Seed dormancy, its causes and applicability

Seed dormancy is defined as a state in which seeds are prevented from germinating even under environmental conditions normally favourable for germination. According to Wareing (1965) the term dormancy is used in the sense where the viable seed of a given species falls to germinate under conditions of moisture, temperature and oxygen supply which are normally favourable for the later stages of germination and growth of that species.
Many seeds do not germinate when placed under conducive conditions for germination viz., optimum temperature, moisture, oxygen. However, seeds can be shown to be viable, as they can be induced to germinate by various special artificial treatments, or under special external conditions. Such seeds are said to be dormant, or to be said in a state of dormancy.
Plants with a long history of domestication generally, show less dormancy than wild or recently domesticated species. When domesticated species exhibit dormancy, they become a problem to the communities involved in seed occupation. However, a degree of dormancy in certain crops is desirable since it prevents pre-harvest germination and helps in the maintaining seed quality. Though, dormancy may be cause seeds of numerous species to maintaining ungerminated in the soil for many years. This explains the presence of unwanted crop plants or weeds in fields that are cultivated regularly and maintain seed quality.

1. Seed Dormancy: Causes And Applicability Pragya Naithani 45979

2. What is s eed d ormancy ? Seed dormancy is defined as a state in which seeds are prevented from germinating even under environmental conditions normally favorable for germination. These conditions are a complex combination of water, light, temperature, gasses , hard seed coats, and hormon es .

3. S.No Types Causes Embryo character Pre-treatment 1 Physical Seedcoat 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. Comprehensive Classification of seed dormancy

4. Harper, 1957 Nature of origination Innate  Induced   Enforced Time/Cause of origin  Primary     Secondary Location of dormancy Exogenous   Endogenous  Combined Most extensively used classification system of seed dormancy

5. Mechanism of Dormancy Hormonal control theory. Temperature and gaseous exchange restriction theory. Metabolic control theory.

6. Hormonal control theory Dormancy causing hormones are ABA and ethylene. Dormancy causing hormones and dormancy provoking hormones (GA & Cytokinin) ratio determine the fate of dormancy. Chilling treatment, light exposure increases Gibberellin l evel in seeds & causes germination.

7. HORMONAL MECHANISM : ABA and GA have antagonistic roles in germination and dormancy respectively. ABA induces and maintains seed dormancy during embryo maturation. The key to ABA metabolism are some specific genes encoding key enzymes are of NCED family ( NCED6 and NCED9 ) ( Nine-cis-epoxy carotenoid dioxygenase) Embryonic identity protein (FUS3), which positively regulates ABA synthesis.

8. Temperature and gaseous exchange theory Restricted oxygen uptake (due to hard seed coat) and high temperature induces dormancy in seeds. Oxygen deficiency leads to inhibition of Cytochrome oxidase activity which leads to dormancy.

9. Metabolic control theory Dormancy in seed is controlled by metabolic energy produced in seeds. Respiratory system blocked ATP synthesis b locked increase in AMP and ADP leads to increase in the activity of Glycolytic pathway. Production of growth inhibitors at lower concentration of oxygen.

10. Causes of Seed Dormancy Hard seed coat Immature embryo Germination inhibitors Dormancy due to specific light requirement. Period after ripening Temprature Excessive Salts

11. Hard s e e d coat 19 Water impermeability : Leguminosae , Gramineae, Malvaceae ,, Geraniaceae , Ohenopodiaceae , Solanaceae, Chenopodiaceae, Convolvulaceae, and Nymphaeaceae etc. Gas impermeability : Xanthium Mechanical resistance (physically prevents the expansion of the embryo) : Alisma , Amaranthus , Capsella

12. Immature Embryo Rudimentary and poorly developed embryo : Emb r y o ful l y de v eloped bu t unable to re s ume growth : 22

13. Rudimentary and poorly developed embryo The embryo does not develop as rapidly as surrounding tissues. Embryos are still immature and rudimentary when the seeds are dispersed. Anemone nemorosa , Fraxinus , Caltha palustris , Ginkgo biloba , members of Orchidaceae , Orobanchaceae etc. Seeds need time for maturity .

14. Emb r y o ful l y de v eloped bu t unable to re s ume growth Iris, cher r y , tulip, p o pla r, apple, pe a ch, pi n es, p e as, e t c . Seeds need Stratification or after- ripening.

15. Specific Light Requirement : There two type of seeds for light sensitivity: Positive photoblastic seeds: R emain dormant in the dark, germinate when exposed to light Ficus, Lettuca sativa ( requires red light (660nm) or white light) Rhododendron, Tomato, Bignonia (requires a photoperiod of 12 or more hours) Negative photoblastic seeds: R emain dormant when exposed to light , germinate only in the dark. Tobacco, Amaranthus, Nigella, Phlox, Nemophila and Silene The light sensitive seeds are called photoblastic .

