1. What is Seed Dormancy? Describe its Types and Causes.
Answer:
Seed dormancy is a condition in which seeds fail to germinate even under favorable environmental conditions. It is an adaptive mechanism that ensures seeds do not germinate prematurely, which could lead to unsuccessful growth or survival. Dormancy can be categorized into different types, each with its specific causes:
Types of Seed Dormancy:
- Physical Dormancy: This occurs when the seed coat is impermeable to water and gases. This barrier prevents the seed from taking in water necessary for germination.
- Physiological Dormancy: It involves biochemical factors, including hormonal imbalances, where the seed contains chemical inhibitors that prevent germination.
- Morphological Dormancy: In this type, the embryo of the seed is underdeveloped and needs time to mature before it can germinate.
- Combined Dormancy: A combination of both physical and physiological factors that inhibit germination.
Causes of Seed Dormancy:
- Seed coat imperviousness: A hard seed coat that prevents water absorption.
- Presence of growth inhibitors: Such as abscisic acid, which prevents the germination process.
- Undeveloped embryos: Seeds in which the embryo has not fully developed or matured.
- Environmental factors: Such as extreme temperatures, light, or moisture, which trigger dormancy.
2. How does the Seed Coat Contribute to Seed Dormancy?
Answer:
The seed coat plays a significant role in physical dormancy. It acts as a physical barrier that prevents water, gases, and sometimes light from reaching the seed’s internal tissues. This imperviousness is crucial for maintaining dormancy under harsh conditions.
Types of Seed Coats and Their Role in Dormancy:
- Hard Seed Coat: Some seeds, such as those of legumes, have thick, tough coats that need to be scarified (damaged) before they can absorb water and begin the germination process.
- Seed Coat Impermeability: The seed coat may not allow the passage of water or oxygen, essential for initiating germination. This impervious nature is often broken by mechanical abrasion, fire, or microbial activity in nature.
3. Explain the Role of Abscisic Acid in Seed Dormancy.
Answer:
Abscisic acid (ABA) is a key plant hormone that promotes seed dormancy by inhibiting the germination process. It works by:
- Inhibiting enzyme activation: ABA prevents the enzymes needed for breaking down stored food in the seed, thus blocking the energy supply needed for growth.
- Maintaining dormancy under stress: ABA levels rise in response to environmental stresses like drought or cold, ensuring that seeds do not germinate when conditions are unfavorable.
- Balancing with other hormones: ABA interacts with other hormones like gibberellins, which promote germination, creating a balance that regulates dormancy and germination.
4. Describe the Mechanism of Cold Stratification in Breaking Dormancy.
Answer:
Cold stratification is a dormancy-breaking process where seeds are exposed to cold temperatures for a period, typically several weeks or months, to simulate winter conditions. This process:
- Mimics natural conditions: Seeds from temperate or cold climates require cold temperatures to break dormancy, as it ensures they do not germinate prematurely in winter.
- Cold-induced biochemical changes: The low temperatures activate certain enzymes and reduce ABA levels, making the seed more responsive to other germination signals, such as warmth or moisture.
- Applications in horticulture: Cold stratification is used in seed germination techniques, especially for species like deciduous trees or some perennials.
5. How Does Gibberellin Help in Breaking Seed Dormancy?
Answer:
Gibberellin (GA) is a plant hormone that plays a central role in breaking seed dormancy by stimulating the processes that lead to germination. It helps by:
- Promoting enzyme activity: Gibberellins stimulate enzymes like amylases that break down starches in the seed, providing energy for the growing embryo.
- Overcoming the inhibitory effects of ABA: GA works antagonistically to abscisic acid, reducing ABA levels, and making the seed more susceptible to germination stimuli.
- Triggering embryo growth: It enhances the growth of the embryo by promoting cell division and elongation, which are necessary for sprouting.
6. What is Scarification, and How Does it Break Dormancy?
Answer:
Scarification is a technique used to break the physical dormancy of seeds that have a hard or thick seed coat. It involves mechanically or chemically damaging the seed coat to allow water, oxygen, and nutrients to enter, thereby promoting germination. Common methods include:
- Mechanical scarification: This involves scratching or nicking the seed coat using tools or sandpaper.
- Chemical scarification: Soaking seeds in acids or other chemicals can break down the tough seed coat.
- Natural scarification: Environmental factors like fire or passage through the digestive tract of animals can scarify seeds in nature.
7. Explain the Importance of Light in Seed Dormancy.
Answer:
Light plays a critical role in breaking dormancy for some seeds. Certain seeds require light to initiate germination, and this is known as photodormancy. The presence or absence of light can affect the germination process in the following ways:
- Light-sensitive seeds: Seeds like those of lettuce or certain grasses need light to trigger germination, making light a critical environmental cue for them.
- Red light exposure: In many seeds, exposure to red light (specific wavelengths) can break dormancy and start the germination process.
- Darkness for other seeds: On the contrary, some seeds require darkness to maintain dormancy and may not germinate in illuminated conditions.
8. What is the Role of Water in Breaking Seed Dormancy?
Answer:
Water is essential for seed germination, but it can also play a significant role in breaking dormancy, especially in seeds with physical dormancy. Water breaks dormancy in the following ways:
- Hydration of seeds: Seeds require water to activate metabolic processes, soften the seed coat, and allow enzymes to break down stored food reserves for growth.
- Activation of enzymatic processes: Water triggers the activation of enzymes that are involved in the breakdown of stored food like starches and proteins.
- Osmotic pressure: Water uptake creates osmotic pressure inside the seed, which can assist in cracking the seed coat or facilitating the movement of growth-promoting hormones.
