Seed Technology - I

 1. Seed and it’s Structure

 

A true seed is defined as a fertilized mature ovule that possesses embryonic plant, stored material, and a protective coat or coats. Seed is the reproductive structure characteristic of all phanerogams.  A seed is an important part of a flowering plant. They give rise to a new plant. They may be of different shapes, colours and sizes. They may be round, wrinkled, winged or hairy. They are in a dormant condition until they receive adequate sunlight, water, and soil.

On the basis of the number of cotyledons in the embryo the angiosperms have been divided into two large groups:

1. Dicotyledons, having embryos with two cotyledons, and

2. Monocotyledons, with only one cotyledon.

Structure of a Dicotyledonous Seed

Unlike monocotyledonous seed, a dicotyledonous seed, as the name suggests, has two cotyledons. It has the following parts:

·         Seed coat: This is the outermost covering of a seed. The seed coat has two layers, the outer testa and the inner tegmen.

·         Hilum: The hilum is a scar on the seed coat through which the developing seed was attached to the fruit.

·         Micropyle: It is a small pore present above the hilum.

·         Embryo: It consists of an embryonal axis and two cotyledons.

·         Cotyledons: These are often fleshy and full of reserve food materials.

·         Radicle and plumule: They are present at the two ends of the embryonal axis.

·         Endosperm: In some seeds such as castor, the endosperm formed as a result of double fertilisation, is a food storing tissue. In plants such as bean, gram and pea, the endosperm is not present in the matured seed. They are known as non-endospermous.

Structure of a Monocotyledonous Seed

A Monocotyledonous seed, as the name suggests, has only one cotyledon. There is only one outer layering of the seed coat. A seed has the following parts:

·         Seed Coat: In the seed of cereals such as maize, the seed coat is membranous and generally fused with the fruit wall, called Hull.

·         Endosperm: The endosperm is bulky and stores food. Generally, monocotyledonous seeds are endospermic but some as in orchids are non-endospermic.

·         Aleuron layer: The outer covering of endosperm separates the embryo by a proteinous layer called aleurone layer.

·         Embryo: The embryo is small and situated in a groove at one end of the endosperm.

·         Scutellum: This is one large and shield-shaped cotyledon.

·         Embryonal axis: Plumule and radicle are the two ends.

·         Coleoptile and coleorhiza: The plumule and radicle are enclosed in sheaths. They are coleoptile and coleorhiza.

 

2. Importance of Seeds

 

Seeds are incredibly important for several reasons:

·         Reproduction: They are the primary way for flowering plants to reproduce.

·         Dispersal: Seeds can travel long distances by wind, water, or animals, allowing plants to colonize new habitats.

·         Dormancy: Seeds can lie dormant for extended periods, which allows plants to survive through unfavorable conditions such as drought or cold weather.

·         Food source: Seeds are a vital food source for humans and animals.

·         Agriculture: Seeds are essential for agriculture, allowing us to grow crops for food, fiber, and fuel.

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3. Seed Vs Grain

 

	Seed	Grain
Definition	A self-contained unit that contains an embryo of a new plant	A type of fruit produced by grasses
Function	Reproduction	Food source
Structure	Can have various structures depending on the plant	Typically small, hard, and dry
Examples	Sunflower seeds, pumpkin seeds, bean seeds	Wheat, rice, barley, corn (maize), oats

 

4. Role and goals of seed technology

 

Seed technology plays a vital role in modern agriculture by focusing on improving seeds for better crop production. Seed technology strives to create a reliable and sustainable system for seed production and distribution, ultimately contributing to a more secure and productive agricultural sector.

 

Role:

         i.            Seed improvement: Seed technology uses various methods to develop seeds with desirable traits. This can involve conventional breeding techniques or even genetic modification to create varieties with:

·         Higher yields

·         Improved resistance to pests and diseases

·         Better tolerance to environmental stresses like drought or salinity

·         Enhanced nutritional value

       ii.            Seed production and management: Seed technology ensures efficient methods for producing large quantities of these improved seeds. This involves:

·         Maintaining genetic purity through proper seed classes (breeder, foundation, certified)

·         Seed processing techniques like cleaning, treating, and coating for better storage and germination

·         Developing efficient storage methods to maintain seed viability

Goals:

·         Increased agricultural productivity: By providing farmers with high-quality seeds that produce higher yields, seed technology helps to meet the growing demand for food.

·         Improved food security: Reliable access to good quality seeds is essential for stable food production, especially in areas facing challenges like climate change.

·         Enhanced food quality: Seed technology can contribute to developing crops with higher nutritional content or better taste.

·         Reduced reliance on external inputs: Seeds with improved resistance to pests and diseases can help farmers reduce their dependence on pesticides and herbicides.

