03 Seed Dormancy

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Seed Dormancy

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Lecture on:

Lecture on SEED DORMANCY

Highlights of the Lecture:

01 Highlights of the Lecture What is Seed Dormancy? Types of Seed Dormancy Innate /Primary Dormancy Secondary Dormancy Advantages & Disadvantages Methods of breaking dormancy

SEED Dormancy:

SEED Dormancy “T he failure of fully developed, mature, viable seed to germinate even under favorable physical conditions (like moisture and temperature). Defination Question Asked In Examination Define Seed dormancy

Classification of Seed Dormancy:

Nature of origination Innate  Induced  Enforced Time of origin Primary Secondary Location of dormancy   Exogenous  Endogenous  Combined Classification of Seed Dormancy Harper (1977)

Classification of Seed Dormancy:

Classification of Seed Dormancy Endogenous / Innate dormancy / Primary dormancy : Embryo characteristics prevents germination - Epicotyl , Hypocotyl , Radical Dormancy may be induced before maturity, during maturity and after maturity but before seed is dispersed from mother plant   Based on Amen (1963)

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Secondary dormancy :  Characteristic of structures - Seed coats, Fruit walls, including endosperm or perisperm prevents germination Whose germination is inhibited, fail to recover even when the inhibitory factor is removed. Adoptive mechanism to pass the adverse environmental condition. Only in a matured and imbibed seed by certain environmental conditions, which are unfavourable to germination. e.g. 1) Spring wheat and winter barley : 50 and 90  o C 2) Storage of winter barely for seven days in high moisture containers at 20 o C

Types of seed dormancy:

Types of seed dormancy

Classification of Seed Dormancy, Causes & Pretreatment:

Sr Types Causes Embryo character Pre-treatment 1 Physical Seed coat impermeable Fully developed, non-dormant Scarification ( Mechanical and acid) 2 Physiological Physiological inhibiting mechanism of germination in the embryo Fully developed dormant Seed soaking in growth regulators (GA3, Ethrel , and chemical solutions ( KNO3, Thiourea ) 3 Combinations 1+2 Fully developed dormant Scarification followed by chemical treatment 4 Morphological Under developed embryo Under developed non-dormant Cold stratification 5 Morpho -physiological Under developed embryo, physiological Under developed dormant Stratification followed by chemical soaking. Classification of Seed Dormancy, Causes & Pretreatment

Causes of Physiological Dormancy:

Causes of Physiological Dormancy Covering restricts oxygen Inhibitors in coverings Embryo cannot break through physical barriers Endosperm restrict embryo growth Interaction between embryo and covering

Causes of Morphological Dormancy:

Causes of Morphological Dormancy Morphology of embryo not developed Temperate families - Apiaceae , Ranunculaceae Tropical families – Annonacease , Arecaceae , Degeneriaceae , Lactoridaceae , Monimiaceae , Myrsticaceae , Winteraceae

Causes of MorphoPhysiological Dormancy:

Causes of MorphoPhysiological Dormancy Underdeveloped embryos Embryo growth and dormancy break required Embryo grows first then dormancy broken or both at same time Vary warm, moist and cold stratification periods

Causes of physical Dormancy:

Causes of physical Dormancy Present in 15 angiosperm families Large embryos with food reserve in embryo not endosperm Impermeable in seed coats - micropyle , hilum , chalazal area, impermeable palisade cells Embryo is not dormant Air drying during development intensifies hardness

Causes of Chemical Dormancy:

Causes of Chemical Dormancy Inhibitors in embryo, endosperm, seed coat Leaching or seed coat removal Seed may have physiological dormancy too so need prechilling Abscisic acid inhibits germination when applied exogenously Nickel (20 mg/liter) increased germination of Picea abies

Causes of mechanical Dormancy:

Causes of mechanical Dormancy Stony endocarps Embryos with deep physiological dormancy - require long prechilling Anacardiaceae , Cornaceae , Juglandaceae , Nyssaceae , Oleaceae

Advanatges & Disadvantages of Seed Dormancy:

Advanatges & Disadvantages of Seed Dormancy ADVANTAGES Storage life of seed is prolonged  Seed can pass through adverse situation Prevents the in situ germination. DISADVANTAGES Long periods of time needed to overcome dormancy (for uniform germination) Contributes to longevity of weed seed. While raising a crop it is very difficult to maintain the population in the field with dormant seed lot

Mehods / Techniques of dormancy break:

Mehods / Techniques of dormancy break Scarification  Hot water treatments  Stratification treatment Leaching of metabolites (Inhibitors) Temperature treatments  Light and Phytochrome Promoters - inhibitors concept Seed Treatment with Growth Regulators/Chemicals

