting plants are inspected for trueness-to-type.

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Real Value of Seed

• The real value of seed represents its seed quality in terms of purity and germination. It combines both parameters into a single, meaningful figure that indicates how much of the seed lot will actually produce plants.
• It can be evaluated by multiplying the purity percentage and germination percentage of a seed sample and dividing the product by 100.

• Real Value (RV) is expressed in percentage known as Utility percentage of seed. This is one of the most practical indicators of seed quality used in the seed trade.

• Conditions affecting the real value of seeds are the method of production, the method of handling and the method of storage.

Summary Cheat Sheet