Plant Growth and DevelopmentSeed and Bud Dormancy

Arnold V. DamasoMS Crop Science – Agronomy

Central Luzon State University

• Dormancy is a mechanism to prevent germination during unsuitable ecological conditions, when the probability of seedling survival is low.

Dormancy

• There are two types of dormancy– predictive dormancy– consequential dormancy

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• One important function of most seeds is delayed germination, which allows time for dispersal and prevents germination of all the seeds at the same time.

• The staggering of germination safeguards some seeds and seedlings from suffering damage or death from short periods of bad weather or from transient herbivores; it also allows some seeds to germinate when competition from other plants for light and water might be less intense.

Importance and Significance 

• Physiology of DormancyDuring the developmental cycle of

the plant, at some phase or the other certain structures like buds, tubers, seeds, etc., go through a period of temporary suspension of growth activity.  Such a state is called dormancy.

• In plant physiology, dormancy is a period of arrested plant growth. It is a survival strategy exhibited by many plant species it may be imposed by certain environmental factors or internal factors or genetic factors included. 

• Generally plants or plant structures, in order to overcome or survive against hostile environmental conditions undergo a period of dormancy with suitable modifications. 

• In lower plants production of endospores, zygospores, auxospores, akinetes, etc, are some of the methods involved in tiding over unfavorable conditions

• Vulnerability and Adaptation

Embryonic development ending in seed dormancy

Seed Dormancy

• Angiosperms produce seeds within the ovary and ovary itself develops into a fruit.  In Gymnosperms only seeds develop.

• The development of ovary into fruit and seed go hand in hand. 

Gene expression during dormancy and breaking dormancy

Role of GA and ABA in antagonistic manner during dormancy and breaking dormancy

• ABA promotes seed dormancy. ABA levels are high when seeds mature, promoting lowered metabolism and synthesis of proteins needed to withstand the dehydration associated with dormancy.

• Seeds germinate when ABA is degraded by some environmental action. Desert seeds must have the ABA washed out of the seed coat; temperate are plants have ABA degraded by light stimulated enzymes.

Analysis of mRNAs and proteins during different stages of seed dormancy.

Categories and Types of Dormancy • Exogenous dormancy• Endogenous dormancy– Physical– Physiological–Morphological

Exogenous dormancy• Exogenous dormancy is caused by

conditions outside the embryo and is often broken down into three subgroups:– Physical dormancy–Mechanical dormancy– Chemical dormancy

Endogenous dormancy• Endogenous dormancy is caused

by conditions within the embryo itself, and it is also often broken down into three subgroups: – physiological dormancy, –morphological dormancy and – combined dormancy,(each of these groups may also have subgroups)

Duration of seed dormancy

• The duration of dormancy in seeds varies from species to species and it is species specific. 

Viability of Seeds• The period for which the embryo

remains healthy within the seed coat and capable of germination under permissible conditions is referred to as seed viability.

Seed dormancy and its advantages and disadvantages• Dormancy has its own advantages as

well as disadvantages for seeds.  • Seeds with a longer period of

dormancy and viability are capable of surviving the worst hazards of environmental conditions.

Causes for seed dormancy

• Prevents water uptake:  Some plants produce seeds with hard seed coat which is also waxy. 

• Prevents oxygen:  Some seeds though they are capable of imbibing water, they are incapable .

• Prevents the growth of the embryo. In this condition, the seeds may remain for months or years until the said seed coat gets cracked or loosened of taking in atmospheric air.

• Mechanical Scarification.  Shaking the seed with abrasives or nicking the seed coat with sharp edged metals or chewing the seed coats without damaging the embryo makes the seed coat to crack open.

• Chemical Scarification. The hard seed coats can be loosened by strong acids or solvent treatments where the hard coat is rendered soft.

Temperature Effect• A large number of plants produce

seeds which germinate under normal temperatures. 

• But some do not germinate if they are stored at room temperatures. 

• They require chilling treatment for a period of time

Inductions of Dormancy by Chemicals• Search for substances that induce

dormancy in seeds resulted in the discovery of a host of compounds like Coumerin, Para ascorbic acid, Hydrogen cyanide, Abscisic acid, etc.

Effect of Light• Besides initiating many other

photobiological processes, radiant energy has a profound influence on seed dormancy and germination.

Interplay of GA and ABA is seed Dormancy• The action of Gibberellins is

breaking seed dormancy is interesting because they are very effective on seeds that require light treatment for germination.

Chemical compounds that break seed dormancy• Plants have unique properties in

synthesizing compounds which can induce dormancy as well as break the dormancy.

• Gibberellins• Cytokinins• Ethylene• Chlorohydrins• Theourea

Bud Dormancy • Perennial plants like shrubs, trees

have to go through different seasons in a year. 

• The onset of winter is always an unfavorable season for the growth and even survival of plants become difficult.

How plants prepare for winter?

 Dormant axillary buds

Rising from dormancy

Dormant cabbage

Site of Perception• Though leaves perceive changes in

photoperiodic effects of the day, it is the buds that act as the sites of perception for inducing dormancy. 

• The induction of dormancy in buds starts only after the falling of leaves.

Mechanism of Induction of Dormancy in Buds• Quantitative estimation by solvent

extraction methods reveal that dormant buds contain greater amounts of ABA than actively growing buds. 

• Abscissin is a well-known growth inhibiting hormone.

Breaking Bud Dormancy• The dormant buds can be induced to

sprout again by treating with cytokinins and gibberellins. 

• But in natural course, the onset of spring and long photoperiods, the dormant buds become active and develop into branches.

• The photoperiodic effect either in breaking the dormancy or induction of dormancy is explained on the basis of phytochrome involvement.

Summary• Dormancy is the state in which a

plant or plant part exhibits little or no growth and in which most, if not all, metabolic activity ceases for a period of time.

• The vast majority of plant life functions best when there is ample water and temperatures are well above freezing throughout the year

• Dormancy evolved as a means of surviving unfavorable environmental conditions. In the temperate zones, buds normally form from spring to midsummer.

• Seed dormancy can also be caused by a number of different factors. For several reasons, the presence of a hard seed coat will very often result in dormancy of the seed.

• Bud dormancy is a suspension of most physiological activity and growth that can be reactivated.

• Seed germination is also dependent on temperature.

• The seeds of almost all species have a minimal temperature below which they will not germinate.

• Gibberellins, cytokinins, and ethylene, all natural plant hormones, have been shown to be involved in breaking seed dormancy, and the gibberellins and other substances, such as thiourea, are used to germinate seeds commercially.

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