9. Discuss the Effect of Temperature on Seed Dormancy.
Answer:
Temperature has a profound effect on seed dormancy. It can both induce and break dormancy depending on the seed type and environmental conditions:
- High temperatures: Some seeds require exposure to high temperatures, such as those experienced in forest fires, to break dormancy and trigger germination.
- Low temperatures: Many seeds, especially those from temperate climates, need a period of cold stratification before they can germinate.
- Optimal germination temperatures: Seeds typically have a specific range of temperatures in which they will germinate, and extreme temperatures outside this range can either prevent dormancy break or kill the seed.
10. How do Environmental Stressors Influence Seed Dormancy?
Answer:
Environmental stressors like drought, extreme temperatures, or low light levels can induce seed dormancy, ensuring that seeds only germinate when the environment is suitable for survival. Stress factors influence seed dormancy in the following ways:
- Drought conditions: Seeds in dry environments may enter dormancy to avoid germinating during periods of water scarcity.
- Temperature extremes: Seeds in cold or hot regions may remain dormant until they experience a temperature shift that signals more favorable growing conditions.
- Low light or shade: Some seeds require a specific amount of light exposure to trigger dormancy break, and lack of light may prolong dormancy.
11. What is Morphological Dormancy in Seeds?
Answer:
Morphological dormancy occurs when the embryo of the seed is not fully developed at the time of seed dispersal. This means that the seed is biologically incapable of germinating until the embryo matures. The process involves:
- Immature embryo development: The embryo must undergo growth or maturation within the seed before it can germinate.
- Environmental triggers: Once the embryo reaches maturity, environmental factors like temperature and moisture will trigger germination.
12. How Does Physiological Dormancy Differ from Physical Dormancy?
Answer:
Physiological dormancy and physical dormancy differ in their causes and mechanisms:
- Physiological dormancy: This type involves biochemical factors, such as high levels of abscisic acid (ABA) or low levels of gibberellins, which prevent the seed from germinating.
- Physical dormancy: This is caused by physical barriers, usually a hard seed coat, that prevent water and oxygen from entering the seed.
13. Explain the Role of Enzymes in Seed Germination and Dormancy Breaking.
Answer:
Enzymes play an essential role in both seed dormancy and germination:
- Breaking dormancy: Enzymes like amylases and proteases help in breaking down stored food reserves in the seed once dormancy is broken.
- Germination activation: During germination, enzymes degrade starches and proteins to release energy for the growing seedling.
- ABA inhibition: Enzymes involved in hormone metabolism, like those that degrade ABA, also help break dormancy.
14. Describe the Concept of Secondary Dormancy.
Answer:
Secondary dormancy is a phenomenon where seeds, after initially being exposed to favorable conditions for germination, enter dormancy again due to unfavorable environmental factors:
- Triggered by stress: Secondary dormancy can be triggered by unfavorable conditions such as drought, extreme temperatures, or poor light exposure.
- Protective mechanism: It prevents the seed from germinating when the conditions are no longer suitable for growth.
15. How do Animal Dispersed Seeds Overcome Dormancy?
Answer:
Animal-dispersed seeds often undergo dormancy-breaking processes during their passage through an animal’s digestive system:
- Scarification: The digestive process helps scarify the seed coat, breaking the dormancy.
- Temperature and moisture conditions: Some seeds need to be exposed to the moist, warm environment inside the animal’s stomach to trigger dormancy break.
16. What is the Role of Ethylene in Seed Dormancy?
Answer:
Ethylene is a gaseous plant hormone that can influence seed dormancy by promoting or inhibiting certain processes:
- Breaking dormancy: In some cases, ethylene can break dormancy by interacting with other hormones like gibberellins, promoting the germination process.
- Seedling growth: Ethylene also helps regulate seedling growth by influencing cell expansion and elongation during germination.
17. What Role Does Nitric Oxide Play in Seed Dormancy and Germination?
Answer:
Nitric oxide (NO) has been identified as a regulator of seed dormancy and germination. It plays several roles:
- Dormancy breaking: NO can interact with hormones like gibberellins and ABA to break seed dormancy.
- Promotion of germination: It helps trigger metabolic processes that lead to seedling emergence, including the activation of certain enzymes.
18. What Are the Agricultural Implications of Seed Dormancy and Its Breaking Mechanisms?
Answer:
Understanding seed dormancy and its breaking mechanisms has significant agricultural implications, such as:
- Optimizing germination: Farmers can use techniques like stratification, scarification, or hormone treatments to overcome dormancy and promote uniform germination.
- Improving crop yields: Controlling dormancy in crops can help achieve better seedling establishment, particularly in challenging environmental conditions.
- Seed storage: Proper knowledge of dormancy helps in seed storage and preservation, ensuring long-term viability.
19. How Do Environmental Cues like Fire and Smoke Break Seed Dormancy?
Answer:
Fire and smoke are important natural cues that can break seed dormancy, particularly in fire-adapted plants:
- Heat from fire: High temperatures from fire can break the seed coat or kill growth inhibitors, leading to dormancy break.
- Smoke exposure: Chemicals in smoke, particularly compounds like karrikinolide, can trigger germination in seeds that have experienced fire.
20. How Does Photodormancy Affect Seed Dormancy?
Answer:
Photodormancy refers to the requirement of light for breaking dormancy in certain seeds. Some seeds have light-sensitive mechanisms that:
- Initiate germination: Light, particularly red light, can trigger the germination of seeds like those of many grasses and herbs.
- Prevent premature germination: Seeds that require light to break dormancy will remain dormant in dark conditions, ensuring that they germinate in the right environmental context.