·         Making agriculture more sustainable: Techniques like seed priming can improve germination rates and establishment, leading to more efficient use of water and other resources

 
5. Characteristics of Quality Seed material
 

Seeds are the foundation of a successful harvest. But not all seeds are created equal. Using high-quality seed material is crucial for maximizing your crop yield and ensuring healthy plant growth.

The characteristics that define good quality seeds are:

a)      Genetic Purity: This refers to the seeds being true to type, meaning they carry the desired genetic traits of the chosen variety. Consistent genetic makeup translates to predictable plant characteristics, like disease resistance or high yields.

b)      High Germination Rate: A good seed lot boasts a high percentage of seeds that can germinate and sprout into healthy seedlings. This ensures a strong stand of plants and avoids the need for excessive replanting.

c)      Physical Purity: Free from weed seeds, other crop seeds, and inert matter like dirt or debris. A clean seed lot minimizes competition from unwanted plants and ensures efficient use of planting resources.

d)      Seed Vigor:  This goes beyond germination. Vigorous seeds exhibit strong seedling growth, rapid emergence, and the ability to withstand environmental stresses. Vigorous plants are more competitive and ultimately produce better yields.

e)      Good Physical Health: Quality seeds are free from visible signs of damage, disease, or insect infestation. Healthy seeds have the best chance of developing into strong, disease-resistant plants.

f)        Appropriate Moisture Content: Seed moisture content plays a crucial role in storage life and germination. Seeds with the optimal moisture level for the specific variety will store well and germinate uniformly when planted.

 

6. Seed Dormancy – Causes

 

Definition: Seed dormancy is the temporary inability of a viable seed to germinate even when environmental conditions are favourable for growth.

 

Causes of Seed Dormancy:

There are two main categories of factors that can cause seed dormancy:

1.       External (Exogenous) Dormancy: This type of dormancy is caused by factors outside the seed itself, typically related to the environment.

·         Impermeable Seed Coat: Some seeds have tough, water-resistant coats that prevent water uptake, a crucial trigger for germination. This can be broken down by natural processes like scarification (abrasion from passing through an animal's digestive tract or exposure to freezing temperatures) or scarification techniques used in cultivation. (Examples: Beans, legumes)

·         Light Requirements: Certain seeds require specific light conditions (darkness or sunlight) to germinate. This prevents them from sprouting under leaf litter or buried too deep in the soil. (Examples: Lettuce, tobacco)

·         Temperature Requirements: Some seeds need to experience a period of cold temperatures (stratification) to break dormancy, mimicking the winter season. This ensures germination happens during a more favourable time for seedling growth. (Examples: Apples, peaches)

2.       Internal (Endogenous) Dormancy: This dormancy originates within the seed itself, due to its internal makeup.

·         Immature Embryo: In some species, the embryo inside the seed might not be fully developed at dispersal. It requires a period of after-ripening, even under favourable conditions, to mature and become capable of germination. (Examples: Oaks, avocados)

·         Presence of Chemical Inhibitors: Certain seeds contain natural growth inhibitors that prevent germination until specific environmental cues trigger their breakdown. Examples include some grasses and shrubs.

7. Breaking of Seed Dormancy

 

Seeds sometimes enter a period of dormancy, a natural adaptation that prevents germination during unfavorable conditions. Luckily, we have methods to overcome seed dormancy and encourage germination, ensuring a successful planting season. Here are some common methods with examples:

 

1. Stratification: This mimics the natural exposure to cold and moist conditions that some seeds require.

Example: Many tree seeds (oaks, maples) and some perennials (lavender, poppies) benefit from cold stratification. Seeds are stored in moist sand or potting mix for several weeks at chilly temperatures (around 4°C).

2. Scarification: This weakens or breaks the seed coat, allowing water and oxygen to penetrate and stimulate germination.

Example: Some seeds with hard coats (honeysuckle, canna lily) benefit from scarification. Techniques include nicking the seed coat with a nail file or soaking in hot water for a brief period.

3. Gibberellic Acid (GA): This plant hormone can stimulate germination in certain seeds.

Example: Some seeds with hormonal dormancy (clematis) might require soaking in a weak GA solution to trigger germination.

4. Light Exposure:  Light can be a critical factor for some seeds.

Example: Light-sensitive seeds like lettuce or some wildflowers benefit from planting them shallowly and not covering them completely with soil.

5. Hot Water Treatment: Soaking seeds in hot water can break down dormancy mechanisms in some species.

Example: Seeds with water-permeable inhibitors (tomatoes, peppers) might benefit from a brief soak in hot water (around 50°C) before planting.