Principles of Propagation from Seeds

Transcription

1 Learning Objectives: Describe the process of germination Compare methods for measuring germination Define the environmental and disease factors influencing germination Describe the types of seed dormancy and how dormancy controls germination Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 1

2 The Germination Process A seed is a ripened ovule. At the time of separation from the parent plant, it consists of an embryo and stored food supply, both encased in a protective covering. The activation of the seed s metabolic machinery leading to the emergence of a new seedling plant is known as germination. Seed coat Cotyledons Endosperm Radicle Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 2

3 Seeds may end seed development and display viviparous, recalcitrant, or orthodox seed behavior. Viviparous and recalcitrant seeds germinate before completing the maturation drying stage of development. Orthodox seeds continue to dry to <10 percent moisture and can be either nondormant or dormant. The Germination Process The transition from seed development to seed germination. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 3

4 For germination to be initiated, three conditions must be fulfilled: 1. The seed must be viable. The Germination Process 2. The seed must be subjected to the appropriate environmental conditions. 3. Any primary dormancy condition present within the seed. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 4

5 Early seed germination is a triphasic increase in seed fresh weight due to increasing water uptake. The three phases are described as follows: Imbibition - an initial rapid increase in water uptake. The lag phase - a period of time where there is active metabolic activity but little water uptake. Radicle protrusion - results from a second period of fresh weight gain driven by additional water uptake. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 5

6 Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 6

7 Imbibition phase Imbibition is the initial uptake of water into the seed. Water uptake is controlled by water potential forces. Water Potential forces = m + + p Matric Osmotic Pressure Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 7

8 Imbibition phase Orthodox seeds are very dry < 10%. Initial water uptake is from matric forces. Water Potential forces = m + + p Matric Osmotic Pressure Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 8

9 Imbibition phase Imbibition is a rapid initial stage followed by a slower linear stage of water uptake is typical for most seeds. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 9

10 Imbibition phase Seeds leak during imbibition. This is due to a lack of membrane integrity in the dry seed. The amount of ion leakage can be diagnostic for seed quality. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 10

11 Imbibition phase Membranes are bilayers made from phospholipids. Water Phospholipids have a hydrophilic polar head and a hydrophobic tail. Head Tail The bilayer is necessary to maintain a semipermeable membrane. Until it is repaired during germination, seeds leak ions. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 11

12 Lag phase The lag phase is a period of time where there is active metabolic activity but little water uptake. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 12

13 The Lag Phase is a period of metabolic activity that prepares the seed for germination. Cellular activities critical to normal germination during the lag phase include: Mitochondrial maturity. Protein synthesis. Lag phase Metabolism of storage materials. Enzyme activity increases. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 13

14 Lag phase - Mitochondria Mitochondria are present in the dry seed and these must be rehydrated, and membranes within the mitochondria must become enzymatically active. Within hours of imbibition, mitochondria appear more normal when viewed by electron microscopy, and both respiration and ATP synthesis increase substantially. Mitochondria Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 14

15 Lag phase Protein synthesis Although mrna is present within the dry seed protein synthesis does not occur until polysomes form after seed hydration. Polysomes Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 15

16 Lag phase Protein synthesis New proteins are formed within hours of the completion of imbibition. Endosperm New protein synthesis during the lag period is required for germination. Nucleolus Aleurone cells Aleurone cells show prominent redstained nucleoli as an indication of new mrna synthesis for proteins. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 16

17 Lag phase Storage reserve metabolism This is the enzymatic breakdown of storage macromolecules to produce substrates for energy production and amino acids for new protein synthesis. Reserve metabolism also produces osmotically active solutes (like sucrose) that can lead to a change in water potential of cells within the embryo in preparation of radicle protrusion. Protein bodies in endosperm cells. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 17

18 Lag phase Enzyme synthesis Specific enzymes including those responsible for cell wall loosening in the embryo or tissues surrounding the embryo can be produced. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 18

