1. Principles of Propagation by Seed
Chapter 7
Principles of Propagation by Seed

2. Principles of Propagation by Seed
Germination process-
Seed must be viable = embryo alive and capable of germination
Seed must receive:
Water
Proper temperature
Oxygen
Light (depending on species)
Primary dormancy must be overcome = after-ripening. Often removed by environmental conditions

3. Phases of germination

4. Principles of Propagation by Seed
Phases of Germination:
I.) Water uptake
By imbibition = a physical process in seeds with a permeable seed coat
Occurs whether seed is alive, dead, dormant or non-dormant
First minutes = rapid uptake
Followed by hours of slow uptake
Seeds generally do not wet uniformly
Volume of seed increases

5. Principles of Propagation by Seed
Phases of Germination:
I.) Water uptake (continued)
Leakage: amino acids, sugars, proteins,... Since cell membranes are not fully functional yet
Quantity leaked is proportional to seed quality
High leakage means the seed is susceptible to attack by insects, fungi, and bacteria (can be measured by an electrical conductivity meter)

6. Imbibition & Lag Phase
Fresh Weight

7. Principles of Propagation by Seed
II.) Lag phase
Mitochondria mature
Proteins are synthesized (enzymes are activated)
Food reserves are metabolized
Enzymes loosen cell walls

8. Principles of Propagation by Seed
III.) Radicle emergence
Result of cell enlargement
Food reserves continue to be used
Enzymes degrade certain cell walls to permit exit of the radicle
GA promotes enzymatic cell wall hydrolysis and radicle emergence
ABA inhibits enzymatic cell wall hydrolysis

9. Radicle emergence

10. Principles of Propagation by Seed
Use of storage reserves
Proteins in protein bodies
In cotyledons and endosperm
Enzymes (proteinases) are required to break down proteins into amino acids
Proteinases synthesized during imbibition

13. Principles of Propagation by Seed
Starch
In endosperm (in grain crops)
Order of events:
Imbibition
GA in embryo & scutellum (protective sheath around cotyledons in monocots) translocated to the aleurone layer
Aleurone layer = a secretory cell layer that surrounds the endosperm
Enzymes are synthesized (-amylase)
Enzymes convert starch to glucose & maltose sugars and then transported to the embryo for use in development and growth
Note: these sugars are important in beer making!

14. aleurone layer

18. Principles of Propagation by Seed
Lipids
Oil bodies in endosperm & cotyledons
Oils = triacylglycerides (glycerol & fatty acids)
Glyoxysomes are organelles found ONLY in seeds! They process stored oils.
Fatty acids are high energy compounds used in the glyoxylate cycle to produce sucrose
Sucrose is then transported to the embryo for use in development and growth

19. Canola Seed Oil

22. Principles of Propagation by Seed
Measures of germination:
Germination percentage (%) = number of seedlings produced in a specified time
Germination rate - T50 value = # of days required to achieve 50% germination of the seed lot

23. Germination curve

24. Standard seed germination curve

25. Seed vigor

26. Principles of Propagation by Seed
Environmental factors influencing germination:
1.) Water - threshold water potential = amount of water needed by the seed for radicle emergence
Rate of water movement in soil depends on:
Texture (pore space)
Packing (pore space)
Closeness of seed/soil contact
Water with high salt content can counter-balance the effects of water imbibition (this is a problem in California with subirrigated fields and high water evaporation)

27. Principles of Propagation by Seed
Seed priming
Regulates water imbibition of seeds
Charles Darwin suggested this possibility in 1855!
Polyethylene glycol (PEG) used today (aerated)
Starts metabolic processes without radicle emergence
Seed is re-dried for short-term storage at cool temperatures
Results in uniform germination
Used on bedding plant plug production (annuals)

