seed dormancy

Seed dormancy, types of seed dormancy, causes of seed dormancy, overcoming of seed dormancy, resting stage, stratification, Scarification

1. Presented by… Maitri . M . Thakor ☻ M.Sc. (Botany) Department of Life Sciences, H.N.G.U., Patan . S eed Dormancy 1

2. contents Introduction Types of Dormancy Types of Seed Dormancy Causes of Seed Dormancy Overcoming of Seed Dormancy 2 ( Source: https ://www.upol.cz/fileadmin/_processed_/4/4/csm_hrach_62f4e13155.jpg)

3. introduction Dormancy : 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. Dormant = not active or growing but able to become active later . 3

4. Wareing (1969) defined dormancy as any phase in the life cycle of a plant in which active growth is temporarily suspended. Seed Dormancy : It is the incapacity of fully developed, mature, viable seed to germinate even under favourable conditions. In such cases, the completely dry ripe seed is physiologically inactive and is said to be in a resting stage . The seed is called dormant and the phenomenon is termed dormancy . 4

5. The main reason behind this condition is that they require a period of rest before being capable of germination. This Conditions may vary from days to months and even years. 5 (Source : https://cdn1.byjus.com/wp-content/uploads/2018/12/Seed-Dormancy.png)

6. types of dormancy According to wareing (1969) the dormancy may be two types : 1) Imposed dormancy or quiescence: The dormancy due to unfavourable environmental conditions is called imposed dormancy or quiescence . 2) Innate dormancy or rest : 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. 6

7. During the entire process, there may be following three phases of dormancy : 1) Pre-dormancy or early rest : During these phase, the dormant organ has capacity to resume growth by various treatments i.e., capacity of germination or growth is not completely lost. It is called predormancy . 2) Full dormancy or mid rest : When a seed or organ becomes completely dormant and germination or growth cannot be induced by changes environmental conditions, it is called full dormancy or mid rest . 7

8. 3) Post dormancy or after rest : When a dormant seed or organ gradually emerges from full dormancy and in it the germination or growth can be induced by changing enviornmental conditions, it is called post-dormancy or after rest . The dormancy may be true, relative or secondary : 1)True dormancy : When in a seed or organ, the germination or growth cannot be induced under any set of environmental conditions, it is called true dormancy . 8

9. 2) Relative dormancy : When in a seed, the germination can be induced under specific conditions even at the time of its deepest dormancy, it is called relative dormancy . 3) Secondary dormancy : When a seed has not fully emerged from dormancy and is again thrown back into full dormancy by certain environmental conditions, e.g., temperature etc., it is called secondary dormancy . 9

10. types of seed dormancy Crocker (1916) divided seed dormancy into two types : 1) Seed coat induced dormancy : The dormancy of seeds due to extreme hardness of seed coat is called seed coat induced dormancy . 2) Embryo induced dormancy : The dormancy of seeds due to rudimentary or complete dormant embryo is called embryo induced dormancy . 10

11. Other types of dormancy may be – 3) Secondary dormancy : When the seed become dormant again after breaking the dormancy, it is called secondary dormancy . It may be due to combination of different kinds of dormancy in a single seed, e.g., Xanthium pennsylvanicum. 4) Special type of dormancy : The failure of seedling development is not always traceable due to dormancy of seed itself. 11

12. In many of the spring wild plants the germination of seed takes place but the growth is restricted due to establishment of young roots. Sometimes the system of epicotyl fails to germinate. In some cases, the epicotyl may be pushed through the seed coat but remains dormant. This dormancy is often broken by exposure to low temperature. 12

13. According to C. Baskin and J. Baskin (1998; 2004) have proposed a comprehensive classification system which include five classes of seed dormancy – (1) Physiological dormancy (2) Morphological dormancy (3) Morpho-physiological dormancy (4) Physical dormancy (5) Combinational dormancy 13

14. 1) Physiological dormancy : Physiological dormancy prevents embryo growth and seed germination until chemical changes occur. It is result of seed requiring some type of physiological condition to be met in order to germination. These chemicals include inhibitors that often retard embryo growth to the point where it is not strong enough to break through the seed coat or other tissues. 14

15. 2) Morphological 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. These embryo do not have physiological dormancy and only require additional time to grow and germinate. Commonly, under favourable conditions, embryos in such seeds begin growth within a period from a few days to several weeks, and seeds germinate within 30 days. 15