19 Radicle Protrusion phase Radicle protrusion results from a second period of fresh weight gain driven by additional water uptake. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 19

20 Radicle Protrusion phase The first visible evidence of germination is protrusion of the radicle. This is initially the result of cell enlargement rather than cell division. However, soon after radicle elongation begins, cell division can be detected in the radicle tip. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 20

21 Radicle Protrusion phase Radicle protrusion is controlled by the opposing forces between the growth potential in the embryo and the physical resistance presented by the seed coverings Growth potential in radicle cells. vs. 2 Physical resistance of the seed coverings. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 21

22 Radicle Protrusion phase Radicle protrusion occurs when: (a) The water potential of the cells in the radicle becomes more negative due to metabolism of storage reserves. (b) Cell walls in the hypocotyl and radicle become more flexible to allow cell expansion. (c) Cells in the seed tissues surrounding the radicle weaken to allow cell expansion in the radicle. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 22

23 Radicle Protrusion phase Water potential A major way that the cell remains turgid is by keeping a negative osmotic potential in the vacuole where the cell stores sugars, amino acids and organic acids. = m + + p Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 23

24 Radicle Protrusion phase Water potential Water moves into or out of the cell due to osmosis. Plant cell Osmosis is the movement of water across a membrane in response to the osmotic potential on either side of the membrane. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 24

25 Radicle Protrusion phase Water potential Water moves to areas with more negative osmotic potential. Plant cell - 10 bars Osmotic potential is measured in bars or megapascals. - 5 bars In this example, water will move into the cell because it has the more negative osmotic potential. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 25

26 Radicle Protrusion phase Water potential Water no longer moves into the cell when the water potential inside and outside are at equilibrium (-5 bars). - 5 bars Plant cell - 5 bars Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 26

27 Radicle Protrusion phase In non-endospermic seeds like radish (Brassica) and lentil (Lens), the seed coat is thin and presents very little resistance to radicle protrusion. Epicotyl Radicle Cotyledon In these seeds, changes in the water potential of the cells in the radicle and cell wall flexibility are responsible for radicle elongation. Seed coat Lentil Radicle Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 27

28 Radicle Protrusion phase In endospermic dicot seeds, the seed coverings (especially the endosperm cap) can be a significant barrier to germination in some species. Endosperm properties are especially important under conditions that reduce germination, like low temperature conditions or seeds with physiological dormancy. Cotyledons Endosperm Seed coat Tomato Radicle Endosperm cap Radicle Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 28

29 Radicle Protrusion phase Balance of forces involved in germination Gibberellin promotes, while ABA inhibits changes in growth potential of the radicle Growth potential in radicle cells. vs. Physical resistance of the seed coverings. Gibberellins promote, while ABA inhibits enzymatic cell wall loosening in the seed coverings. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 29

30 Radicle Protrusion phase In seeds like tomato and tobacco, hydrolytic cell wall enzymes (like endo-b-mannanase and extensins) soften endosperm cell walls, and other cell wall enzymes (like β-1,3- glucanases) cause cell-to-cell separation. This permits germination by reducing the force of the seed coverings restricting radicle elongation and, finally, releasing the radicle for germination. Cotyledons Endosperm Tomato Radicle Endosperm cap Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 30

31 Radicle Protrusion phase In some seeds germination proceeds in two stages. (a) Seed coat (b) Seed coat First, radicle elongation initiates seed coat cracking while the endosperm stretches over the radicle. Endosperm (c) Seed coat (d) Endosperm Seed coat In the second step, the endosperm ruptures releasing the radicle. Hormones may act differently in each step. Radicle Petunia Endosperm Radicle Endosperm Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 31

32 Seedling Emergence Seedling emergence begins with elongation of the root and shoot meristems in the embryo axis, followed by expansion of the seedling structures. The embryo consists of a shoot axis bearing one or more cotyledons and a root axis (radicle). Shoot axis Root axis (radicle) Cotyledon Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 32

