Germination and Dormancy - Royal Botanic Gardens, Kew

Improving the identification, handling 

and storage of ‘difficult’ seeds 

Germination and Dormancy 

Supported by 

Also supported by 

1

What does germination and dormancy 

have to do with ‘difficult’ seeds? 

Inherently difficult 

• recalcitrant or 

intermediate seeds 

• orthodox but dormant 

• orthodox but short-lived 

• Orthodox but enclosed 

by a hard fruit coat eg 

Terminalia 

Handling and storage difficulties 

• immature orthodox seeds dried 

too rapidly 

• orthodox seeds insufficiently 

dried prior to storage and/or 

stored under poor conditions 

• Seeds damaged by insects 

• Seeds damaged during 

processing 

© Copyright Board of Trustees of the Royal Botanic Gardens, Kew 

Germination and 

dormancy 

Monitoring the viability of collections is one of the most important routine 

tasks for all seed bank managers. Critically, it enables managers to plan 

regeneration before viability has fallen to a level where important genotypes 

might be lost. 

Although alternative viability methods such as the tetrazolium test have an 

important role to play, the germination test remains the most reliable and 

effective method for assessing the viability and vigour of collections. 

Depending on factors such as taxonomy, life form, habitat, climate 

preference and seed structure, the specific conditions required for 

germination vary considerably amongst species and in many cases the 

situation is further complicated by the presence of seed dormancy. 

2

Objectives 

That you know and understand: 

• How water, temperature and light can affect 

germination 

• The reason for dormancy and how it may be 

expressed 

• How to select practical treatments to overcome 

dormancy 

• How to interpret tests where dormancy breaking 

treatments have been applied 



dormancy 

This lecture and practical exercise will explain and demonstrate how 

environmental factors affect seed germination. The importance of seed 

dormancy, the two most important ways that dormancy is expressed and 

methods to overcome dormancy will also be examined using examples of 

‘difficult’ species identified in the earlier stakeholder workshops. 

3

Environmental factors affecting 

seed germination 

• Water 

• Temperature 

• Light 

• Gases 



dormancy 

The four principal environmental factors that affect seed germination are: 

water; temperature; light and gases. It could be argued that water is the 

most important because all seeds must take up water for the embryo to 

enlarge and break through the covering structures. However, temperature 

and light are also extremely important and seeds can vary considerably in 

their responses to these factors. Although less understood, the gaseous 

environment surrounding seeds is also important and could be critical for 

some seeds during germination under natural conditions. 

4

Problems associated with water 

uptake (imbibition) 

• Water sensitivity - impaired respiration, made 

worse by low temperatures 

• Imbibition injury - due to too rapid water 

uptake resulting in solute leakage 



dormancy 

Imbibition is a 3-stage process. The first phase is a rapid uptake of water. 

This is a purely physical process related to the hygroscopic nature of seeds 

and their very low water potential when dry. As the seeds become more 

and more hydrated the water potential inside the seeds increases and the 

water potential difference between the seeds and the wetted filter paper 

diminishes. This causes the rate of uptake to slow and eventually stop 

when the seeds are fully hydrated. 

There are two practical, problems that can arise associated with the 

process of imbibition: 

Water sensitivity: 

Put simply, this is when seeds ‘drown’ because excess water impairs 

respiration. Low temperatures tend to exacerbate this because the whole 

process of germination is slowed down and therefore the seeds are 

stressed for longer. This can be a practical problem for growers of 

temperate crops in cold wet springs. 

Imbibition injury: 

The initial phase of water uptake is very rapid. This can be problematic 

when very dry seeds of certain species are sown. Because the process of 

drying causes membranes to become ‘leaky’ there is a risk that sugars and 

electrolytes can leach out the seeds during this period of rapid uptake 

before the membranes have restored their normal function. The leakage 

itself may not be lethal but the leachate provides a perfect substrate for 

microbial pathogens that kill the seeds before germination can occur. 

5

Problems associated with water 

uptake (imbibition) 

Imbibition damage: prevented by RH conditioning. 

