Model Answer
0 min readIntroduction
Seed dormancy is a crucial adaptation that prevents germination under unfavorable conditions, ensuring seedling survival. It is defined as the inability of a viable seed to germinate even when environmental conditions such as adequate water, oxygen, and suitable temperature are present. This phenomenon is widespread across plant species and plays a vital role in synchronizing germination with optimal growth periods, preventing overcrowding, and facilitating seed dispersal. Understanding and overcoming seed dormancy is essential for successful plant propagation, conservation efforts, and agricultural practices.
Understanding Seed Dormancy
Seed dormancy is a complex trait influenced by both internal and external factors. It can be categorized into several types:
Types of Seed Dormancy
- Physical Dormancy (Hard Seed Coat): This is caused by a mechanically resistant seed coat that restricts water uptake and gas exchange. Common in plants of the Fabaceae family (legumes).
- Physiological Dormancy: This is due to internal factors within the embryo, often involving plant hormones like abscisic acid (ABA) and gibberellins (GA). It requires specific environmental cues to break.
- Morphological Dormancy: The embryo is underdeveloped at the time of seed dispersal and requires time to mature.
- Combinational Dormancy: A combination of two or more types of dormancy.
- Morphophysiological Dormancy: Combination of morphological immaturity and physiological block.
Methods to Overcome Seed Dormancy
Various techniques are employed to break seed dormancy, depending on the type of dormancy present:
1. Scarification
This involves physically or chemically weakening the seed coat to allow water penetration.
- Mechanical Scarification: Abrading the seed coat with sandpaper, filing, or shaking with abrasive materials. Used for seeds with hard seed coats like Acacia and Lupinus.
- Chemical Scarification: Treating seeds with concentrated sulfuric acid or other chemicals to dissolve part of the seed coat. Requires careful control to avoid damaging the embryo.
2. Stratification
This involves exposing seeds to a period of cold, moist conditions to break physiological dormancy. It mimics natural winter conditions.
- Warm Stratification: Some seeds require a period of warm, moist conditions followed by cold stratification.
- Double Stratification: Alternating periods of warm and cold stratification.
Example: Apple seeds require a prolonged period of cold stratification (typically 60-90 days) to break dormancy.
3. Hormonal Treatments
Applying plant hormones, particularly gibberellic acid (GA), can overcome physiological dormancy by promoting germination.
- GA promotes the synthesis of enzymes that mobilize stored food reserves in the seed.
- Used for seeds of Lettuce and other light-sensitive seeds.
4. Light Exposure (Photodormancy)
Some seeds require exposure to light to germinate, while others are inhibited by light.
- Red light promotes germination in many small-seeded species.
- Far-red light inhibits germination.
Example: Grandiflora seeds require light for germination.
5. After-ripening
This is a natural process where seeds are stored under dry conditions for a period of time, allowing physiological changes to occur that break dormancy.
6. Leaching
Washing seeds to remove germination inhibitors present in the seed coat or surrounding tissues. Common in some desert plant species.
7. Smoke Treatment
Exposure to smoke can stimulate germination in some species, mimicking the effects of wildfires. Smoke contains chemicals like karrikinolide that act as germination cues.
Table Summarizing Dormancy Types and Breaking Methods
| Dormancy Type | Cause | Breaking Method | Example Plant |
|---|---|---|---|
| Physical | Impermeable seed coat | Scarification (mechanical/chemical) | Lotus corniculatus |
| Physiological | Internal hormonal balance (ABA/GA) | Stratification, Hormonal treatment | Malus domestica (Apple) |
| Morphological | Immature embryo | After-ripening | Asarum europaeum |
| Photodormancy | Light sensitivity | Light exposure/darkness | Lactuca sativa (Lettuce) |
Conclusion
Overcoming seed dormancy is a critical aspect of plant propagation and conservation. The appropriate method depends on the specific type of dormancy exhibited by the seed. Understanding the underlying physiological and morphological mechanisms governing dormancy allows for the development of effective strategies to enhance germination rates and ensure successful plant establishment. Continued research into seed dormancy will be crucial for addressing challenges related to food security, biodiversity conservation, and ecological restoration in a changing climate.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.