Plant Propogation and Nursery Management

Types of Dormancy (Endogenous Dormancy)

2. Endogenous Dormancy

This type of dormancy is imposed by rudimentary or undeveloped embryo at the time of ripening or maturity. This can be of different types such as morphological, physiological, double dormancy and secondary dormancy.

A. Morphological dormancy (Rudimentary and linear embryo): Dormancy occurs in some seeds in which the embryo is not fully developed at the time of seed dissemination. Such seeds do not germinate, if planted immediately after harvesting. Plants with rudimentary embryos produce seeds with little more than a pro-embryo embedded in a massive endosperm at the time of fruit maturation. Enlargement of the embryo occurs after the seeds have imbibed water but, before germination begins. Formation of rudimentary embryo is common in various plant families such as Ranunculaceae (Ranunculus), Papavaraceae (poppy). Some plants of temperate zone like holly and snow-berry have also rudimentary embryos.

B. Physiological dormancy

a) Non-deep physiological dormancy: After ripening time is required for seeds in dry storage to lose dormancy. This type of dormancy is often transitory and disappears during dry storage. Temperate fruits such as apple, pear, cherry, peach, plum and apricot, cultivated cereals, vegetables and flower crops, have this type of physiological dormancy which may last for one to six months and disappears with dry storage.
b) Photo dormancy: Seeds that either require light or dark condition to germinate are termed as photo-dormant seeds. It is due to photo-chemically reactive pigment called phytochrome widely present in some plants. When imbibed seeds are exposed to red light (660-760 nm), the phytochrome changes to red form (P_fr), thereby substituting the germination process. However, when seeds are exposed to far-red light (760-800), P_fr is changed to Pf which inhibits germination process.
c) Thermo dormancy: Some seeds have specific temperature requirement for their germination, otherwise they remain dormant. Such seeds are called as thermo dormant .For example seeds of lettuce, celery and pansy do not germinate if the temperature is below 25^oC.

Physiological dormancy is of 3 types:

I) Intermediate physiological dormancy: The seeds of some species require a specific period of one-to-three months of chilling, while in an imbibed and aerated state, commonly called as moist chilling. For example, most of temperate fruit seeds require moist chilling to overcome seed dormancy. This requirement led to the standardization of world famous, horticultural practice of stratification. In this process, the seeds are placed between layers of moist sand in boxes and exposed to chilling temperatures (2 to 7⁰C) for the period varying from 3-6 months to overcome dormancy.
II) Deep physiological dormancy: Seeds, which usually require a relatively long (>8 weeks) period of moist chilling stratification to relieve dormancy as in peach.
III) Epicotyl dormancy: Seeds having separate dormancy conditions for the radicle hypocotyl and epicotyl, is called as epicotyl dormancy e.g. Lilium, Hepatica antiloba and Trillium.

C. Double dormancy

• In some species, seeds have dormancy due to hard seed coats and dormant embryos.
• For instance, some tree legumes seed coats are impervious and at the same time their embryo are also dormant.
• Such seeds require two years for breaking of dormancy in nature. In the first spring, the microorganisms act upon the seed making it weak and soft and then embryo dormancy is broken by chilling temperature in the winter next year.
• Combination of two or more types of dormancy is known as ‘double dormancy’. It can be morpho-physiological i.e. combination of under developed embryo and physiological dormancy or exo-endodormancy i.e. combination of exogenous and endogenous dormancy conditions i.e. hard seed coat (physical plus intermediate physiological dormancy).
D. Secondary dormancy

Secondary dormancy is due to germination conditions. It is a further adaptation to prevent germination of an imbibed seed if other environmental conditions are not favorable. These conditions can include unfavorably high or low temperature, prolonged darkness and water stress. It is of two types:

I) Thermo dormancy: High temperature induced dormancy.
II) Conditional dormancy: Change in ability to germinate related to time of the year.

Advantages:

1. Permitting germination only when environmental conditions favour seedling survival as in fruit plants of temperate region.
2. Helpful in creation of a “seed bank”
3. Dormancy can also synchronize germination to a particular time of the year.
4. Seed disposal can be facilitated by specialized dormancy conditions. For example modification of seed covering through digestive tract of a bird or other animals.