Seeds held 

above water in 

sealed box for 

1-2 d at 20°C 



dormancy 

The risk of imbibition injury can be easily avoided by allowing seeds to take 

up mositure gently in a saturated atmosphere before they are sown. 

Holding seeds above water in a suitable sealed container for 1-2 days at 

ambient temperatures (20-25°C) before sowing is a very effective method 

to prevent imbibition injury. 

6

Effect of high humidity conditioning on 

germination of dry-stored Lathyrus sphaericus 

seeds, chipped to remove physical dormancy 

Conditioning at 

100% RH for 24 h 

prevents damage 

The drier the 

seeds the more 

susceptible they 

are to imbibition 

damage 



dormancy 

Species with large seeds in the Leguminosae appear to be particularly 

susceptible to imbibition injury. 

The graph shows the results of an experiment carried out by John Dickie of 

the MSB some years ago which looked at the effect of RH conditioning in 

preventing imbibition injury in a Lathyrus species. The results drastically 

show that imbibition damage is very dependent on initial moisture content. 

Very dry seeds at around 3% MC were very susceptible whereas seeds 

with an initial moisture content of around 9% were affected much less. 

7

The effect of temperature on 

germination 

• Speed of germination. 

• Range of temperatures over which germination 

can occur. 

• Seasonal physiological changes as seeds become 

more or less dormant. 



dormancy 

There are a number of ways that temperature affects seed germination. 

Temperature controls the rate of metabolic processes. 

The range of temperatures over which germination can occur can vary 

widely amongst species, amongst populations or ecotypes and even for a 

single seed collection through time. 

In nature, seasonal changes in temperature control physiological changes 

in some species as seeds become more or less dormant. In most cases, 

seeds are indifferent. 

8

The effect of temperature on 

germination 

Minimum Optimum Maximum 

Germination % 

100 

80 

60 

40 

20 

0 

End of expt 

1 week 

0 10 20 30 40 50 

Temperature 


Imagine an experiment to investigate the range of temperatures for 

germination of a typical non-dormant seedlot. Samples of seeds would be 

germinated in a range of temperature controlled incubators. Germination 

would then be scored at time intervals for example, weekly. We could then 

plot the % germination occurring at each temperature for those time 

intervals. The graph illustrates the results we could expect if the experiment 

was scored after say one week and then at the end of the experiment 

several weeks later. 

The graph shows that in the initial stages (1 week) germination is restricted 

to a few temperatures and that only a proportion of the seeds are able to 

germinate. At the end of the experiment (6 weeks), a high percentage of 

seeds have germinated over a wide range of temperatures. As we have 

already discussed it will take longer for seeds to germinate at lower 

temperatures. At the end of the experiment, we are able to define three 

important so-called ‘cardinal’ temperatures: 

The minimum temperature below which no seeds are able to germinate 

The maximum temperature above which no seeds are able to germinate 

and 

The optimum temperature which is the temperature that enabled all 

seeds to germinate first. Put another way it is the temperature that allows 

the fastest germination. 

9

Variation in temperature range for 

germination in plants in the same desert 

habitat 

Germination % 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Winter plants Summer plants 

0 10 20 30 40 50 

Temperature (deg C) 


The range of temperatures over which germination can occur can vary 

according to climatic and ecological factors. For example, Annual species 

growing in deserts that experience both summer and winter rainfall were 

found to vary in their temperature requirements depending on the season 

when they grew. Summer germinating species required warmer 

temperatures for germination than winter species. 

In summary: winter species require cool temperatures to trigger 

germination and summer species require warm temperatures. 

10

Requirement for alternating 

temperatures 

Temp 

Seeds near soil surface experience wide 

diurnal variation in temp 

D N D N D N D N 

SOIL 

Buried seeds experience constant 

temperature 


As the diagram illustrates, sensitivity to the amplitude of diurnal (day / 

night) variation in temperature probably acts as a depth sensing 

mechanism. Temperature variation is greatest on or very near the soil 

surface and the insulating effect of soil and litter means that this variation 

decreases with increasing depth with temperatures more or less constant 

below about 10-15 cm depending on geographic location. The requirement 

for alternating temperatures is very common in annuals, temperate 

grasses and wetland species. 