28. Seeds primed and pregerminated in aerated PEG

29. Effects of seed priming on germination

30. Principles of Propagation by Seed
2.) Temperature
The MOST important environmental factor that regulates TIMING of germination
Boil-treat seeds to control disease. This won’t damage the seed as long as the seed is DRY
Store seed at low temperatures to prolong viability
Temperature affects germination percentage and germination rate
Germination rate increases with an increase in temperature (up to a point)
Germination percentage is constant in the mid-temperature range and low on either end

31. Principles of Propagation by Seed
Temperature ranges:
Minimum - lowest temperature for germination
Maximum - highest temperature for germination
Optimum - a range where the greatest percentage of seedlings are produced at the highest rate

32. Principles of Propagation by Seed
Temperature categories
Cool-temperature tolerant - native to temperate zones prefer °F
Ex: broccoli, cabbage, carrot, peas, alyssum
Cool-temperature requiring - native to a Mediterranean climate. No germination if > 77 °F
Ex: celery, lettuce, onion, delphinium

33. Principles of Propagation by Seed
Temperature categories (continued)
Warm-temperature requiring - native to subtropical and tropical regions
Must be > 50°F for sweet corn & tomato
Must be > 60°F for beans, pepper, cucumbers, cotton

34. Principles of Propagation by Seed
Temperature categories (continued)
Alternating temperatures
Day/night temperature fluxes are better than constant temperatures
Used in seed testing labs
18°F (10°C) difference often used
Imbibed weed seeds deep in soil do not germinate since there is little temperature flux, however, they will germinate if the soil is cultivated and seeds are brought to the surface where there is temperature flux

35. Principles of Propagation by Seed
Aeration effects on germination
Oxygen uptake is proportional to the amount of metabolic activity
Oxygen diffuses through water slowly therefore waterlogged soils slow/inhibit germination

36. Principles of Propagation by Seed
Light effects on germination
Involves quality (wavelength) and photoperiod (duration)
Light-sensitive seed are generally small in size or are epiphytes (grow on other plants)
Ex: alyssum, begonia, coleus, orchids

37. Principles of Propagation by Seed
Light effects on germination
A few plants have germination inhibited by light
Ex: amaranthus, allium, phlox
Some require dark to germinate
Ex: calendula (pot marigold), delphinium, pansy
Some require a specific daylength
Ex: birch, hemlock

38. Principles of Propagation by Seed
Disease during germination
Damping-off
Pythium ultimum
Rhizoctonia solani
Botrytis cinerea
Phytophthora spp.
Drying, salts and excess heat at the soil surface can also look like damping off
Optimum growth between 68-86°F
Pb. on warm-requiring seeds
Secondary pathogens

39. Damping-off in tomato & soybean

40. Principles of Propagation by Seed
Dormancy - regulation of germination
Quiescent seeds - only need to be imbibed and an appropriate temperature for germination. NO dormancy!
Primary dormancy - a type of dormancy where seeds will not germinate despite adequate environmental conditions
Secondary dormancy - induced under unfavorable environmental conditions

41. Principles of Propagation by Seed
Dormancy is important to propagators because it allows storage, transport and handling of seed
After-ripening - changes in the dry seed during storage that allow the seed to germinate following favorable conditions

42. Principles of Propagation by Seed
Types of Primary Dormancy
Exogenous dormancy - factors outside the embryo (seed coat or parts of the fruit)
Inhibits water uptake
Physical restriction on embryo expansion or radicle emergence
Controlling gas exchange (O2/CO2)
Preventing leaching of internal inhibitors
Supplies inhibitors to the embryo

43. Principles of Propagation by Seed
Exogenous physical dormancy (“seed coat”)
Outer integument becomes hard or fibrous during dehydration and ripening (Ex: coconut, honeylocust, Kentucky coffeetree)
In drupes (cherry, peach, etc.). Have a hardened endocarp (“pit” or “stone”)
In nature, hard seed coats are softened by:
Microorganisms
Passage through an animal’s digestive tract
Abrasion -freeze/thaw
Fire

