CHapter_7_Seed_dormacy_&_pretreament_ppt

This chapter mainly focuses on seed dormancy and its pretreatment mechanisms.

1. CHAPTER SEVEN SEED DORMACY & PRESOWING TREATMENT 1

2. Outline of the chapter Introduction Classification of dormancy Pretreatment mechanisms Trade and Distribution 2

3. 7 .1. Introduction… Seed dormancy refers to a state in which viable seeds fail to germinate when provided with conditions normally favourable to germination ( adequate moisture, appropriate temperature regime, a normal atmosphere and in some cases light). ‘ Resistance’ to germination can have different degrees, ranging from very slight , causing just a short delay of germination, to very strong/deep , in which seeds need a strong pretreatment to initiate germination. Sometimes the development or degree of dormancy changes during the lifetime of the seed, usually as a response to external conditions . Hence , dormancy may be innate (inborn), develop, be broken and redevelop in seed . 3

4. 7 .1. Introduction… In nature, dormancy is broken gradually or by a particular environmental event. In seed handling, the natural dormancy-breaking mechanism is applied or simulated during the process of pretreatment. ‘ Pretreatment ’ is a term used for conditions or processes applied to break dormancy prior to germination, while ‘treatment’ is used for application of pesticides for control of pest and diseases. The type of pretreatment is thus closely related to the type of dormancy, and in seed handling the classification of dormancy is linked to the pretreatment procedure . 4

5. 7 .1. Introduction… Dormancy can occasionally be advantageous, e.g . to prevent presowing germination of recalcitrant seed ; however, for orthodox seed, where germination is easily prevented by drying, dormancy is mostly considered as an inconvenience or constraint in seed handling . Seeds which have not been given an appropriate pretreatment to overcome dormancy: may fail to germinate altogether , germination may be slow or germination of individual seeds in a seed lot may take place over a lengthy period . Pretreatment thus has a dual purpose, viz. to ensure both that seeds will germinate , and that germination is fast and uniform . 5

6. 7 .1. Introduction… Dormancies are products of different dispersal modes, regeneration strategies and taxonomic relation. For example, animal-dispersed fleshy fruits usually have dormancy caused by inhibitory substances , pioneers are often light-dependent and leguminous species often develop hard impermeable seed coats . Knowledge of species taxonomy, morphology, dispersal and regeneration biology thus often gives a clue to dormancy type . However, variation in dormancy between and within species, and variation within the same seed lots, implies a current challenge to refine and adopt the pretreatment method to maximize germination . Pretreatment methods thus often have to be adjusted to individual species and seed lots on the basis of experience and experiments. Pretreatment is often a matter of adjusting the strength or duration of already-known methods , rather than adopting new ones . 6

7. 7 .1. Introduction… Pretreatment is normally undertaken shortly before sowing, i.e. after storage , because some types of dormancy may redevelop if pretreatment is undertaken earlier, and because pretreatment and germination conditions are often interlinked. In some cases dormancy is overcome by providing appropriate germination conditions, rather than a special pretreatment. An example is where physiological dormancy is overcome by light or fluctuating temperature . Like any other seed handling operation pretreatment is subject to economic considerations. Where dormancy is weak, the gain of pretreatment may be marginal. Where several alternative methods exist, the technically most effective may not always be the most economical one. 7

8. 7 .1. Introduction… Some pretreatment procedures are not directly related to seed dormancy , but are carried out in order to speed up the germination process or promote seedling establishment. Various hormones and nitrogenous compounds may help in breaking dormancy under certain conditions, and may simultaneously have a direct impact on germination . 8

9. 7.2. Dormancy in a regenerational context The ecological purpose of dormancy is to delay germination until the chances of seedling survival are high and/or to spread germination over a prolonged time. This makes sense for species regenerating in a seasonal climate and under variable conditions . Dormancy here serves to ‘save’ seed from waste germination efforts under conditions. Some of these situations for example are: Germination conditions in terms of temperature and moisture are suitable for germination, but survival conditions for young seedlings are poor because of subsequent low winter temperatures. Light showers may be sufficient to trigger germination, but not to provide adequate moisture for the seedlings to establish themselves . Regeneration in fire-prone areas is greatly improved after fire. Seeds that happen to be buried under a thick layer of soil may be unable to reach the soil surface during germination, only germinate if they are uncovered. Seedlings of light-demanding pioneers cannot survive the shaded conditions under a forest canopy. They regenerate only after gap formation. 9

10. 7.3. Classification of dormancy Several types of dormancy classifications exist. Traditionally physiologists , ecologists and seed handlers have used different classifications. A plant-physiological approach classifies dormancy according to which part of the seed is responsible. Dormancy related to the embryo, e.g. immature development or chemical inhibitors located in the embryo, is referred to as endogenous or embryo dormancy . Analogously , mechanical resistance, physical impermeability, inhibitors or light sensitivity associated with the seed coat are called exogenous or seed-coat (enhanced) dormancy . An ecological approach prefers classification based on development : ( 1) innate (or primary) dormancy is when dormancy is developed before dispersal (e.g. hard seed coat or chemical inhibitors), and (2 ) induced (or secondary) dormancy is dormancy developing as a response to external environmental factors (e.g. drought or cold ). 10

11. 7.3. Classification of dormancy… Seed practitioners relate dormancy to the physiological nature and the method of pretreatment used to overcome them simplified here to six main types : Mechanical dormancy is used for structures which impede expansion of the embryo ; Physical dormancy refers to impermeability or serious delay in water absorption; T hermodormancy here encompasses all types of temperature-related dormancies , whether high, low or fluctuating ; Photodormancy encompasses all light-related dormancy phenomena ; Chemical dormancy is used for all types of dormancy based on chemical inhibitors ; Immature embryo dormancy is used for the delayed germination caused by an undeveloped embryo at dispersal. Where two or more dormancy types occur in the same seeds it is called ‘ double dormancy ’ or ‘ combined dormancy ’. 11