11

Alternating temperatures 

• Selection pressure for germination of small seeds near 

soil surface. 

• Difference between day and night temperature 

(amplitude) decreases with depth of burial. 

• Requirement for alternating temperatures acts as 

depth sensing mechanism. 



dormancy 

If very small seeds germinate when they are deeply buried in the soil it is 

likely that food reserves would be exhausted before the seedling could 

reach the soil surface and the seedling will die. Thus, selection pressure 

has resulted in the evolution of mechanisms in species with small seeds to 

make sure that they only germinate when they are close to the soil surface. 

Their dependence on two environmental cues: light and alternating (diurnal) 

temperatures ensures that this is the case. 

12

Alternating temperatures 

• In the lab: 

– 25/10 (8h/16h) temperate 

– 35/20 (8h/16h) tropical 

– illumination during warm phase 



dormancy 

Using Alternating Temperatures in the Laboratory 

When using alternating temperature regimes, attention should be paid to 

the amplitude (the difference in °C between the component temperatures) 

and the relative period spent at each phase. 

At the MSB, 25/10°C for temperate species and 35/20°C for tropical 

species are used with either 8 h /16 h or 12 h /12 h spent at each phase. 

Light is provided during the warm phase thus mimicking daytime. 

These are diurnal cycles which are applied throughout the germination 

test. 

Some species respond to single temperature shifts (sometimes referred to 

as heat shock treatments) involving brief periods at high temperatures. 

e.g. a single 2 h shift from 15 to 35°C can be as effective as daily cycles of 

say 25/10°C (8h/16h). MSB researchers have shown that heat shock can 

alleviate a form of dormancy induced by drying in Papaya seeds. 

13

The effect of light on germination 

• Most cases seeds are indifferent 

• Many require light e.g. small-seeded annuals 

• Few are inhibited 

• Seeds sensitive to duration, intensity and especially 

quality 

• All light responses controlled by phytochrome 



dormancy 

Many growers believe that most seeds require darkness for germination - 

this is wrong. In fact most seeds germinates equally well in light or dark. 

Many seeds only germinate in the light and only a a few will only germinate 

in the dark. Even in a single batch of seeds, the response may vary 

depending on other environmental factors. e.g. temperature. Seeds may 

be insensitive at one temperature but require light at another. 

Sensitivity to light increases during imbibition; very dry seeds cannot 

respond to light. 

Response depends on duration, intensity and especially on light quality and 

in all cases the response to light in seeds is controlled by phytochrome. 

14

Practical implications for seed testing 

• Low energy, white fluorescent tubes used for 

germination testing. 

• Photoperiod usually 8 or 12 hours per day. 

• Incandescent lamps avoided - too much FR light and 

heat. 



dormancy 

15

Evidence that some relatively large 

seeds may be inhibited by light 

• Light inhibition has been 

reported in some 

Cucurbitaceae 

Acanthosicyos naudinianus 



dormancy 

Prolonged high intensity irradiations can inhibit germination by a process 

called the 'high irradiance reaction' (HIR). Ecologically, the HIR probably 

serves to prevent germination when seeds are exposed on the soil surface 

where there is a high risk of desiccation. This might be more important in 

species with large seeds. 

It is possible that some dryland species have evolved so that the seeds 

only germinate when they are below the soil surface where the soil is less 

likely to dry out. Such species are more likely to germinate best in darkness 

and be susceptible to the HIR reaction. 

16

The effect of gases on germination 

• Reduced O 2 or elevated CO 2 usually reduces 

germination. 

• Except some submersed aquatics where germination 

is stimulated by anaerobic conditions. 

• Nitrogen dioxide gas may have potential for 

dormancy breaking. 



dormancy 

Since germination depends on metabolic activity it is not surprising that the 

gaseous environment surround seeds is important for germination. 

As a rule, lowering O 2 or raising CO 2 reduces germination. However, some 

aquatic species have been reported to be stimulated by low O 2 levels, e.g. 

seeds of Zostera species (marine angiosperms) germinate best under 

anaerobic conditions. 

