Model Answer
0 min readIntroduction
Seed dormancy is a crucial adaptation in plants, ensuring survival under unfavorable conditions. It's the condition where seeds fail to germinate even when provided with seemingly suitable environmental factors like moisture, oxygen, and temperature. While dormancy prevents premature germination, "hard seeds" represent a related but distinct phenomenon. These seeds possess a tough seed coat that physically restricts water imbibition and gas exchange, hindering germination. Understanding the physiological basis of both is vital for efficient agricultural practices and conservation efforts, particularly given the impact of climate change on seed viability.
Defining Dormant and Hard Seeds
Dormant seeds are viable seeds that are prevented from germinating due to internal physiological factors, even under favorable conditions. These factors can be hormonal, morphological, or biochemical. Hard seeds, on the other hand, are non-dormant but are physically prevented from germination due to a very hard, impermeable seed coat.
Physiological Basis of Seed Dormancy
The physiological basis of seed dormancy is complex and often involves a combination of factors:
- Hormonal Imbalance: A high abscisic acid (ABA) to gibberellic acid (GA) ratio often induces dormancy. ABA inhibits germination, while GA promotes it.
- Embryo Immaturity: In some species, the embryo isn't fully developed at the time of seed dispersal, requiring a period of after-ripening for germination.
- Physical Barriers: A water-impermeable seed coat (as in hard seeds) restricts water uptake necessary for germination.
- Chemical Inhibitors: The seed coat or surrounding tissues may contain chemical inhibitors that prevent germination.
- Light Sensitivity: Some seeds require light exposure for germination (positive photoblastic), while others are inhibited by light (negative photoblastic).
Methods to Overcome Seed Dormancy
Breaking dormancy requires addressing the underlying physiological factors. Methods can be broadly categorized:
Physical Methods
- Scarification: This involves weakening the seed coat to allow water imbibition. Techniques include:
- Mechanical Scarification: Abrading the seed coat with sand or grit.
- Hot Water Treatment: Soaking seeds in hot water (typically 80-90°C) for a specific duration.
- Fire Scarification: Exposing seeds to fire, common in some plant species adapted to wildfires.
- Stratification: Exposing seeds to a period of cold, moist conditions to break dormancy. This mimics winter conditions and is crucial for many temperate species.
Chemical Methods
- Gibberellic Acid (GA) Treatment: Applying GA externally can overcome ABA-induced dormancy.
- Kinetin/Cytokinin Treatment: These plant growth regulators can also stimulate germination in some dormant seeds.
- Ethanol Treatment: Brief exposure to ethanol can disrupt the seed coat and promote germination.
Other Methods
- After-Ripening: Allowing seeds to age under dry conditions can sometimes break dormancy.
- Alternating Temperature: Exposing seeds to alternating periods of high and low temperatures can stimulate germination.
| Dormancy Type | Cause | Overcoming Methods |
|---|---|---|
| Physiological Dormancy | Hormonal imbalance (ABA/GA), embryo immaturity, chemical inhibitors | GA treatment, after-ripening, alternating temperatures |
| Physical Dormancy (Hard Seeds) | Impermeable seed coat | Scarification (mechanical, hot water, fire), ethanol treatment |
The National Food Security Mission (NFSM) in India, for example, sometimes incorporates seed treatment techniques to address dormancy issues in pulses and oilseeds to improve crop yields.
Conclusion
In conclusion, seed dormancy and hard seeds represent distinct challenges to germination. Understanding the underlying physiological mechanisms—hormonal control, physical barriers, and chemical inhibitors—is crucial for developing effective strategies to overcome dormancy. A combination of physical, chemical, and environmental manipulations can be employed, tailored to the specific species and the nature of the dormancy. Further research focusing on the genetic control of dormancy and the development of more efficient breaking methods remains essential for enhancing agricultural productivity and biodiversity conservation.
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.