12. 7.4. Dormancy types and pretreatment methods Table 7.1. Classification and characteristics of seed dormancy 12 Dormancy type Characteristics Examples of occurrence Dormancy breaking stimulus Natural Seed handling *Immature embryo Seeds are physiologically immature for germination Fraxinus excelcior, Gingko biloba Post-dispersal development After- ripening Mechanical dormancy Embryo development physically restricted due to hard seed/fruit coat Pterocarpus , some Terminalia spp., Melia volkensii Gradual decomposition of hard structures, e.g. by termites Mechanical cracking of restricting structure; Cold water treatment  drying  soaking Physical dormancy Imbibition impeded because of impermeable seed-coat or fruit Mainly hard seed Leguminosae , plus some Myrtaceae and others Acacia , Prosopis , Albizia and many lowland species Abrasion by sand, high temperatures , temperature fluctuations, ingestion by animals, or other mechanical or chemical impact Mechanical scarification (e.g. abrasion or burning), boiling water or acid pretreatment

13. 7.4. Dormancy types and pretreatment methods… Table 7.1. Classification and characteristics of seed dormancy 13 Dormancy type   Characteristics   Examples of occurrence   Dormancy breaking stimulus Natural Seed handling Chemical dormancy Fruit and seed contain chemical inhibitory compounds that prevent germination Fleshy fruit such as berries, drupes and pomes, plus some dry seeds Ingestion by frugivores, leaching by rain, gradual decomposition of fruit pulp Removal of fruit pulp plus leaching with water Photo dormancy Seeds fail to germinate unless exposed to appropriate light conditions/regime. Many temperate species, Humid tropical pioneer species e.g. Spathodea and some eucalypts Exposure to light conditions likely to promote seedling survival viz. white light or light relative rich in red light Exposure to light, normally during germination, sometimes a distinct light- dark cycle of variable duration Thermo dormancy Germination low without pretreatment with appropriate temperatures Most temperate species, e.g. Pinus Dry zone tropical - subtropical pioneers e.g . Eucalyptus, Humid tropical pioneers Exposure to low winter temperature. Exposure to grass, bush or forest fires. Diurnal fluctuating temperature in gaps Stratification or chilling High temperature, e.g. kiln or light burning Fluctuating temperature

14. 7.5. Accelerated Germination Whether seeds are sown in the nursery or directly in the field, an accelerated germination process is desirable since its helps to shorten the total establishment period and to overcome the most vulnerable period of seedling establishment . The germination process can be accelerated by different means during the three main germination phases : Stage 1: imbibition rate. The higher the water pressure, the faster the imbibition . And the faster the imbibition, the faster the seed enters into the next phase. Stage 2: lag phase. The second phase of germination is mobilization of the metabolic system. The seeds use primarily their internal resources, but experience shows that various hormones can help shorten the germination process. Stage 3: growth phase. The third stage of germination is the growth expansion of the embryo, which continues into the seedling growth phase , where seeds become increasingly reliant on their own absorption. 14

15. 7.5. 1 . Soaking in Water For non-dormant seed, soaking serves to ensure fast imbibition and thus entrance into the second phase of germination. Species with thin and fragile seed coats become more sensitive to mechanical damage after imbibition. 15

16. 7.5. 2. Growth Regulators Some compounds stimulate individual metabolic processes during germination without being directly linked to dormancy . The effect of germination stimulants is often most evident under suboptimal germination temperatures. Total germination percentage, germination speed and seedling vigour may be promoted by application of germination stimulants. The main hormone group responsible for suppression of germination (dormancy hormone) is ABA; the main hormone group that stimulates germination and growth is gibberellic acid (GA ). Various nitrogenous compounds such as potassium nitrate (KNO3) and thiourea promote the germination process. Neither hormones nor other germination compounds are much used in practical seed propagation, but as more knowledge is generated, they may become suitable alternatives to overcome complicated physiological dormancy constraints. 16

17. 7.5. Trade and Distribution The supplier or collector ought to have adequate information recorded about the seeds starting from identification of mother trees. There are four main documents used in distribution/ trade: 1. Consignment Note : Designates the sender and the consignees, net weight of the seed lot. It is used by carriers, and needed in the customs, quarantine. 2. Record of origin : Specifying the stand, locality 3. Certificate of quality : Date of test, results of quality parameters, moisture content treatment applied etc. 4. Certificate of Health: phytosanitary certificate 17

18. 7.5. Trade and Distribution…. Any biological material imported or exported is subject to Quarantine Regulations. These are rules and regulations governing the healthiness of biological material carried in to or out of a country. The document bears Date, name and signature of Government authority certifying that “the seed lot has been thoroughly examined and found to be substantially free from injurious disease and pests It may also specify the type of treatment/ chemical applied ( e.g fumigant) and the duration. Quarantine regulations may vary from country to country, and its important to know such rules and regulations of the country of interest before conducting trade /distribution. 18

19. 7.5. 1 Sample of seed lot label (for customer use ) Seed lot number………………….. Species…………………….. Local name…………………Language….…….. Seed source No……………Locality…………….. Woreda …………………..Zone………….Region………… Seed source category…………………… Date of latest test…………………. Purity……………………………Moisture content…………………% No. of seeds per Kg…………………………. Germination percent…………… Pre-sowing treatment…………………. 19

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