Seed Dormancy

1. Introduction

• Seed dormancy is an important adaptive mechanism that prevents seeds from germinating until conditions are suitable for seedling survival.
  It helps plants to avoid unfavorable seasons (such as drought or cold) and ensures that germination occurs only under optimal environmental conditions.
• Understanding dormancy is essential in seed technology, crop production, and storage management, as it directly influences seed germination, vigor, and field establishment.

2. Definition

• Seed dormancy is a condition in which viable seeds fail to germinate even under favorable environmental conditions such as adequate moisture, temperature, oxygen, and light.
• In other words, the seed possesses life and viability, but remains metabolically inactive or quiescent due to internal or external inhibitory factors.

3. Importance of Seed Dormancy

• Ensures survival: Prevents germination during adverse climatic conditions (e.g., frost, drought).
• Synchronizes germination: Ensures that seeds germinate when environmental conditions are favorable.
• Promotes seed dispersal: Allows seeds to spread over a longer period and distance.
• Prevents pre-harvest sprouting: Especially in cereals during humid conditions.
• Aids in crop establishment: Uniform germination leads to even crop stands.
• Contributes to seed longevity: Dormant seeds often store better over time.

4. Causes / Types of Seed Dormancy

Seed dormancy is caused by several physical, physiological, and biochemical factors.
It is broadly classified into three major types: Exogenous, Endogenous, and Combinational Dormancy.

4.1 Exogenous Dormancy (Seed Coat or External Dormancy)

Dormancy caused by external structures such as seed coat, pericarp, or fruit wall.

• Physical Dormancy: Caused by impermeable seed coat which prevents entry of water and gases. Common in Leguminosae, Malvaceae, and Convolvulaceae Example: Crotalaria, Cassia, Lathyrus.
• Mechanical Dormancy: Hard seed coat mechanically restricts the expansion of embryo. Example: Canna indica.
• Chemical Dormancy: Presence of germination inhibitors like abscisic acid (ABA), phenolic compounds, and coumarins in fruit or seed coat. Example: Tomato, Citrus, Apple.

4.2 Endogenous Dormancy (Internal Dormancy)

Dormancy caused by factors within the embryo or its metabolism.

• Morphological Dormancy: Embryo is underdeveloped or immature at the time of seed dispersal. Requires additional time to mature before germination. Example: Ginkgo, Orchids, Datura.
• Physiological Dormancy; Embryo is fully developed but metabolically inactive due to internal inhibitors. Common in temperate fruits and cereals. Example: Wheat, Apple, Barley.
• Morphophysiological Dormancy: Combination of underdeveloped embryo and physiological inhibition. Example: Ranunculus, Celery.
• Combinational Dormancy: Caused by a combination of physical (seed coat) and physiological (embryo) Example: Nelumbo nucifera (Lotus), Rosa spp.

5. Mechanisms Responsible for Dormancy

6. Methods of Breaking Seed Dormancy

Dormancy can be broken using mechanical, physical, chemical, or biological treatments.
These treatments are used to stimulate germination and obtain uniform seedling emergence.

6.1 Mechanical Methods

Scarification: Scratching, nicking, or abrading the hard seed coat using sandpaper, file, or knife to allow water penetration. Used in Crotalaria, Cassia, Albizia.

6.2 Physical Methods

6.3 Chemical Methods

Biological and Environmental Methods

• After-ripening: Storing freshly harvested seeds for some time under dry conditions to complete physiological maturation. Example: Wheat, Barley.
• Microbial action: Soil microorganisms degrade hard seed coats naturally over time.
• Fire or frost exposure: In some wild species, exposure to fire or cold breaks dormancy (ecological adaptation).

7. Significance of Seed Dormancy in Agriculture

• Prevents pre-harvest sprouting in cereals under humid conditions.
• Ensures uniform germination after treatment during sowing.
• Improves seed storage life by delaying germination.
• Assists in scheduling sowing according to season.
• Facilitates long-distance seed transport without loss of viability.
• Aids in natural regeneration and species survival in forest ecosystems.

8. Measurement of Dormancy

• Dormancy is measured indirectly by comparing germination percentage before and after dormancy-breaking treatment.
• Dormancy Index (%) =Germination after treatment − Germination before treatment / Germination after treatment × 100

9. Differences Between Dormant and Non-dormant Seeds

10. Examples of Seed Dormancy in Crops