Why Seed Dormancy Matters: Plant Survival and Human Benefits
Seed dormancy is a condition in which viable seeds do not germinate even when the environment seems favourable (adequate moisture, temperature, and oxygen). It ensures that seeds wait for the right moment to begin growing into new plants, helping them survive in changing or harsh conditions. In this article, we will discuss what is seed dormancy, the types of seed dormancy with examples, the causes of seed dormancy, and various ways of breaking seed dormancy. We will also explore the importance of sustaining plant populations.
What is Seed Dormancy?
Seed dormancy refers to the state in which a seed is unable to germinate under ideal conditions. This mechanism prevents seeds from sprouting at inappropriate times, increasing their chance of survival. Dormancy can last from a few days to several years, depending on the plant species and environmental factors.
Also Read: Monocots and Dicots
Causes of Seed Dormancy
A seed may remain dormant due to a variety of reasons, which include:
Light Requirement: Some seeds need exposure to light or darkness before they can germinate.
Temperature: Extremely low or high temperatures can halt seed germination.
Hard Seed Coat: A thick, rigid seed coat can stop water or gases from entering the seed.
After-ripening Period: Some seeds are shed before their embryos are fully mature and need more time to complete internal development.
Germination Inhibitors: Certain chemicals within the seed (or its coat) can prevent germination until they are leached out or neutralised.
Immature Embryo: If the embryo has not fully developed, the seed remains dormant.
Impermeability to Water: Some coats do not allow water to enter.
Impermeability to Oxygen: The seed may fail to get sufficient oxygen for respiration.
Mechanically Resistant Seed Coat: Tough coats may restrict embryo expansion.
Presence of Concentrated Solutes: High solute levels within the seed can cause dormancy until they are diluted.
Types of Seed Dormancy with Examples
Seed dormancy can be broadly classified into three major types:
Innate Dormancy
Seeds are incapable of germination even if all ideal conditions are provided.
Example: Many Orchid seeds are released with immature embryos, requiring more time or specific fungal associations before they can germinate.
Enforced Dormancy
Seeds cannot germinate because of unfavourable external conditions such as limited moisture, incorrect temperature, or lack of oxygen.
Example: Seeds of certain desert plants that await rainfall for adequate moisture.
Induced Dormancy
Occurs when seeds have absorbed water but are then subjected to conditions highly unsuitable for germination, causing them to revert to a dormant state.
Example: Some cereal seeds can enter induced dormancy if they experience prolonged cold and damp conditions after partial imbibition.
Breaking Seed Dormancy
Breaking seed dormancy is essential for seed germination and plant propagation. It can be done through natural or artificial methods:
Natural Methods
Favourable environmental conditions: Adequate moisture and temperature can naturally break dormancy.
Weathering of seed coats: In nature, seed coats become permeable when exposed to soil microbes, temperature changes, or when passing through the digestive tracts of animals.
Completion of after-ripening: The embryo matures inside the seed over time.
Leaching of inhibitors: Rainwater or irrigation can wash away germination inhibitors present in the seed coat.
Inactivation of inhibitors: Exposure to cold, heat, or light can neutralise chemicals that prevent germination.
Production of growth hormones: Hormones like gibberellins can counteract the effect of inhibitors.
Artificial Methods
Scarification: Physically or chemically weakening the seed coat (e.g., by filing, chipping, or using abrasive material).
Hot water treatment: Immersing seeds in hot water to remove waxes and surface inhibitors.
Temperature treatments: Placing seeds in a controlled environment (cold or heat) to break dormancy.
Hydraulic pressure: Subjecting seeds to high pressure for a short time to weaken tough seed coats.
Acid treatment: Treating seeds with concentrated sulphuric acid to dissolve part of the coat.
Importance of Seed Dormancy
Below are 5 importance of seed dormancy (plus a bonus point):
Survival in harsh conditions: Dormant seeds withstand extreme climates (cold winters or hot summers) and survive droughts.
Synchronised germination: Seeds can wait for suitable conditions, leading to uniform growth when favourable weather arrives.
Seed dispersal: Delayed germination gives seeds more time for wind, water, or animal dispersal.
Extended viability: Seeds remain viable in the soil for many years, providing a continuous supply of new plants.
Avoidance of competition: By spreading out germination over time, seeds reduce overcrowding among seedlings.
Storage for future use: Dormant seeds can be stored safely and used by humans and animals at a later date.
Quick Quiz
Test your knowledge with the following questions:
1. Which type of dormancy occurs when seeds have absorbed water but still fail to germinate due to unfavourable conditions?
A. Innate dormancy
B. Enforced dormancy
C. Induced dormancy
D. None of the above
2. Which of the following is NOT a common cause of seed dormancy?
A. Immature embryo
B. Excess oxygen in the soil
C. Hard seed coat
D. Presence of germination inhibitors
3. Which artificial method of breaking seed dormancy involves using abrasive surfaces or acid to remove tough seed coats?