In most species, water logging tends to reduce germination and under 

natural conditions elevated CO2, depressed O2 coupled with darkness 

could be important in the induction of dormancy. Such problems could 

account for the delay or failure of germination when seeds are sown in 

seed compost in the nursery. 

Certain gases for example, nitrogen dioxide and ethylene, may be used to 

overcome dormancy in some species. 

17

Seed Dormancy 

Definition: failure of viable seeds to 

germinate under ‘favourable’ conditions 

Function: to synchronise germination with 

environmental conditions suitable for plant 

growth. 



dormancy 

Seed dormancy is the failure of viable seeds to germinate under favourable 

conditions. Dormancy has evolved to synchronise germination with 

climatic/environmental conditions, to ensure a high probability of seedling 

establishment and development of the plant to reproductive maturity. 

Although there is an underlying genetic basis for the control of dormancy 

the quantitative expression of dormancy is strongly influenced by 

environmental factors operating during the growth and development of the 

parent plant. 

18

Why is seed dormancy a problem 

for gene bank managers ? 

Monitoring: Can result in underestimate of true 

viability 

Utilisation: must be able to turn conserved seeds 

back into plants 



dormancy 

Dormancy is relevant to all seed banks in routine viability tests. In routine 

germination testing there is a need to remove seed dormancy to ensure 

that all viable seeds are identified. A failure to break dormancy could mean 

that viability will be underestimated. 

It is often said that there is little point in conserving seeds if they cannot be 

turned back into plants for use. Thus protocols for breaking seed dormancy 

are vital for the effective use of conservation collections. Examples of use 

include plant breeding programmes, research, reintroduction (of 

endangered species) and habitat restoration. 

When dormancy cannot be overcome, cut tests or TZ tests can be used to 

distinguish between dead and dormant seeds. Dormant, viable seeds will 

present themselves in a cut test as having firm, usually white, internal 

tissues. By contrast, dead seeds usually appear soft, will be surrounded by 

exudate and microbial infection and the internal tissues will have changed 

colour, usually to brown. In TZ tests, the vital tissues of dormant seeds 

usually stain uniformly red. 

19

The plasticity of dormancy 

• A population of seeds will display a normal 

distribution of dormancy states 

• Some species or genera or even families will be 

characterised by a particular type of seed dormancy 

BUT 

• Just because a species usually displays a particular 

form of dormancy; non-dormant populations could 

exist AND 

• The dormancy status of a seed population will change 

through time 



dormancy 

A population of seeds will display a normal distribution of dormancy states. 

Some seeds may exhibit no dormancy or be very weakly dormant and 

some seeds will be deeply dormant but the majority of individuals will 

possess an average level of dormancy. 

Some species or genera or even families will be characterised by a 

particular type of seed dormancy. For example, physical dormancy is 

widespread in the Fabaceae. But this does not mean that every species in 

the Fabaceae has physical dormancy. Nor does it mean that a species that 

usually shows physical dormancy will always be dormant. Moreover, 

individual seeds will undergo changes in the depth and expression of 

dormancy through time. 

20

Seed dormancy types 

• Endogenous 

(embryo related) 

• Physiological 

• Morphological 

• Morphophysiological 

• Exogenous 

(seed/fruit coat related) 

• Physical 

• Combinational 



dormancy 

The dormancy classification shown is the one described by Baskin and 

Baskin (2003) based on original ideas of M. G. Nikolaeva. 

The three forms of endogenous dormancy are due to some property of the 

embryo that prevents germination. For example, the embryo may be 

underdeveloped or there is some inhibitory mechanism present. 

The two forms of exogenous dormancy relate to some property of the seed 

or fruit coat that prevents germination. 

Baskin and Baskin (2003) sampled 5250 species, representing all major 

taxonomic groups of seed plants from vegetation regions around the world. 

69.6 % were dormant 

30.4 % were non-dormant 

Of those dormant seeds: 

64.8% possesed physiological dormancy 

20.8% possesed physical dormancy 

1.6% possesed morphophysiological dormancy 

2.1% possesed morphological dormancy 

Physiological dormancy is the most frequent type of dormancy. It is also the 

most frequent type of dormancy found in collections at the MSB and 

arguably the most difficult to overcome. 