16. 3) Morpho-physiological 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. Thus, to germinate these seeds require time for embryo growth and a dormancy breaking treatment. Morphological dormancy can be divided into two types- epicotyls dormancy and double dormancy. 16 ( Sources: http://botanico.uclm.es/wp-content/uploads/2015/10/Captura-de-pantalla-2015-10-13-a-las-11.20.21.png) Double dormancy

17. 4) 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, Cherry etc. (hardened endocarp of seed). 17

18. 5) Combinational dormancy : This class groups seeds with simultaneous physiological and physical dormancy. In this case, physiological dormancy is generally characterized as non- deep. A cold stratification treatment of seed after scarification to permit imbibitions is a common dormancy breaking treatment in this class of seeds. 18

19. causes of the seed dormancy Bewlay and Black (1994) have divided seed dormancy into two categories, seed coat based and embryo based . The dormancy of seeds may be either due to single or a combination of many different following factors. 1) Seed Coat induced dormancy / Hard seed coat : The seed coat is mostly formed by the integumentary layers of ovules. The seed coat consists of two layers – the outer is testa and the inner is tegmen . 19

20. The testa particularly is composed of a complex mixture of polysaccharides, hemicelluloses, fats, waxes and proteins. During the maturity of seed dehydration takes place in the seed coat and thus it forms a tuff protective layer. Hard seed coat prevents germination due to following reasons. (i) Water impermeability : Seed coats of many plants belonging to families Leguminosae, Chenopodiaceae, Malvaceae, Convolvulaceae, Solanaceae and Nymphaeaceae etc. have very hard seed coats so, it cannot permeable to entry of water in seed. 20

21. (ii) Gas impermeability : The seed coat of certain seeds are impermiable to gases such as oxygen(o 2 ) and carbon-di-oxide (co 2 ). Since oxygen is required for early respiratory activity in germinating seeds, the seeds fail to prolong germination. e.g., Xanthium. iii) Mechanical resistance : In certain wild plants the such hard and tough seed coat physically prevents the expantion of the embryo. Thus they remain dormant. E.g., Alisma , Amaranthus , Capsella etc. 21

22. 2) Embryo Induced Dormancy : Dormancy due to embryo condition may be of two types- (i) Rudimentary and poorly developed embryo (ii) Embryo fully developed but unable to resume growth. (i) Rudimentary and poorly developed embryo : Instance the embryos are still immature and rudimentary when the seeds are dispersed. This is seen in many species like Anemone nemorosa , Ginkgo biloba , members of Orchidaceae , Orobanchaceae etc. In such seeds the embryo does not develop as rapidly as surrounding tissues. 22

23. (ii) Embryo fully developed but unable to resume growth : In many species, e.g., Seeds of apple, peach, Iris, cherry, tulip, poplar, pines, peas, etc., although the embryos are completely developed in ripe seed but the seeds fails to germinate even when the environmental conditions for germinations are favourable. The embryo of such seeds does not germinate even if the seed coats are removed. The germination in such seeds can be induced if they are stored in moist, well aerated and low temperature conditions. This process is called Stratification or after- ripening . 23

24. 3) Dormancy due to Specific Light Requirement : The seeds of certain plant species such as Lactona sativa , Lythrum salicaria , Nicotiana tabacum etc. have specific light requirement for germination. Light not only qualitatively but also quanti-tativly. The germination of certain seeds requires a specific photoperiod , e.g., Bignonia requires a photoperiod of 12 or more hours for seed germination. The light sensitive seeds are called photoblastic . 24

25. 4) Dormancy due to Germination Inhibitors : Seeds of certain plants contain compounds which inhibit their germination. Such natural germination inhibitors have been found in the pulp of the fruits, seed coat, endosperm and embryos or structures surrounding them etc. (e.g., in tomatoes, in glumes of Oats etc.). A number of chemical substunces such as organic acids, phenolics, tannins, alkaloids, unsaturated lactones, ammonia and cyanide releasing substunces, indoles and gibberellins etc. have isolated from seed which are germination inhibitors. Besides, other inhibitors are ABA, ammonia, phthalides, coumarin and parascorbic acid. 25

26. overcoming of seed dormancy The dormancy of seeds, though very useful to man, is not liked by the farmers who would like the seeds to germinate soon after they have been harvested. A number of methods are, therefore, employed for the breaking of dormancy. The methods employed vary from species to species depending upon the cause of the dormancy . There are main two types of overcoming of seed dormancy : 1) Natural overcoming of seed dormancy 2) Artificial overcoming of seed dormancy 26