01) Scarification:

01) Scarification “Any treatments may be physical or chemical that weakens or softens the seed coat” More applicable to Malvaceae and Leguminaceae group of seeds. Types of Scarification Acid scarification :  H 2 SO 4  @ 100 ml/kg of seed for 2-3 minutes Duration of treatment will vary and it depends on type and nature of seed coat. E.g.  Tree crops 1-3 hours, Rose seeds, treat the seed partially with acid and then given with warm stratification

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Mechanical scarification : Seeds are rubbed on a sand paper or with a help of mechanical scarifier or by puncturing on seed coat with the help of needle to enhance / increase the moisture absorption by seeds. E.g. Bitter gourd for sand scarification Sand and seed 2:1 ratio should be followed. Rub against hard surface of seed for 5 to 10 minutes .

02) Hot water treatments:

02) Hot water treatments It is effective in case of leguminous crop seeds. The seeds should be soaked in boiled water for 1-5 minutes for 60-80 minutes. Some crops like Bengal gram and Groundnut , hot water treatment for more than 1 minute is found injurious to seed.

03) Stratification:

03) Stratification Cold stratification  Incubate the seed at low temperature of 0-5  o C  over a moist substratum for 2-3 days to several months. It depends on the nature of seed and kind of dormancy. e.g. Cherry and oil palm seeds. . Warm stratification  Some seeds require temperature of 40-50 o C  for few days : e.g. paddy. Oil palm : 40-50 o C for 2 months Care should be taken during the treatment and moisture content of seed should not be more than 15% When seed dormancy is due to embryo factor , seeds can be subjected to stratification treatments.

04) Leaching of metabolites (Inhibitors):

Seeds soaked in water for 3 days. But once in 12 hours fresh water should be changed to avoid fermentation or seeds can be soaked in running water for a day to leach out the inhibitors. e.g. Coriander ( Coumarin ), Sunflower (Hydrocyanic acid) 04) Leaching of metabolites (Inhibitors)

05) Temperature treatments:

a) Low temperature treatments  Plants which grow in temperate and cooler climates, require a period of chilling for breakage of dormancy.  E.g.  Apple seed : storing the seeds at 5 o C 05) Temperature treatments b) High temperature treatment  Early flowering "winter" annuals. E.g.  Blue bell ( Hyacinthoides nonscripta ). Their seeds are shed in early summer and do not germinate until they have been exposed to the heat during high summer .

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c) Alternate temperature treatments  Most of the plant species which grow in temperate and cool temperate regions require alternate temperature for breakage of dormancy e.g. Bull rush ( Typha ). d) Fire Treatment Sub tropical and semi-arid regions have extremely hard seeds in which the seed coat is very impervious to water. Dormancy in such seeds is clearly coat imposed, and maybe broken by exposure to extreme heat such as fire.  E.g.  Seeds of  Calluna vulgaris - dormancy is broken by fire .

06) Light and phytochrome  :

06) Light and phytochrome    Inactive Phytochrome Active Phytochrome (Dormant) (Non Dormant) (Pr) ( Pfr ) Blue Green Color Yellow Green Color Red Light 670 nm Far Red Light 730 nm Red light promotes germination and Far Red Light affects germination

07) Promoters - inhibitors concept   :

07) Promoters - inhibitors concept    For regulation of germination the promoters and inhibitors present in the seed should be in a balanced manner. GA helps in translocation of food reserve materials to active site of meristematic activity. GA also helps in cell division. Cytokinin is a natural endogenous hormone which controls germination through DNA to RNA transcription system. Abscisic acid is an inhibitor that can prevent germination by affecting RNA synthesis.

08) Seed Treatment with Growth Regulators/Chemicals   :

08) Seed Treatment with Growth Regulators/Chemicals    If the endogenous dormancy is due to the presence of inhibitors, we can apply growth regulators at the low level to break dormancy. GA & Cytokinin and kinetin : 100-1000 ppm GA is light substituting chemical.  KNO3 2 % (e.g.) Oats, Barley and Tomato. Thiourea : (e.g.) lettuce - thiourea @ 10-2 to 10-3 M Ethrel : cotton seed (ABA in pericarp of seed)

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09) Infra red radiation treatment Infra red rays can be passed on to the seeds and dormancy can be released. 10) Pressure treatment Dormant seeds can be kept in autoclave and required pressure can be employed for breaking dormancy 11) Magnetic seed treatment Seeds can be kept in the magnetic field for about 1 to 10 days for breaking dormancy

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