For the rest of this lecture we will focus on the two most important forms of 

dormancy; Physiological (PD) and Physical (PY). 

21

Physiological Dormancy 

Due to a physiological inhibiting mechanism of 

the embryo or an embryo covering structure such 

as the endosperm and seed coat, that prevents 

radical emergence. 

Examples: Gramineae, 

Iridaceae Liliaceae, 

Capparaceae, 

Papaveraceae 

Cleome gynandra 



dormancy 

Cleome gynandra (Capparaceae) is an under-used leafy vegetable whose 

seeds can be difficult to germinate as a result of physiological dormancy. 

Physiological dormancy occurs in all kinds of seeds irrespective of the 

nature and size of the embryo. Although seeds may be have lost their 

physiological dormancy and be ready to germinate in a particular season, 

they may require particular environmental triggers such as light, alternating 

temperatures or smoke before germination will occur. As we have already 

discussed, the requirement for light and alternating temperatures ensures 

that seeds only germinate when they are close to the soil surface and not 

shaded by other plants. The requirement for smoke in dryland species 

ensures that germination only occurs after fire when competing vegetation 

will be cleared and nutrient levels will be favourable for seedling 

establishment and healthy plant growth. 

22

‘Difficult’ seeds with physiological 

dormancy 

• Physiological dormancy is common in 

tropical grasses such as Panicum, Eragrostis, 

Eleusine 



dormancy 

23

Physiological Dormancy 

Physiological dormancy (PD) occurs in all kinds of 

embryos. 

endosperm embryo 



dormancy 

Physiological dormancy occurs in all kinds of seeds irrespective of the 

nature and size of the embryo. 

24

Physiological dormancy 

• PD enables seeds to avoid seasons unsuitable 

for seedling establishment 

• Hence seasonal patterns of emergence in 

many species 



dormancy 

25

Seasonal patterns 

• Many species have evolved seeds that cycle in and out 

of dormancy 

• Seeds are dormant in seasons unfavourable for 

establishment 

• Seeds are non-dormant in the season when 

germination occurs naturally 

• However, seeds may not germinate if other factors 

are unfavourable 



dormancy 

Although seeds may be have lost their physiological dormancy and be 

ready to germinate in a particular season, they may require particular 

environmental triggers such as light, alternating temperatures or smoke 

before germination will occur. As already discussed, the requirement for 

light and alternating temperatures ensures that seeds only germinate when 

they are close to the soil surface and not shaded by other plants. The 

requirement for smoke in dryland species ensures that germination only 

occurs after fire when competing vegetation will be cleared and nutrient 

levels will be favourable for seedling establishment and healthy plant 

growth. 

26

Seasonal synchronisation: 

when is germination favoured ? 

• Mediterranean and tropical dryland species: 

– Wet season 

• Temperate species: 

– Spring or Autumn germination 



dormancy 

27

Understanding natural patterns of 

germination and emergence 

• Seed burial experiments 

• Seeds recovered at intervals and tested for 

germination in the lab 



dormancy 

28

Soil temperature 

Soil moisture 

Seed fall 1 Seed fall 1 

After-ripening 2 After-ripening 2 

Warm stratification 3 

Germination stimulant 5 

Cold stratification 4 

Germination 

Dormancy loss Dormancy induction Dormancy loss 

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 

Summer Autumn Winter Spring Summer 

Pattern of dormancy loss and response to dormancy treatments postulated for 


Western Australia species 

This diagram illustrates timing of seed dispersal (seed fall) and germination 

of species in a typical Southern hemisphere Mediterranean ecosystem. 

The figure was generated by Merritt and co-workers to illustrate the 

behaviour of Western Australian species with physiological dormancy but 

the principles probably apply to Mediterranean and tropical dryland species 

in general. 

