1. Imbibition and germination
Seed dormancy,
Imbibition and germination

2. Seed development and germination
Vivipary: No dormancy the seed continues growth.
Orthodox seeds: The seed enters dormancy, dehydrated seed- tolerance to water deficiency
Recalcitrant seeds: the seed enters dormancy but is not tolerant to water deficiency

3. Mangrove tree- Rhizophora- vivipary

4. Dormancy is an adaptive trait The consequence of natural selection
Drought and stress resistance
Maximal germination under existing environmental conditions

5. Dispersal in time of germination between different
seeds on the same plant in order to maximize survival
a. reduced risk of mortality
b. Reduced sibling competition.
Dormancy break as a consequence of sensing
favorable environmental conditions, seasonality, water
And nutrients availability

6. Seed dormancy
Dormant seed: a seed that does not germinate under optimal water ; oxygen and temperature conditions
Germination: Growth of embryo, breakage of seed coat,
Water absorption and cell division in the embryo.

7. Breaking seed coat barriers by the root
Endosperm
Cersis canadensis

8. Types of dormancy - Non-dormant seeds
- Primary dormancy – gradual time dependant
Release from dormancy – post ripening
- Secondary dormancy – seeds do not germinate under unfavorable environmental conditions
- Enforced dormancy – temp. water stress

9. Dormancy classification
Seed dormancy
Types of dormancy
Exogenous Coat imposed dormancy
Mechanical dormancy: growth constraint.
Physical dormancy: permeability to H2O , O2, inhibitors.
Chemical dormancy: inhibitors in endosperm, seed coat.
Endogenous Embryo dormancy
Morphological dormancy: Under-developed embryo.
Physiological dormancy : Physiological inhibition.
Dormancy classification
Nikolaeva (1977), Black & Bewley (1980)
Baskin & Baskin (1998)

10. Harsh seed coat inhibits germination. Oleaceae
Coat imposed dormancy
Mechanical dormancy
Harsh seed coat inhibits germination.
Oleaceae
(Rosaceae)
(Juglandaceae)

11. Morpho-physiological dormancy

12. Coat imposed dormancy:
Chemical dormancy
ABA
Phenols (coumarin, ferulic acid)
Na Cl: Atriplex , Zygophyllum ,
- Leaching

13. water permeability barrier
3) Coat imposed dormancy:
Physical dormancy
water permeability barrier
, Musaceae, Cannaceae.

14. Bottom cavities in
legumes

15. Relation between air humidity%
seed
water
content
Relation between air humidity
and seed water content
RH %

16. Relation between air humidity
and seed water content %
% RH
X -hard seeds
O -scarified

17. Barrier to gas exchange
1. Seed coats are impermeable to oxygen the difusion coefficient of oxygen in water is very low, could be a limiting factor to germination
Excess water can inhibit germination
2. Seed coats contain phenolic compounds that fix oxygen molecules

18. %
Effect of oxygen concentration on seed germination
% O2

19. Effect of Hillum plugging on germination
in melon varieties

20. Effect of oxygen concentration on germination of melon seeds

21. Techniques for breaking seed dormancy
1. Scarification - mechanical abrasion
2. Stratification -. Chemical treatments
Treatment with concentrated sulfuric acid (15 min.)
3. Hot water or dry air

22. Physiological dormancy Non-deep physiological dormancy
The dormancy can be broken by a short (days) exposure to either low or high temperatures, light or hormonal treatments GA ו- CK.
- The requirements is nullified gradually in storage

23. Lettuce
Embryo in endospem

24. Datura
Tomato
Lettuce
lateral
endosperm
micropylar
endosperm

25. Physiological dormancy
Intermediate/deep physiological dormancy
Seeds require 1-5 months of stratification to break dormancy
During stratification hormonal changes will occur in the embryo axis

31. Factors that determine dormancy
Factors related to seed coats
Water penetration
Mechanical resistance for radical break
Gas exchange    
2. Factors related to immature embryos  
Stratification by heat and cooling
3. Physiological factors related to imbibition and germination
Hormonal treatments

32. A classification system for seed dormancy
Baskin JM & Baskin CC
A classification system for seed dormancy
Seed Science Research 14:

33. Seed Imbibition
Phase I (imbibition)- Water uptake is a physical process driven by water potential gradient between the seed and its environment.
Phase II (lag phase)- Seed water reaches a plateau level and remains constant. Metabolic processes are activated. Duration depends on Temp.
Phase III (Embryo growth)- Point of no return, seed absorbs large quantities of water. Sensitive to dehydration. Embryo growth and radicle emergence.

35. Imbibition Temperature
Temperature at imbibition is critical especially in large seeds.
Low temp. at Imbibition cause imbibition injury.
Membrane deterioration- leakage of cellular contents

37. PHASE TRANSITION DIAGRAM
Phospholipid
Phospholipid
Free Fatty Acid

38. Phospholipid Phospholipid Free Fatty Acid Gel Phase Liquid Crystalline

39. Respiration and metabolic activation
Mitochondria – oxidative phosphorylation
Generation of ATP prior to radicle emergence.
Anaerobic respiration is possible.
Protein and nucleic acid formation
Repair of DNA damaged in dehydration

40. Weakening of covering tissue-embryo growth
Cell division and cell expansion
Cell division does not occur before radicle emergence
Weakening of the cover by water and physical pressure of the radicle
Cell wall hydrolases are expressed
Activation is controlled by higher GA lower ABA levels.