44. Principles of Propagation by Seed
Exogenous chemical dormancy
In fleshy fruits
Contain chemical inhibitors such as ABA (Ex: citrus, cucumbers, apples, pears, grapes, etc.)
Desert plant fruits have chemical inhibitors that must be leached away by rains that then provide enough water for germination and seedling development

45. Principles of Propagation by Seed
Endogenous dormancy
Morphological dormancy
Rudimentary embryo = araliaceae (ginseng), papaveraceae (poppy), ranunculaceae (anemone)
Linear embryo = ericaceae (rhododendron), annonaceae (pawpaw)
Overcome by:
Alternating temperatures
Treat with KNO3 or GA

47. Principles of Propagation by Seed
Endogenous dormancy (continued)
Physiological dormancy
Non-deep = after-ripening. Fresh seeds of herbaceous plants (annuals and many perennials) lose dormancy during standard storage
Photodormancy
Seeds require either light or dark conditions
Involves phytochrome (in most plants) which is photoreversible
There is often an interaction between light and temperature
Light requirement can sometimes be offset by cool temperatures or alternating temperatures (Ex: lettuce seed, can germ. in dark if temp. below 73°F)

48. Principles of Propagation by Seed
Endogenous dormancy
Physiological dormancy
Photodormancy (continued)
Seed coat or underlying endosperm act as light sensors (if removed, light control disappears)
Hormones (GA) can overcome a light requirement
Red > far-red in natural sunlight (2:1) therefore phytochrome is active (PFr form) and seed stimulated to germinate
Under foliage, far-red light penetrates more than red light, therefore phytochrome is inactive (Pr form) and seeds fail to germinate
Red light does not penetrate soil as deeply as far-red light, therefore light-sensitive seeds stay dormant until they get closer to surface (Ex: weed seeds)

49. Lettuce seed is light sensitive
Dark Light

53. Tanada effect Exposed to red light Exposed to far- red light
(phytochrome active)
Exposed to far- red light
(phytochrome inactive)

54. Principles of Propagation by Seed
Endogenous dormancy
Physiological dormancy
Intermediate/deep physiological dormancy
Stratification (moist-chilling)
Must be aerated
Moisture should be constant
Temperature should reflect the plant’s native habitat for the winter/spring (≈ ºF) with a minimum of 23ºF
Time for seed-chilling requirements are related to bud-chilling requirements!

55. Effects of stratification on germination

56. Seeds hormone levels during stratification

57. Effects of vernalization/stratification on seedling development

58. Principles of Propagation by Seed
Endogenous dormancy
Physiological dormancy
Intermediate/deep physiological dormancy (continued)
For intermediate dormancy, if embryo is removed from the seed, it will readily germinate (dormancy is mostly seedcoat) and moist-chilling time required is short
For deep dormancy, an excised embryo will not germinate readily nor will it form normal plants (physiological dwarfs) and these seeds require a long moist-chilling period (> 2mo.)

59. Principles of Propagation by Seed
Double dormancy
Seed has two kinds of dormancy
Ex: a rudimentary embryo & seed coat dormancy = morphological dormancy & physical (exogenous) dormancy
Thermodormancy
Different from thermal inhibition
Once exposed to high temperatures, will not germinate when temperatures return to the optimal germination range
Ex: lettuce in summer, therefore often primed (in PEG) at cool temperatures to allow germination prior to sowing

60. Principles of Propagation by Seed
Advantages of seed dormancy
Seedling survival - permits germination only when the environmental conditions are favorable
Creates a “seed bank” - not all seeds for a species germinate in a single year. Spreads germination out over time
Synchronize germination - for a particular time of year
Seed dispersal - especially after being carried in the digestive tract of animals

61. Summary of types of germination

62. Epigenous seed germination

63. Epigenous seed germination

64. Epigenous seed germination

65. Hypogenous seed germination