It is common observations that seeds of many plants species do not germinate when freshly harvested even under favourable conditions. They need a period of rest/ storage before they become capable of germination.
Inability of viable and mature seeds to resume growth immediately after harvest in an environment normally favourable for the germination of the concerned plant species is known as seed dormancy.
The period of rest after harvest that is necessary for germination is referred as after ripening period. The period of dormancy varies from a few days to several years depending on the plant species.
1. True dormancy/primary dormancy/innate dormancy: due to chemicals/ anatomical features of seed.
2. Enforced dormancy/imposed dormancy/quiescence/ secondary dormancy: due to unfavourable environmental conditions.
Ex:Exposure of dry barley seed to temperature of 50-900C
Seven days storage of winter barley at high moisture content at 200C
Placement of seed under water in dark condition for 3 days 20C
Biological significance of Dormancy
1. Storage of seeds is prolonged, it is a survival mechanism
2. Seed can pass through adverse situation /conditions
3. Prevents the insitue germination i.e., vivipary
1. No uniform germination
2. Difficult to maintain plant population
3. Interferes in seed testing procedure
Nikolaeva (1969 and 1977) classified dormancy into three broad class are as below;
I. Exogenous Dormancy: Dormancy is due to some features of the seed located outside the embryo
a. Impermeability of seed coat to water: due to seed coat structure, which is hard enough to restrict the entry of moisture into the seeds, thereby preventing seed germination. Ex: Malvaceae, Leguminoseae, Lilaceae
b. Impermeability of seed coat to gases; is related to the insufficient intake of oxygen by seeds due to impermeability of seed structure enclosing embryo. Ex: Graminaceae, fruit crops & forest trees
c. Mechanical resistances of seed coat: growth of embryo is checked due to extremely hard seed/fruit structure such as seed coat, endosperm per carp etc., Ex: Acacia species.
d. Inhibitors present in seed coat/endosperm: biochemical substances present in seed coat or endosperm block the germination of embryo.
Ex: Iris app- inhibitors present in the endosperm
Tomato-Feruline and Caffeie acid
All spp.- Coumarin
II. Endogenous dormancy: the reason for dormancy is present within the embryo
a. Incomplete embryo development: due to an incomplete development of the embryo. In such cases, germination does not occur until the embryos develop to their normal size. Ex: Palmaceae, Amgnoliaceae
b. Inhibitors present within the embryo: Dormancy arises from metabolic blocks produced by biochemical substances called inhibitors present within the embryo. In such cases germination can commence only when these inhibitors are leached out of the embryo Ex: Xanthium, Fraximus
III. Combined Dormancy: dormancy is produced by a combination of two or more factors which act in complementary fashion.
Methods of breaking dormancy
I. Natural breaking of dormancy: in nature dormancy terminates when embryo gets suitable environment such as adequate moisture, aeration and temperature. The impermeable seed coat present in many species became permeable due to the rupturing of softening action of natural agents like micro organism, high or low temperature, humidity fiber and abrasion due to wind or digestive tracts of birds and animals which feed on these seeds. Ex: Rhizoctonia damages seed coat
II. Treatments to break Dormancy: the various treatments for overcoming dormancy may be divided into the following three groups
1. Seed coat treatments: These treatments aim at making hard seed coat permeable to water or gases either cracking or softening them. The process is usually referred as scarification. These treatments are either physical or chemical in nature.
i. Acid scarification: treating seeds with concentrated acids like sulphuric acid, Hydrochloric acid etc.,
ii. Thermal scarification: the seeds are treated with different temperatures and gases
iii. Mechanical scarification: The seed coat is damaged using mechanical means. Viz.,
- rubbing seeds on sand paper or by using mechanical scarifier as in subabul
- Making small incision by piercing a needle as in bittergourd
- Removing of entire seed coat as in rubber
2. Embryo treatments:
1. Stratification: the incubation of seeds at a suitable low temperature (Usually 0-50C) over a moist substratum before transferring them to a temperature optimum for germination. Ex. Cherry (Prunus cerasus), Mustard and rape seeds
2. High temperature treatment: in some species, incubation at 40-500C for few hours to 1-5 days may be effective in overcoming dormancy. Ex. Rice seeds more than 15% seed moisture treated in hot water of 400C for 4-5 hours.
3. Chemical treatments: alternatively growth regulators or other chemicals may be applied to induced germination growth regulators commonly used GA3 (100ppm), kinetin (10-15ppm) and thio-urea (0.5-3%)
3. Miscellaneous approaches:
i. Exposing seeds to light
ii. Pressure treatment
iii. Infra red radiation treatment
iv. Magnetic treatment
In seed germination process, the seed’s role is that of reproductive unit; it is the thread of life that assures the survival of all plant species. Furthermore, because of its role in stand establishment, seed germination remains a key to modern agriculture.
Seed would normally germinates only after they have undergone a predefined period of growth and development accumulating food reserves and finally becoming air dry.
Seed germination depends on a favourable combination of several external and internal factors; in nature, seed must wait for this combination to occur for their germination.
Definition: Seed germination is the resumption of active growth of the embryo that results in the rupture of the seed coat and the emergence of the young plant under favourable conditions.
Types of germination:
1. Hypogeal germination: the cotyledons or storage organs do not emerges above the soil surface; only plumule emerges above the ground. Ex: Most of the monocots and pea
2. Epigeal Germination: The cotyledon or storage organs emerge above the soil surface. Ex: Most of the dicots and pine
Phases of Seed Germination
1. Imbibiton: Rapid water uptake
2. Active Metabolism: Major metabolic events begin
2. External factors: a) Water b) Oxygen c)Temperature d)Light and e) Soil Factors
a. Water: Water is a basic requirement for germination. It is essential for enzyme activation, breakdown, translocation and use of reserve storage materials.
b. Oxygen: atmospheric air is composed of 79.9 % Nitrogen, 20% oxygen and 0.03 % carbon dioxide. Oxygen is required for germination of most of species. If CO2 concentration is higher than 0.03 % it retards germination. Respiration increases sharply during seed germination. Since respiration is essentially an oxidative process, an adequate supply of oxygen is a must.
c. Temperature: Seed germination is a complex process involving many individual reactions and phases, each of which is affected by temperature. The effect on germination can be expressed in terms of cordial temperature i.e., minimum, optimum and maximum temperature. The optimum temperature for most of the seeds is between 15 to 30 0C. maximum temperature is between 30 to 400C. Some species will germinate even at freezing point also ex. Alpine
d. Light: Some species required light for seed germination. Both light intensity (lux) and light quality (colour and wavelength) influence seed germination
e. Soil factor: Soil structure, soil texture and soil temperature influences on seed germination.