Note that rainfall is highly seasonal, occurring during the cooler winter 

months when most plants grow, and flower. Seed set and seed fall occurs 

during the Spring and early Summer as temperatures rise sharply and the 

soil dries out. These warm dry conditions will cause the decline in PD in 

some species by the process we call dry after-ripening. Other species may 

lose their PD as a result of warm moist stratification which occurs as 

temperatures begin to fall in the autumn and soil moisture is restored. Thus 

germination is programmed to occur during late autumn and winter. The 

diagram also reveals that some species may respond to cold moist 

stratification in the winter and that additional triggers (germination 

stimulants) such as smoke may be required for some cases. 

29

Seeds of some species require several 

seasons before germination occurs 

Embryo 

Germ 

Seasonal changes in temperature 

W Sp Su A W Sp Su A W Sp 

Cardiocrinum cordatum (Japan): embryo grows 

in second autumn, visible germination following 

spring 



dormancy 

Some species, especially those that possess tiny embryos, can exhibit 

more complicated forms of PD. For example, a temperate woodland 

species from Japan, Cardiocrinum cordatum, takes more than a year to 

germinate. In this species, embryo development does not occur until the 

second Autumn after dispersal and germination itself is delayed until the 

beginning of the following Spring. 

30

Overcoming physiological dormancy 

(mimicking the seasons) 

Dryland species and 

temperate winter annuals: 

avoiding a hot dry season 

‘dry’ after-ripening 

(30-50°C, 2-4 weeks at 

ambient RH) 

Tropical grasses such as 

Eleusine indica will 

probably respond 

favourably to ‘dry’ afterripening 

germination conditions 

( 25 - 30°C) 



dormancy 

Dry after-ripening mimics the loss of physiological dormancy that would 

occur in nature during the dry season or summer months in a temperate 

climate. We can simulate and accelerate this process in the laboratory by 

holding dry seeds at 30-50°C at ambient relative humidity for a few weeks. 

Tropical grasses such as Panicum sp would be expected to respond 

favourably to this treatment and we have witnessed after-ripening occurring 

in collections of Eragrostis held in the dry room for several months. The 

probem with after-ripening treatments is that the conditions used (high 

temperature and moderate RH) also accelerate the ageing process and 

therefore there is a risk that some seeds may lose viability. Warm 

stratification of imbibed seeds also works in some cases. This treatment 

involves holding imbibed seeds at temperatures above about 25-30°C for 

several weeks followed by transfer to normal germination temperatures. 

31

Overcoming physiological dormancy 

(mimicking the seasons) 

Some high altitude 

species experiencing a 

‘temperate’ climate 

may respond to cold 

stratification. 

(Seeds programmed to 

germinate after the 

cold season has passed) 

Cold, moist 

stratification (chilling) 

5°C, 8-16 weeks 

germination 

conditions 

(15 - 20°C) 


Cold stratification, or chilling, of imbibed seeds is a very effective dormancy 

breaking treatment for spring germinating temperate species. There is 

some evidence that the treatment can also be effective for some 

Mediterranean and tropical dryland species and it is worth considering. 

32

Overcoming physiological dormancy: 

assisting the embryo 

Seed surgery: 

Excision of tissue close to 

embryo. 

Removes mechanical 

constraint enabling the 

embryo to grow 

Very effective for tropical grasses such as Eragrostis sp. 


In many species with physiological dormancy the inhibiting mechanism 

results in the embryo having insufficient growth potential to break through 

the covering structures. Thus careful surgical treatments that aim to 

remove a portion of seed coat close to the embryo can be extremely 

effective when all else fails. 

33

Overcoming physiological dormancy: 

assisting the embryo 

Surgical treatment needs 

to be applied close to 

root tip in some cases. 

Important to 

understand seed 

structure to avoid 

damage! 

Pennisetum foermerianum 



dormancy 

Surgical treatments often have to be performed under a dissecting 

microscope. There is a significant risk of embryo damage. 

34

Response of a range of problem 

collections to surgical treatment 

% Germination with Surgical 

Treatment/Chipping 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Effect of Surgical Treatment/Chipping 

-10 

-10 0 10 20 30 40 50 60 70 80 90 100 

% Germination without Surgical 

Treatment/Chipping 

ns effect of ST 

sig + effect of ST 

sig - effect of ST 

14 out of 16 

grasses tested 

showed 

significant 

positive effect 



dormancy 

At the MSB we investigated the effect of surgical treatments on seed 

germination in a range of problem collections representing several families. 

