Fin 1

72問 • 1年前

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During the developmental cycle of the plant, at some phase certain structures like buds, tubers, seeds, go though a period of temporary suspension of growth activity or slow down for a period of time or deep sleep, such a state is called ?

Dormancy

This minimizes metabolic activity and therefore helps an organism to conserve to energy.

Dormancy

not active or growing but able to become active later.

Dormant

defined dormancy as any phase in the life cycle of a plant in which active growth is temporarily suspended.

Wareing (1969)

It is the incapacity of fully developed, mature, viable seed to germinate even under favourable conditions.

Seed Dormancy

In such cases, the completely dry ripe seed is physiologically inactive and is said to be in a ?

resting stage

The seed is called ________ and the phenomenon is termed ________.

dormant, dormancy

is defined as a state in which seeds are unable to germinate even under environmental conditions normally favorable for germination.

Seed dormancy

provides a mechanism for plants to delay germination until conditions are optimal for survival of the next generation (Finkelstein et al. 2008).

Seed dormancy

Studies with ABA and GA biosynthesis and signaling pathway demonstrated that these two hormones have important and antagonistic roles in germination and dormancy respectively.

HORMONAL MECHANISM

induces and maintains seed dormancy during embryo maturation. The key to ABA metabolism are some specific genes encoding key enzymes are of NCED family

ABA

positively regulates ABA synthesis.

Embryonic identity protein

Due to internal causes even if all external environmental conditions are duly present.

Primary Dormancy

When dormancy caused due to external factors like absence of sunlight, CO2, very high or low temperature.

Secondary Dormancy

only germinate if there is presence of smoke.

Nicotiana attenuata

Classification of seed dormancy

Exogenous dormancy, Endogenous dormancy

Exogenous dormancy

Physical Dormancy, Mechanical Dormancy, Chemical Dormancy

Endogenous dormancy

Physiological Dormancy, Morphological Dormancy, Combined Dormancy

Dormancy that is caused by an impermeable seed coat. ➤ It is the result of impermeable layer that develops during maturation and drying of the seed or fruit. ➤ It prevents the seed from taking up water or gases. ➤ As a result, the seed is prevented from germinating until dormancy is broken. ➤Generally, physical dormancy is the result of one or more palisade layers in the fruit or seed coat.

Physical Dormancy

Seed coats are impermeable in water due to macrosclereid cells, mucilaginous outer cell layer or hardened endocarp. ➤ Depth of the puncture to the seed coat increased, so did the permeability of seed coat of water. ➤ E.g., Olive, Peach, Plum, (hardened endocarp of seed).

Physical dormancy

Mechanical dormancy occurs when seed coats or other coverings are too hard to allow the embryo to expand during germination.

Mechanical dormancy

Presence of inhibitors within the seed (seed coat) can cause dormancy. E.g.: Cotton seed pericarp contains Abscisic acid Coriander seed pericarp contains Coumarin Paddy hull contain Abscisic acid

Chemical dormancy

Cotton seed pericarp contains? Coriander seed pericarp contains? Paddy hull contain?

Abscisic acid, Coumarin, Abscisic acid

prevents embryo growth and seed germination until chemical changes occur.

Physiological dormancy

Physiological dormancy prevents embryo growth and seed germination until chemical changes occur.

Photo Dormancy, Thermo Dormancy

light sensitivity affects germination of some seeds. These photoblastic seeds need a period of darkness or light to germinate.

Photo dormancy

seed sensitivity to heat or cold. Some seeds germinate only at high temperature. like Amaranth.

Thermo dormancy

the embryo is underdeveloped or undifferentiated.

Morphological dormancy

These seeds have both morphological and physiological dormancy.

Combined dormancy

This class of seed dormancy refers to seed with under development and differentiated embryos, this includes embryos in which the cotyledons and hypocotyls, radical axis are differentiated, but small in size.

Morphological dormancy

In this class, seeds have embryos that are underdeveloped (in size), but differentiated (e.g., into cotyledons and hypocotyls radical) as well as a physiological component to their dormancy.

Morpho-physiological dormancy

Morphological dormancy can be divided into two types

epicotyls dormancy, double dormancy

TYPES OF DORMANCY According to wareing (1969) the dormancy may be two types

Imposed dormancy or quiescence, Innate dormancy or rest

The dormancy due to unfavourable environmental conditions is called

Imposed dormancy or quiescence

The dormancy due to conditions within the dormant plant or organ is called

Innate dormancy or rest

It is a condition in which germination or growth fails to occur even though the external environmental conditions are favourable.