33 Seedling Emergence The seedling stem is divided into the hypocotyl, cotyledonary node, and the epicotyl. The hypocotyl is the stem section between the cotyledons and the radicle. The epicotyl is the section between the cotyledons and the first true leaves. Cotyledon Hypocotyl Epicotyl Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 33

34 Seedling Emergence Initial seedling growth usually follows one of two patterns depending on whether the hypocotyl or epicotyl emerges above the ground first. Hypocotyl Epicotyl Epigeous germination Hypogeous germination Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 34

35 Seedling Emergence In epigeous germination, the hypocotyl elongates, forms a hypocotyl hook, and raises the cotyledons above the ground. Hypocotyl hook Melon Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 35

36 Seedling Emergence - Epigeous germination In members of the cucurbit family, there is a structure on the hypocotyl called a foot that separates the seed coat to allow the cotyledons to emerge. Hypocotyl hook Hypocotyl Foot Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 36

37 Seedling Emergence - Epigeous germination The hypocotyl hook does not open until the seedling emerges and is exposed to light. This is an example of photomorphogenesis. Tomato Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 37

38 Seedling Emergence Hypogeous germination is characterized by a lack of hypocotyl expansion so only the epicotyl emerges above the ground, and the cotyledons remain within the seed coverings. Epicotyl Pea Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 38

39 Storage Reserve Utilization Initially, new embryo growth is dependent on the storage reserves manufactured during seed development and stored in the endosperm, perisperm, or cotyledons. The major storage reserves are Proteins Carbohydrates (starch) Lipids (oils) Grass endosperm Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 39

40 Storage Reserve Utilization Storage reserves are converted to amino acids or sugars to fuel early embryo growth. The embryo is dependent on the energy and structural materials from stored reserves until the seedling emerges into the light and can begin photosynthesis. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 40

41 Storage Reserve Utilization The general pattern of seed reserve mobilization leading to germination. These include the conversion of (a) starch to sugar; (b) lipids to sugar; and (c) storage protein to amino acids. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 41

42 Storage Reserve Utilization The cereal grain model for starch mobilization in seeds. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 42

43 Storage Reserve Utilization The cereal grain model for starch mobilization in seeds. Aleurone cells Endosperm cells containing starch Wheat Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 43

44 Storage Reserve Utilization Lipid conversion to starch is a complex set of enzymatic reactions coordinated between the oil body, mitochondria, and glyoxysome. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 44

45 Measures of Seed Germination Important aspects of seed germination can be measured by three parameters: Percentage Speed (Rate) Uniformity Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 45

46 Measures of Seed Germination Normal distribution The number of seeds that germinate per day can usually be represented as a normally distributed curve. The area under the curve is the germination percentage. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 46

47 Measures of Seed Germination Germination can also be represented as a sigmoidal distribution. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 47

48 Measures of Seed Germination - Percentage Number of seeds germinated / seeds sown x 100. Germination percentage (100%) Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 48

49 Measures of Seed Germination Speed (rate) Time to reach 50% germination or emergence in those seeds that germinate. Germination speed (T 50 = 75 hr) Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 49

50 Measures of Seed Germination Uniformity Germination uniformity measures how close in time seeds germinate. T 75 T 25 Germination uniformity = 45 T 75 T 25 = 45 hr Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 50

51 Measures of Seed Germination Comparing an untreated and primed seed lot shows how the three measures of germination describe their performance. Percentage Speed Uniformity Untreated 95% 4.4 days 3.5 days Primed 95% 3.2 days 2.4 days Both seed lots show the same germination percentage, but the primed seed lot germinates faster and more uniformly. Hartmann and Kester s Plant Propagation, Principles and Practices 8 th ed. Hudson Hartmann, Dale Kester, Fred Davies, Jr. and Robert Geneve 51