For grasses we found that a removing a small portion of pericarp directly 

above the embryo was very effective in a number of tropical grasses. In 

fact 14 out of 16 grasses tested responded well to this treatment. 

35

Physical Dormancy 

Present in at least 15 families of angiosperms and is 

primarily due to seed or fruit coat impermeability to 

water. 

Examples: Cistaceae, Fabaceae, Geraniaceae, 

Malvaceae, Rhamnaceae 

Pelargonium cucullatum 

Cassia sieberiana 



dormancy 

After physiological dormancy, physical dormancy (PY) is the next most 

important form of dormancy confronting seed bank managers. 

Physical dormancy is simply due to the seed/fruit coat acting as a 

permeability barrier to water. Under natural conditions, the permeability 

barrier is broken by a slow scarification process acting on the seed coat. 

The following are examples of natural processes that overcome PY: 

Weathering 

Microbial attack 

Impaction by soil particles 

Fire (heat) 

Extreme diurnal temperature variation in the dry season 

Freezing and thawing 

Acid scarification after ingestion by animals 

Some species have a natural point of weakness on the seed coat, the 

location of which varies across families. In Papillionoid legumes for 

example it is the strophiole or lens. 

Families where there is a high frequency of PY: Fabaceae, Cistaceae, 

Geraniaceae, Malvaceae, Rhamnaceae, Convolvulaceae, Cannaceae 

36

‘Difficult’ seeds with physical dormancy 

• Corchorus (Tiliaceae) 

• Vigna, Afzelia (Fabaceae) 



dormancy 

The earlier stakeholder workshops identified a number of problematic 

species with physical dormancy. 

37

Overcoming physical dormancy in 

the laboratory 

Mechanical scarification 

• Most reliable method 

for small samples 

• Small portion of testa 

removed to aid water 

uptake 

• Care taken to avoid 

damage to embryo 

Chipping, nipping, filing 

Baskin & Baskin, 2006 



dormancy 

The treatments used to overcome PY are straightforward and simple. 

Examples of practical methods to overcome PY are shown in the following 

slides. 

38

Overcoming physical dormancy in the 

laboratory 

Scarification treatment 

needs to be applied 

away from embryo to 

avoid risk of damage 

Important to 

understand seed 

structure ! 

Scarification 

treatment 

needs to be 

applied here 



dormancy 

39



Wet heat, boiling 

• Can be applied to larger 

samples 

• Seeds dipped in boiling 

water for seconds and 

then rapidly cooled 

• Risk of damage to some 

seeds 

Baskin & Baskin 2006 



dormancy 

40



Dry heat (oven) 

• Can be applied to large 

samples 

• Dry seeds exposed to 

>100°C for minutes 

• Treatment mimics 

exposure to fire in 

nature 


seeds 




dormancy 

41



Acid scarification 


• Can be applied to large 

samples 

• Hazardous 

• Seeds exposed to conc. 

sulfuric acid for up to 60 

mins 

• Optimum exposure time 

can be seedlot dependent 


seeds 



dormancy 

42

Mimicking natural fires to 

overcome dormancy 

• Physical effect of dry heat: 

100°C + 

• Higher the temperature 

shorter the exposure time 

• Chemical effect of smoke: 

applied as aerosol or 

liquid extracts, or NO 2 gas. 

• Combinations of heat and 

smoke can be effective in 

some cases 


In the drylands the seeds of many species are adapted to germinate after 

natural fires. Research has shown that seeds may respond to the extreme 

heat of fires acting on physical barriers to germination or to the subtle 

chemical cues contained in smoke. 

43

MSBP study on effects of smoke 

and dry heat on problem species 

Method 

• Seeds were soaked for 24 h in Kirstenbosch 

“Instant Smoke Plus” aqueous smoke solution at 

the germination temperature. 

• Seeds were then sown onto plain agar (10 g l-1) 

and incubated at constant or alternating 

temperatures depending on the species. 

