Seed dormancy is the physiological inability of the intact viable seed to germinate even under favorable environmental conditions.
Source: Seed Dormancy
Some of the major causes of seed dormancy have been mentioned below.
a) Hard seed coat: Hard seed coat may make the seed
- Impermeable to water
- impermeable to oxygen
- or may act as a physical barrier to the expansion of the developing embryo.
Source: Hard seed coat
b) Immature embryo
In some species, the seeds are liberated when the embryo is not fully developed and needs time to develop fully for it to begin its germination.
c) Period of after-ripening
Seeds of apple, peach, iris and many other species don’t germinate even in favorable conditions and need a period of rest before they can germinate. This period is known as after ripening.
d) Germination inhibitors
The presence of inhibitors such as ABA, ammonia, etc in the seed coat, endosperm, embryo, etc prevents the germination of the seed.
e) Presence of unfavorable conditions also results in secondary dormancy.
Some of the well-known methods to break the dormancy are:
a) For breaking dormancy of hard seed coat
-Scarification: the hard seed coat is ruptured or weakened by mechanical or chemical methods such as threshing, rupturing, or cutting of seed coat or soaking the seed in hot water, acids, etc.
b) For breaking the dormancy of chilling requirement
-Stratification: Exposure of seed to low temperature and moisture for a period of time to overcome their chilling requirement
-Use of gibberellic acid
c) For breaking the dormancy of light-sensitive seeds
for positively photoelastic seed
-exposure to white light
- exposure to red and far-red light
- use of gibberellic acid and kinetin
d) Use of germination promoting substances such as potassium nitrate, thiourea and ethylene chlorohydrin, etc
e) Exposure to alternating temperature, pressure, etc
Metabolic Changes in Seed During Germination
Germination of seed is a complex metabolic, physiological and biochemical process. Some of the major metabolic changes that occur during the germination of the seed have been mentioned below.
a) Initiation of respiration in the hydrated seed.
After the absorption of water by hydrophilic groups present in the seed coat, the seed becomes hydrated which is then followed by the rapid increase in the rate of respiration.
Initially, the respiration is anaerobic and then is replaced by aerobic respiration. The respiration rate is slower-faster-slow.
Increased respiration leads to the synthesis of ATP which helps in the subsequent growth.
b) Mobilization of reserve food materials
Hydration of the seed activates the hydrolytic enzymes such as ἀ-amylase, proteases etc which with the help of ATP from aerobic respiration breaks down complex and insoluble food such as
- starch: to soluble sucrose then to glucose
- lipid: beta-oxidation of FA to acetyl CoA
- proteins: to amino acids
- inorganic materials such as calcium, phosphorous, etc are also liberated
Which are simple and soluble and are then mobilized to growing epicotyl, hypocotyl, plumule, and radicle as
- building blocks for further development
- energy for biosynthesis.
c) Change in the plant hormones
It occurs simultaneously with respiration and mobilization where
- increase in free IAA and decrease in conjugated IAA
- increase in the concentration of cytokinin, GA3, and ethylene
- decrease in the concentration of ABA
d) Cell differentiation and elongation
After the translocation of food and its subsequent assimilation, the cells of the embryo in the growing regions become metabolically active and they grow in size and begin division to form seedlings. Finally, the seed emerges and establish itself.