27. 1) Natural overcoming of seed dormancy : Nature of dormancy stops when the embryo gets appropriate environment such as adaptive moisture and temperature. The seed coat that exists in many species becomes permeable due to the rupturing of smoothing action of natural agents like microorganism, temperature and abrasion by the digestive tracts of birds and animals that feed on these seed. Completion of over-ripening period. 27

28. Leaching of inhibitors present in the seed coat. Inactivation or oxidation of inhibitors by heat, cold and light. Production of growth hormones which can counteract the effect of inhibitors. Attainment of maturity of embryo in case the dormancy is due to incomplete development of embryo. 28

29. 2) Artificial over coming of seed dormancy : i) Scarification : 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 . Mechanical Scarification is done by shaking the seeds with sand or by scratching or nicking the seed coat with knife. 29

30. The treatment of seed coat with strong mineral acids or other chemicals is called Chemical Scarification . Chemical scarification is usually done by dipping the seed into strong acids like H 2 SO 4 or into organic solvents like acetone or alcohol. It can also be done by boiling the seeds in water. 30 Sources :( http://www.culture-acre.com/wp-content/uploads/2017/07/seed-scarification.jpg) (Sources : http://www.usa-gardening.com/seed-scarification/germinating-seeds/scarification-techniques.jpg) ( Sources: https ://image.slidesharecdn.com/botanyforgardeners-2014-final-140202175042-phpapp02/95/botany-for-gardeners-2014-70-638.jpg?cb=1391363643) Scratching by Knife Shaking by Sandpaper By Chemical

31. 31 (Sources : http://slideplayer.com/4694655/15/images/22/Seed+Scarification.jpg)

32. ii) 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 (0°C to 10°C) for weeks to months. This treatment is called Stratification or after-ripening . 32 ( Sources: https ://image.slidesharecdn.com/methodsforbreakingseeddormancy-150208101506-conversion-gate02/95/methods-for-breaking-seed-dormancy-5-638.jpg?cb=1423390603)

33. During this treatment, anatomical and biochemical changes takes place in the seed. Growth promoting hormones increases and growth retarding hormones decreases. iii) Alternating temperature : In some seeds, the dormancy is broken by the treatment of an alternating low and high temperatures.e.g ., Poa pratensis The difference between the alternating temperature should not be more than 10°C-20°C. This method is beneficial in those seeds in which the dormancy is due to immature embryos. 33

34. Alternating temperatures of 15°C and 25°C is useful in breaking the dormancy of photoblastic seeds like Rumex crispus . iv) Impaction : In some of the seeds a strophiolar plug blocks (testa pores) the entry of water and oxygen into the seed. In order to remove the plugs the seeds are shaken vigoursly and this treatment is known as Impaction . E.g., Crotolaria , Trigonella seed. 34 (Source : https://classconnection.s3.amazonaws.com/345/flashcards/6176345/gif/seedia-148D474B9CE6A431CC3.gif)

35. v) Light : Dormancy of photoblastic seeds can be over-come by light exposure. In positive photoblastic plants light induces germination. E.g., Amaranthus, Betula, Capsella, Epilobium, Lactuca etc. The dormancy of positive photoblastic seeds can be broken by exposing them to red light (660 nm). E.g., Lactuca sativa . In negative photoblastic plants light infact prevents germination. In such cases seeds have to be stored in dark for some time and allow to germinate in darkness before they are shifted to light. 35

36. vi) Pressure : Davies (1928) 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. It is strongly believed that the pressure increases the permeability of the seed coats to water. 36

37. vii) Growth Regulators : Kinetins and gibberellins have been used to induce germination in positively photoblastic seeds like lettuce and tobacco etc. Counteracting the effect of growth inhibitory by soaking the seeds in KNO3, ethylene chlorohydrin, thiourea, gibberellins. 37 (Source : https://historicjamestowne.org/wp-content/uploads/tobaccoseed.jpg) Tobacco seed

38. 1) A textbook of Plant Physiology, Biochemistry and Biotechnology Author : S.K. Verma, Mohit Verma Edition : 2007 (Sixth Edition) 2) Textbook of Plant Physiology Author : V. Verma Edition : 2007 (First Edition) 3) www.biologydiscussion.com 4) www.slideshare.net 38 References

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