A. Stratification
B. Scarification
C. Induced germination
D. None of the above
Quiz Answers
C. Induced dormancy
B. Excess oxygen in the soil
B. Scarification
Related Topics
FAQs on Seed Dormancy in Biology: Causes, Types, and How to Overcome It
1. What is seed dormancy and what are its main types?
Seed dormancy is a natural condition where a viable seed is prevented from germinating, even when provided with favourable environmental conditions like water, oxygen, and temperature. It is a survival mechanism. The main types are:
- Physical Dormancy (Seed Coat Dormancy): Caused by an impermeable seed coat that prevents water and gas exchange.
- Physiological Dormancy: The most common type, where the embryo itself has a metabolic block. It often requires specific hormonal changes or environmental cues like cold treatment to be broken.
- Morphological Dormancy: Occurs when the embryo is underdeveloped or undifferentiated when the seed is shed from the parent plant.
2. What are the primary causes of seed dormancy in plants?
The primary causes of seed dormancy are factors that can be either internal (endogenous) or related to the seed coat (exogenous). Key causes include:
- Hard and Impermeable Seed Coat: Physically restricts the entry of water and oxygen or prevents the embryo from expanding.
- Presence of Chemical Inhibitors: Hormones like Abscisic Acid (ABA), phenolic acids, and coumarin within the seed can inhibit the germination process.
- Immature Embryo: The seed is dispersed before its embryo is fully developed and requires a period of rest to mature.
- Specific Light or Temperature Requirements: Some seeds need exposure to specific light wavelengths or periods of cold (stratification) or heat to trigger germination.
3. Which plant hormone is primarily responsible for inducing and maintaining seed dormancy?
Abscisic acid (ABA) is the plant hormone primarily responsible for inducing and maintaining seed dormancy. It acts as a growth inhibitor, counteracting the effects of growth-promoting hormones like gibberellins. High levels of ABA accumulate during seed maturation, signaling the embryo to enter a dormant state and preventing premature germination under unfavourable conditions.
4. What are the common methods used to overcome or break seed dormancy?
Methods to break seed dormancy are designed to overcome its specific cause. Common techniques include:
- Scarification: This involves scratching, chipping, or weakening the hard seed coat to allow water and gases to enter. It can be done mechanically (with sandpaper), chemically (with acids), or with hot water.
- Stratification: This method involves exposing seeds to a period of moist-cold or moist-warm conditions to break physiological dormancy. It mimics the natural wintering process.
- Leaching: Soaking seeds in water to wash away chemical inhibitors present in the seed coat.
- Light Exposure: Providing specific light conditions (e.g., red light) required by certain light-sensitive seeds.
5. What is the difference between scarification and stratification?
Scarification and stratification are two distinct methods for breaking seed dormancy. Scarification targets physical dormancy by physically or chemically breaking the hard, impermeable seed coat. In contrast, stratification targets physiological dormancy by providing a period of moist-cold or moist-warm treatment, which triggers the necessary internal hormonal changes in the embryo to allow germination.
6. How is seed dormancy different from seed quiescence?
Seed dormancy is different from seed quiescence based on the seed's internal state. A dormant seed will not germinate even when external conditions are ideal because of an internal block (e.g., hormonal inhibitors, immature embryo). A quiescent seed, on the other hand, is a non-dormant, viable seed that is simply waiting for favourable external conditions (like water or warmth). It will germinate immediately once these conditions are met.
7. What is the ecological importance of seed dormancy for plant survival?
Seed dormancy is a crucial survival strategy for plants. Its ecological importance includes:
- Timing Germination: It ensures that seeds only germinate when environmental conditions are most favourable for seedling survival (e.g., after a harsh winter or a dry season).
- Dispersal Over Time: It allows seeds to remain viable in the soil seed bank for many years, creating a buffer against catastrophic events that might wipe out a single year's population.
- Avoiding Competition: By staggering germination over time, plants can reduce competition for resources among their own offspring.
8. Can seed dormancy be beneficial for agriculture and horticulture?
Yes, seed dormancy is highly beneficial in agriculture and horticulture. Controlled dormancy allows for the long-term storage of seeds without them losing viability or germinating prematurely in storage. It also prevents seeds from sprouting in the field before harvest (a phenomenon called vivipary) during unseasonal rains, thus protecting crop yield and quality.
9. Why don't seeds of mangrove plants typically exhibit dormancy?
Seeds of many mangrove plants, like Rhizophora, do not exhibit dormancy because they show a special phenomenon called vivipary. Vivipary is the germination of a seed while it is still attached to the parent plant. The embryo grows out of the seed coat and fruit, forming a propagule. This is an adaptation to their saline, water-logged environment, ensuring the seedling has a higher chance of successfully establishing itself when it falls into the soft mud.
10. Do all seeds from the same plant have the same level of dormancy?
Not necessarily. It is common for seeds from a single plant to exhibit varying degrees of dormancy. This phenomenon, known as heteroblastic dormancy, is an evolutionary strategy. By having some seeds that germinate quickly and others that remain dormant for longer, the plant diversifies its risk. This ensures that even if one cohort of seedlings perishes due to an unexpected event like a frost or drought, there are still dormant seeds left in the soil to germinate in a future, more favourable season.