16. Germination Inhibitors F ound in the pulp of the fruits, seed coat, endosperm and embryos or structures surrounding them Tomato fruit pulp – Caffeic acid and Ferulic acid Avena sp.- glumes Chemical substances Orga n ic Acids, P h enol i cs, Tan n ins, Alkaloids, Cyanides , Indoles, ABA, Ammonia, Phthalides, Coumarin, Parascorbic Acid , Cyanides, Ferulic Acid , Dinitrophenols, Aldehydes, Fluorides, U n satur ated Lac t o n es

17. S.No Species Location of inhibitor Name of inhibitor 1 Gossypium spp. Pericarp, testa Absicic acid (ABA) 2 Coriandrum sativum Pericarp Coumarin 3 Helianthus annus Pericarp, testa Hydrocyanic acid 4 Oryza sativa Hull Probably ABA 5 Triticum spp. Pericarp, testa Catechin, catechin tannins, several unknowns 6 Hordeum vulgare Hull Coumarin, Phenolic acids. scopoletin 7 Elaegnus angustfolia Pericarp, testa Possibly coumarin 8 Beta vulgaris Pericarp Phenolic acids, Possibly ABA, high concentration of inorganic ions 9 Avena sativa Hull Unknown Location and name of germination inhibitors in certain species Top

18. Temperature Some seeds require chilling treatment for germination . Apple, Walnut, and Pinus (chilling temperature of 1 -5 ºC for few weeks)

19. Dormancy factor DOG1 transcription levels are enhanced by low temperatures during seed maturation.

20. Excessive Salts S eeds contain a high concentration of solutes which do not allow the embryo to resume its growth Atriplex Osmotic stress

21. Applicability of Seed dormancy Species survival after natural disasters, pandemic, abiotic stress More time for widespread seed dispersal and escape from unfavorable situations Hard seed coat s allow seed to come out of the alimentary canals of birds and other animals uninjured e.g., Guava

22. Germination inhibitors present in the seed coats of desert plants dissolves away after the rainfall . This way dormancy ensures that seed gets proper supply of water during its germination Facilitates conservation of wild, rare, endangered plant species in seed bank / gene bank artificially for the future use Allow storage of food grains for longer period of time

23. Groundnut:- Genetic manipulation of Spanish and Valencia bunch varieties by introducing genes  from the semi-spreading and spreading varieties (Virginia types) for seed dormancy of three to four weeks. - Patro ( 2016) OUAT,Oddisha Induction of Dormancy in Spanish Groundnut Seeds Using low doses of Cobalt-60 Gamma Irradiation (50Gy) remain dormant for five months after harvest - Benslimani,N (2009)

24. Effect of environmental conditions on the dormancy and germination of volunteer oilseed rape seed ( Brassica napus ) Effect of time of incubation (5, 14, and 28 d), water potential (+ = 0 and -1,500 kPa), and light conditions (FR and darkness) on induction of secondary dormancy in seed of two cultivars of oilseed rape. Germination test at 12 C in darkness. (Data are logit transformed values with percentage of dormant seed in parentheses) Incubation time Potential = 0 kPa = 0 kPa Potential =1,500 kPa 1,500 kPa (d) Cultivar FR Dark FR Dark 5 Libravo - -1.09 (11.9) - -2.19 (0) -1.62 (4.7) - -1.88 (2.2) Falcon - -1.87 (1.9) - -2.19 (0) -1.95 (1.1) -2.13 (0.3) Mean - -1.48 (6.9) - -2.19 (0) -1.78 (2.9) -2.01 (1.21) 14 Libravo - -1.06 (12.8) - -2.19 (0) - -1.11 (10.8) - -1.41 (6.9) Falcon - -0.95 (15.3) - -2.19 (0) - -1.68 (3.3) -2.01 (0.8) Mean - 1.01 (14.1) - -2.19 (0) -1.40 (7.1) -1.71 (3.9) 28 Libravo ___b - -2.19 (0) -0.49 (26.7) -0.7 (20.3) Falcon - - -2.19 (0) -0.95 (13.1) - -1.51 (5.0) Mean - - -2.19 (0) -0.72 (19.9) -1.1 (12.6) Francisca et al .,1998

25. Exploiting Weed Seed Dormancy for weed control Soil fertility levels Sodium nitrite was more effective than nitrate in breaking dormancy of dehulled red rice. Cohn et al., 1983 In wild oats, potassium nitrate stimulated germination of dormant seeds in the light. Hilton et al., 1985 Ammonium nitrate increased emergence of some moderately dormant ecotypes about 30% in pot experiments. Agenbag et al., 1989 Light effects cultivation during daylight serves to increase weed populations Wesson and Wareing , 1969 Significantly less weed emergence takes place in plots cultivated at night than during daylight, suggesting that this approach may have practical applications for weed management Hartmann and Nezadal

26. Thank you 28