41. Reserves mobilization
Starch, other carbohydrates and lipids are converted to sucrose.
Sucrose is transported to the growing axis.
Proteases breakdown storage proteins into free amino acids.
Phytase is activated to break down phytin and release of minerals.

45. Germination, dormancy and Hormonal control
Temperature – Cardinal temperatures for dormancy break and germination
Water potential – At low water potential seeds will not germinate regardless of temperatures.
Light – Stimulate germination of many species. Red light (Phytochrome response)
GA required for seed germination. Induces Beta mananase and other cell wall hydrolyzing enzymes.
Ethylene – stimulates germination.
Nutrients – Most seeds do not require nutrients for germination except for nitrates. KNO3 induces germination.

49. Control of seed germination by environmental factors

50. Temperature effects on the germination of melon seeds
Min. temp.
Optimal temp.
Max. temp.
P-202 NY

51. Seed categorization into temperature-requiring groups
1) Cool-temperature tolerant
Range: 5-30/40oC. Optimal: 25-30oC.
2) Cool-temperature requiring
Range: < 20-25oC.
3) Warm-temperature requiring
Range: > 10-15oC.
4) Alternating temperatures
Day/night temp. amplitude about 10oC.

52. Temperature can affect germination in different ways:
1- Dry - High temperature (seed coat permeability)
2- Wet- Low temperature : chilling, stratification.
3- Temperature shifts (low => high temperature)
4- Temperature alternation (daily cycles)
Temperature effects on germination interact
frequently with light effects.

53. Effect of stratification temperature on germination of apple seeds
85 days at low temp => to 20oC

54. Duration of stratification at 5oC on seed germination
of Corylus avellana and Acer spp.
Germ 5 => 200C
A. pseudoplatanus
אדר דולבני
אדר
Corylus
A. platanoides ש
Germ at 50C

55. Effects of stratification on germination of Rumex obtusifolius at 25oC

56. Chilling effects on germination temperature of
Delphinium דורבנית
6oC x 2 weeks

57. Euonymus europaea

58. Effect of number of temperature cycles on seed
germination of Rumex obtusifolius

59. Temperature amplitude for 50% germination

60. Temperature effects on germination

61. Light can affect temperature in several ways
1- Light sensitive seeds (photoblastic seeds)
a) positive photoblastic: light promoted germination.
b) negative photoblastic: light inhibited germination.
c) dual effect.
2) Light insensitive seeds (non-photoblastic seeds)
- Effect of light depends on duration (sec to days), intensity and spectral quality of the light (R/FR).
Phytochrome is the main photo-system involved in light control of germination.

62. Effects of light on germination of Oryzopsis
(דגן-נשרן הדוחן): dual effect
Short irradiation – 5 min
promotion
Darkness
Continuous
light
inhibition

63. Phytochrome photosystem
R m
FR 730 m R FR
Pfr is the promotive form of phytochrome

64. a

65. Dark germination - In light requiring seeds germination in the dark
depends on Pfr accumulated in the seeds during their
development on the mother plant.
- Decay of Pfr to Pr reduces germination in darkness.

66. Effects of light and temperature on germination of Datura stramonium
Continuous light
20oC => 30oC
Short light
irradiation
Darkness

67. Escape time (escape from inhibition by FR)
R => dark period => FR %
Pr Pfr
Lettuce seed germination
Hours between R and following FR irradiation

68. Pr Pfr Red 20oC => 30oC => 20oC Darkness Dark reversion
Destruction
Red
20oC => 30oC => 20oC
Darkness
Red light
Pr Pfr
Action
Dark reversion
(thermal process)
Thermal dark reversion
of Pfr explains inhibition
of light promotion by
high temperature.

69. חסה, סלרי,(summer sowings)
Secondary dormancy
seeds that after primary dormancy fail to germinate can reenter dormancy => secondary dormancy.
Thermodormancy:
Induced by imbibition at high temperatures (> 25oC).
In contrast to thermal inhibition, will not geminate if transferred to lower temperature
חסה, סלרי,(summer sowings)
b) Skotodormancy:
Induced by prolonged dark .
c) Continuous light, anoxia

70. Seed priming
Controlled seed hydration treatment to reduce time to germination and seedling emergence, and improve rate and uniformity of germination.
Seeds are kept in hydrated condition (priming solution) that allows metabolic processes, but prevents cell elongation and radicle emergence.
After a period of time seeds are dried to original water content.
Growth regulators can be added to the priming solution to promote processes.

71. Processes during germination and priming

72. Effects of priming on rate and uniformity of germination

73. Priming requirements
Seed hydration under controlled conditions that prevent germination
low water potential: -0.5 to –1.5 MPa 9 (-5 to 15 bars)
low temperature : oC
during a few days (2-5 days)
Seeds are activated metabolically but do not germinate.

74. Techniques for seed priming
1) Osmotic priming (osmopriming, osmoconditioning)
by imbibing seeds in osmotic solutions: salts , PEG. Then
dried. washed
2) Matrix priming (matriconditioning, solid matrix priming)
solid imbibed matrix: vermiculite, silica, mixed with seeds.
Better for larger seeds.
3) Drum priming:
hydrates seeds with measured amount of water in a
tumbling drum.