Seed dormancy: A natural mechanism of seed protection

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Dormancy is the temporary cessation of growth. Seed dormancy may be defined as the condition in which a seed is unable to germinate under favourable conditions. In other words, dormancy is the physical or physiological condition of the seeds that prevents germination even in the presence of favourable conditions for germination. According to Copeland and McDonald (1999), seed dormancy is the ability of seeds to delay their germination until the time and place are right. It is an important survival mechanism in plants. For the seed analyst and the seed researcher, seed dormancy may be a complex and puzzling challenge. But it is the way through which plants can survive and adapt to their environment. 

                                                                        Fig. A dormant seed

1. Types of seed dormancy

i. Imposed dormancy: It is a type of seed dormancy in which seeds are dormant due to unfavourable conditions. Eg, Xanthium spp.

ii. Innate dormancy/Dip dormancy: It is a type of seed dormancy in which active growth is arrested even under favourable conditions. It is due to the structure (hard seed coat), chemical compounds (growth inhibitors like ABA and phenolic compounds), and the physiological properties of the seed.

iii. Complex dormancy: Dormancy caused due to other factors than imposed and innate dormancy. This type of dormancy includes an immature embryo, after the ripening period in apples, etc.

2. Classification of dormancy

i. Primary dormancy: Also called innate dormancy or endogenous dormancy. Seeds that are incapable of germination just after ripening/harvesting, even by providing favourable environmental conditions, are called primary dormancy. Seeds enter into dormancy before they are harvested, i.e., they are still in the mother plant. Eg, 2-3 months in Virginia runner groundnut, 40 days in sunflower, etc.

ii. Secondary dormancy: It is also called induced dormancy. In this type of dormancy, seeds are capable of germination under favourable conditions just after ripening, but when these seeds are stored under unfavourable conditions for even a few days, they become incapable of germination. Eg, Mustard seed exposed to high concentration of CO2, wheat is stored in high moisture content in air-tight containers at 50°C, etc.

iii. Special type dormancy: Sometimes seeds germinate, but the growth of the sprout is found to be restricted because of very poor development of roots and coleoptile. This kind of dormancy is known as a special type of dormancy

3. Merits of dormancy

i. Dormancy helps to protect the seed and enhance its survival during adverse climatic and environmental conditions.

ii. Helps in ex-situ conservation of seed for the long term.

iii. Help to prevent seeds from sprouting.

iv. Provide sufficient time for the farmer to harvest, thresh, and store the crop plant.

v. Helps in the preparation of the seed catalogue for study purposes.

4. Demerits of dormancy

i. We cannot use the seed for germination purposes immediately after harvest.

ii. Sometimes we have to invest more time, labor, and cost to break dormancy of certain crops.

5. Factors affecting dormancy/causes of dormancy

1 Internal factor

I. Embryo factor

a. Immature embryo: eg, apple, pear, cherry, etc.

b. Rudimentary embryo: eg, orchid seed.

c. After the ripening period. Eg. apple

ii. Seed coat factor

a. Seed coat impermeable to water: Water is the basic requirement for seed germination. Some crops have seed coats that are impermeable to water, due to which dormancy arises. Eg, crops belong to the families Fabaceae, Malvaceae, Chenopodiaceae, and Convolvulaceae, etc.

b. Seed coat impermeable to oxygen: The Seed requires oxygen for germination, as respiration is the basic physiological activity in seed germination. But some crop seeds are impermeable to oxygen, due to which the seeds do not germinate. Eg, crops belong to the families Gramineae and Asteraceae.

c. Mechanically resistant seed coat: Due to the hard seed coat embryo is not able to break the seed coat and germinate. Eg, peach, walnut, coconut, etc.

iv. Presence of chemical inhibitors or germination inhibitors

Various germination inhibitors are present in various crops, due to which they possess dormancy.

a. Coumarin is present in some parts of the seeds, including pulp, testa, and endosperm, in tomato, which causes dormancy.

b. Abscisic acid is present in groundnut.

v. Genetic dormancy

The physical and physiological factors controlling the dormancy of the seed are under the control of a specific gene. Even in the same species but different varieties of rice dormant period is different due to different combinations of gene action. 

6. External factor

i. Light requirement/light-sensitive seed

There are two types of seed, i.e., positive photoblastic and negative photoblastic seed. Positive photoblastic seed requires light for germination, and negative photoblastic seeds germinate only in the absence of light.

a. Positively photoblastic seed: tobacco, tomato, lettuce etc.

b. Negatively photoblastic seed: onion, datura, silene, nigella, etc.

ii. Low temperature requirement/ Chilling requirement

Some crops require a low temperature for chilling requirements for growth. Stratification is done to break the dormancy of the seed.

Stratification: Cold treatment for the seed to break dormancy. Eg, apple.

Vernalization: Cold treatment for the plant part. It is used to break bud dormancy.

7. Method to break seed dormancy/ control measures

i. Mechanical scarification

Rupturing or weakening of the seed coats by mechanical means. It involves rotating the seeds in machines having drums with an abrasive surface, piercing, and pricking with a needle. Eg, sorghum, coriander, etc.

ii. Chemical scarification

Use of various chemicals such as sodium hydroxide, sulphuric acid, nitric acid, etc. Eg, treatment of 3% nitric acid for 6-8 hours helps to break the dormancy of rice.

iii. Stratification/Low temperature treatment

The process of treating seeds with low temperature is called stratification. It is generally done in temperate crops. Eg, 5-10 °C treatment for cold season crops like oat, barley, and apple, and sometimes in hot season crops like sorghum, Rosa family.

iv. Application of pressure

To rupture the seed coat, we give artificial pressure. 2000 atmospheric pressure at 18 ⁰C for 5-20 minutes to break dormancy of sweet clover.

v. High temperature treatment

To break dormancy, apply high temperature. 50- 60⁰C in oil palm, 40⁰C in Cyprus rotundus.

vi. Alteration of temperature treatment

Alternate application of high and low temperatures to break dormancy of the seed. The difference between high and low temperatures should not be more than 10-20 ⁰C. Eg, Chinese red pine, Japanese black pine, etc.

vii. Germination-stimulating compound/hormonal treatment

Hormones or chemicals like thiourea, GA3, KNO3, ethrel, kinetin, benzyl adenine, etc., are used to break the dormancy of the seed. Eg, 1000 ppm GA or 10-15 ppm kinetin or 2 ppm benzyl adenine or 250 ppm ethrel helps to break the dormancy in sunflower.

viii. Light treatment

For positively photoblastic seeds, light treatment, like exposure in the red light zone, is used. Eg, tobacco, lettuce, etc.

ix. Soaking in water or washing in running water

Eg, rice, various forest species, etc.

x. Combination of treatments

Any combination of more than one of the above-mentioned treatments at a time can break dormancy more effectively.

Keywords: seed dormancy, seed, hard seed, definition of dormancy, complex dormancy, seed coat, resistant seed, germination inhibitors, genetic dormancy, Stratification, Vernalization,

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