Latent life- dormancy: Importance and types of dormancy

Latent life- dormancy: Importance and types of dormancy, 

Dormancy

         

As stated earlier, seeds of some plants, fail to germinate even if placed under all the favorable conditions such as moisture, air, suitable temperature and proper sequence of light and dark.  In most of the cases, the germination may be delayed for days, weeks, months or even years. Such seeds are said to be in the state of dormancy. Dormancy may be defined as any rest period or reversible interruption of the phenotypic development of an organism or a state, in which seeds fail to germinate under conditions of moisture, temperature, and oxygen favorable for vegetative growth

Importance

The phenomenon of dormancy is widely spread in living organisms. Bacteria, fungi, insects, and animals also undergo periods of reduced growth activity i.e. dormancy. Looking to the differences in growth habits, nutrition, temperature and water regulation mechanism, and metabolism of these groups, it is unlikely that the same basic mechanism is involved in establishing the dormant condition in these different groups.

The significance of dormancy may be considered as-

·         It enables them to endure periods of unfavorable environmental conditions.

·         In temperate zones, it helps the seeds to pass through the severe colds.

·         In tropical regions, the impermeable seed coats ensure good chances of survival.

·         Dormant seeds and organs in perennial plants resist unfavorable conditions for their development

·         It enables the seed for long distance dispersals.

·         The seeds form a measure of the quantity and duration of rainfall, both of which determine the amount of soil- moisture available for plant growth.

·         The dormancy of seeds of cereals is most important to mankind. If these seeds germinate immediately after harvest, they will be quite useless to mankind.

Types

                       

There are several classifications of seed dormancy, proposed by    different authors.

The main types of dormancy are:

The Imposed Dormancy (Quiescence)

                       

Germination needs favorable environmetal conditions like sufficient water, proper temperature, etc. in absence of such conditions seed does not germinate or becomes dormant which is sometimes considered as inhibition. This type of inhibition is known as imposed dormancy (Quiescence).however, under favorable conditions this kind of dormancy disappears and the seed germinates immediately.

The Innate (Deep) Dormancy

                       

Sometimes a seed unable to germinate due to structural, chemical or physiological properties of the seed itself, this phenomenon is known as Innate (deep) dormancy and is distinguished as physical, chemical, morphological or physiological types of dormancy. These terms describe the main mechanisms that are responsible for the inability of embryo to grow: physical barrier preventing water and/or oxygen access, presence of growth inhibitors, and morphological or physiological immaturity of embryo. These mechanisms can operate separately or in various combinations.

The fully dormant condition of a seed is a gradual process and is not attained suddenly. During the entire process, there may be following three phases of dormancy.

(I)Predormancy or early rest :

 During this phase, the dormant organ has capacity to resume growth by various treatments i. e. , capacity of germination or growth is not completely lost.

(II) Full dormancy or mid rest :

When a seed or organ becomes completely dormant and germination or growth can not be induced immediately by changing environmental conditions.

(III) Post dormancy or after rest :

 When a dormant seed or organ gradually    emerges from full dormancy and in it the germination or growth can be  induced by changing environmental conditions.                                                                                                                                                        

 Complex types of dormancy

                       

These are caused by several different mechanisms, which make germination difficult or hinder it altogether. These mechanisms act as a set of barriers, which prevent germination.

          

The dormancy may be true, relative or secondary.

    

(I) True dormancy :

when in a seed or organ , the germination or growth can not be induced under any set of environmental conditions.

(II) Relative dormancy :

when in a seed, the germination can be induced under specific conditions even at the time of its deepest dormancy.

    

 (III) Secondary dormancy :

when a seed has not fully emerged from dormancy and is again thrown back into full dormancy by certain  environmental conditions.