Innate dormancy or rest

TYPES OF SEED DORMANCY Crocker (1916) divided seed dormancy into two types:

Seed coat induced dormancy, Embryo induced dormancy

The dormancy of seeds due to extreme hardness of seed coat is called

seed coat induced dormancy

The dormancy of seeds due to rudimentary or complete dormant embryo is called

embryo induced dormancy

According to C. Baskin and J. Baskin (1998; 2004) have proposed a comprehensive classification system which include five classes of seed dormancy

Physiological dormancy, Morphological dormancy, Morpho-physiological dormancy, Physical dormancy, Combinational dormancy

Mechanism of Dormancy

Hormonal control theory, Temperature and gaseous exchange restriction theory, Metabolic control theory

➤Dormancy causing hormones are

ABA and ethylene

Oxygen deficiency leads to inhibition of ___________ activity which leads to dormancy.

Cytochrome oxidase

Dormancy in seed is controlled by metabolic energy produced in seeds.

Metabolic control theory

Factors causing dormancy of seeds

➤ Hard seed coat, ➤Dormancy due to temperature, ➤Immature Embryo, ➤ Light Sensitive Seed, ➤Dormancy due to growth inhibitory chemicals, ➤ Excessive Salts

There two type of seeds for light sensitivity:

Positive photoblastic seeds, Negative photoblastic seeds

Seeds of some species like lettuce remain dormant in the dark, they germinate when they are exposed to light.

Positive photoblastic seeds

Seeds of some species like Phlox, Nemophila and Silene remain dormant when exposed to light. They germinate only in the dark.

Negative photoblastic seeds

There are main two types of overcoming of seed dormancy:

Natural overcoming of seed dormancy, Artificial overcoming of seed dormancy

Nature of dormancy stops when the embryo gets appropriate environment such as adaptive moisture and temperature.

Natural overcoming of seed dormancy

IN NATURE, CERTAIN ENVIRONMENTAL AGENTS SOFTEN SEED COATS:

Microorganisms, Forest fly, Weathering

ARTIFICIAL OVERCOMING OF SEED DORMANCY

Mechanical scarification, Impaction, Cold stratification, Removal of inhibitors, Treatment with growth hormones, Specific light requirement, Pressure

includes treating seeds with strong acids and organic solvents. E.g. Sulphuric acid.

Chemical treatment

The method is used for breaking dormancy of seeds caused by hard seed coats which become impermeable to water and gases etc. The method employed in softening or weakening the seed coat is called?

Scarification

When mechanical breaking of seed coat is done at one or more places, it is called ?

Mechanical Scarification

is done by shaking the seeds with sand or by scratching o nicking the seed coat with knife.

Mechanical Scarification

The treatment of seed coat with strong mineral acids or other chemicals is called ?

Chemical Scarification

It is process of shaking of seeds to clear blockage of testa pores. E.g. Seeds of Crotolaria egyptica (legume)

Impaction

In order to remove the plugs the seeds are shaken vigoursly and this treatment is known as?

Impaction

➤Treatment of seed at low temperature under moist condition. ➤ During this anatomical and biochemical changes takes place in the seed. ➤Growth promoting hormones increases and Growth retarding hormones decreases.

Stratification

This method is used to break the dormancy of seeds caused due to condition of embryo.

Stratification

Stratification: ➤ This method is used to break the dormancy of seeds caused due to condition of embryo. ➤ In this process the seeds are exposed to well aerated, moist conditions under low temperature ______________. This treatment is called Stratification or after-ripening.

(0°C to 10°C) for weeks to months

reported the seed germination in certain plants like sweet clover (Melilotus alba) and alfalfa (Medicago sativa) can be greatly improved after being subjected to hydraulic pressure of about 2000 atm. at 18°C for about 5-20 minutes.

Davies (1928)

have been used to induce germination in positively photoblastic seeds like lettuce and tobacco etc.

Kinetins and gibberellins

promotes germination in lettuce seeds.

Red light

The breaking of dormancy

GERMINATION

Low temp. treatment/ stratification

(5-10°C) FOR 5-30 DAYS

High temperature treatment

(OIL PALM, 50-60°C, NUT GRASS, 40°C FOR 3-6 DAYS OR 7-8 YRS)

Altering temperature treatment

(NOT MORE THAN 10- 20°C)

Methods of Breaking the Seed Dormancy

LIGHT TREATMENT, CHEMICAL, APPLICATION OF PRESSURE TO SEEDS

FACTORS AFFECTING THE LIFE SPAN OF SEEDS

Genetic factors, Initial seed Quality, Seed Moisture, Pre and post harvest conditions

includes treating seeds with strong acids and organic solvents. E.g. Sulphuric acid.

Chemical treatment

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