Seed Dormancy – Significance, Causes and Breaking
It is defined as, “In an inactive state, when growth and development of a seed, bud, etc slow or cease, in order to survive adverse environmental conditions”.
Seeds of numerous plants do not germinate readily even if all the conditions favorable for their germination are provided to these. The germination may be delayed for days, weeks, or even months. The seed of such plants are said to in a dormant condition and the condition is called seed dormancy.
SIGNIFICANCE OF SEED DORMANCY
- The dormancy enables the plants to overcome periods of unfavourable environmental conditions, thus providing the plants with a mechanism for survival
- In temperate zones where temp falls below freezing in winter, most plants could not survive in the vegetative or flowering state. Thus, in ay plants seeds and buds become dormant. The dormancy begins at the onset of winter cold allowing the plant to pass the winter with little or no damage.
- Moisture is a pre-condition for the germination of seeds, and seeds of plants growing in arid regions remain dormant until a sufficient amount of water is available.
CAUSES OF SEED DORMANCY
Following are the main causes:
Dormancy due to the condition of the embryo:
Two kinds of dormancy have been observed due to the condition of the embryo.
Dormancy due to immature embryo:
Failure of seed to germinate may be due to poor development of the embryo at seed maturity. Such embryos continue their development during the dormant period. The seeds germinate only when the embryos complete their development.
In the seeds of rosaceous plants (apple, pear, cherry, blackberry, etc) and conifers, the embryo appears to be fully developed but they fail to germinate when all the conditions necessary for germination are provided to them. The embryos in these cases require some period of development during which changes occur within the seeds that result in the breakdown of dormancy.
Dormancy Due to Seed Coat:
The seed coats of most seeds are composed of several layers of cells derived from integument (an outer protective layer on a seed). During seed ripening, some chemicals lose water and form a tough, protective layer around the embryo. Such seed coats cause dormancy in the following ways:
A hard seed coat prevents absorption of water, thus inhibiting germination and causing dormancy. It is common in Leguminosae.
Some seeds are permeable to water but impermeable to gasses esp to oxygen eg Xanthium. On the other hand, some seeds are permeable to oxygen but impermeable to CO2 hence produced inside the seeds during respirations inhibits seed germination and causes dormancy.
Development of Special Structures:
In some seeds, a special cork-like structure called strophiolar plug develops tat blocks micropyle (small opening in the seed coat). Therefore, the seeds are unable to absorb water and oxygen necessary for germination and remain dormant.
In some seeds, the seed coat offers resistance and hence the embryo cannot expand and seed remains dormant.
Dormancy due to Chemical Inhibitors:
Some compounds like the pulp of fruit or glumes block some processes essential for growth and productivity growth abnormality. These compounds are referred to as chemical inhibitors.
Dormancy due to Environmental Factors:
Certain environmental factors esp light and temp affect seed germination including dormancy.
Effect of Light:
- Seeds response to light variously
- Effect of radiant energy on seed germination is quite diverse
- Some seeds require light for germination while others germinate only in the dark
- Some seeds require exposure to altering light and dark periods for germination
- Seeds that require light for germination are said to be
Effect of Temperature:
- Many seed esp of rosaceous spp like peach, plum, cherry, etc; many other deciduous plants will not germinate until they are exposed to low temp in moist conditions
- Many seeds respond to a higher temp and several respond best when daily temp alternate b/w high and low
- About 62 cold requiring spp have been recorded so far
BREAKING SEED DORMANCY:
The following measures can be adapted to break the seed dormancy so these can germinate when conditions favorable for germination are provided to them.
- Breaking the seed coat is called Scarification
- Used when the seed is impermeable or having mechanical resistance
- The seed coats are scarified or cracked either by mechanically, chemically or by the fire
- Mechanically by shaking the seeds with sand or sandpaper; or scratching by a knife, or even by chewing; naturally by microbial action or passage of a seed through the digestive tract of a bird or an animal; exposure to alternating temp, or movement of water across sand or rocks.
- Chemically by concentrated acids, alcohols which dissolve away waxy materials that block water entry. Seeds of many legumes are soaked for a few minutes to an hour in concentrated sulphuric acid to soften the hard seed coat to make these permeable to water and gasses.
- Fire plays an important role in scarification esp for spp like Albizzia spp.
- Impaction means to strike something with force
- This method implies to the vigorous shaking of the seeds
- This method is used when the micropyle is blocked by a cork-like substance, strophiolar plug, which prevents the penetration of oxygen and water
Prechilling or Stratification
- Seeds of many plants like rosaceous spp, conifers, and herbaceous spp do not germinate until these are exposed to low temp in the moist conditions in the presence of oxygen from weeks to months
- Stratification is the process in which when the seeds shed from parent plants in the autumn, these are covered with cold soil, debris, and snow layers
- This process can be done artificially by layering the seeds during winter in flats containing moist sand and peat. The seeds in flats are cooled before they will germinate
- Prechilling is used for Stratification nowadays
The process accumulates food and stores moisture which finally helps in germination thus breaking the seed dormancy.
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