• Dry heat treatment also applied to some species 

before smoke involved exposing dry seeds to 100 

or 110°C for 5 mins 



dormancy 

At the MSB we have investigated the effect of smoke on its own or 

combined with dry heat on the germination of 45 problem collections across 

a number of families. 

44

MSBP study on effects of smoke and 

dry heat on problem species: 

response to aqueous smoke treatment 

% Germination with Smoke 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Effect of Smoke 

0 10 20 30 40 50 60 70 80 90 10 

0 

% Germination without Smoke 

18 out of 45 

collections 

showed 

positive effect 

of smoke 



dormancy 

The graph shows the % germination following smoke treatment plotted 

against the corresponding response of untreated seeds. All of the points in 

pink above the diagonal line denote collections where there was a 

significant positive effect of smoke. The blue points indicate collections 

where there was no effect and the two yellow points represent the only two 

collections that showed a negative effect of smoke. 

45

MSBP study on effects of smoke and 

dry heat on problem species 

Summary 

• Of 39 species tested, 15 showed a significant positive 

response to smoke applied on its own. 

• Of 28 species that also received a dry heat pre-treatment 

followed by smoke, 13 showed a significant increase 

compared with smoke on its own. In 8 species, dry heat 

did not change the response to smoke and in 7 species 

there was a significant reduction. 

Conclusion 

• Smoke applied on its own or in combination with dry 

heat has the potential to overcome germination 

problems in conservation collections BUT the level of 

response is likely to be both species and collection 

specific. 



dormancy 

46

Combinational Dormancy 

Seeds may possess a combination of 

physiological and physical dormancy. 

For germination to occur, both types of 

dormancy must be overcome. 

Examples include: Ceanothus (Rhamnaceae), 

Tilia (Tiliaceae), Rhus (Anacardiaceae) 



dormancy 

47

Main seed dormancy types: summary 

• Physiological 

• Physical 

• Usually endospermic seeds 

– cold or warm stratification 

– dry after-ripening 

– surgical treatment 

• Apiaceae, Iridaceae Liliaceae, 

Papaveraceae, Ranunculaceae 

• Usually non-endospermic seeds 

– scarification 

– dry heat 

• Cistaceae, Fabaceae Geraniaceae 

Malvaceae, Rhamnaceae 


48

Factors that may predict 

germination requirements 

• Taxonomy: family trends 

• Life form: tree / herb / annual / perennial 

• Habitat: terrestrial (wet or dry) / aquatic 

• Climate: temperate / mediterranean / tropical 

• Seed structure: endospermic or non-endospermic, 

nature of covering structures, location and size 

embryo 



dormancy 

Seed morphology and structure can provide important clues in predicting 

germination requirements and the presence or not of dormancy. As a rule 

physiological dormancy tends to occur in seeds with small embryos and 

copious endosperm (endospermic seeds), whereas physical dormancy 

tends to occur in seeds with highly developed embryos with little or no 

endosperm (non-endospermic seeds). 

49

Germination requirements are 

determined by an integration of: 

• What kind of plant it is ? 

• The habitat and climate it lives in ? 

• What kind of seed it has ? 



dormancy 

50

Typical process 

• Use data sources and / or climate data to determine 

optimum temperature 

• Apply dormancy breaking pre-treatment if dormancy 

is known 

• Score at regular intervals until germination stops 

• Perform a cut test on ungerminated seeds 

• Apply a TZ test if a high proportion of dead seeds is 

indicated 



dormancy 

51

Always dissect a few seeds before 

you start 



dormancy 

The value of observations derived from simple dissection tests cannot be 

overstated. 

When confronted by ‘seeds’ for the first time it is important to perform a 

dissection test to check the internal structure: 

Is it a seed, or a fruit containing several seeds? 

Does it appear fully mature? 

Is it endospermic or non-endospermic? 

What does the embryo look like? 

Where is it located? 

Is the seed coat thick/hard, likely to be impermeable ? 

Is there any evidence of insect infestation or other damage ? 

This approach, which requires no more than a forceps and scalpel and 

possibly a dissecting microscope for very small seeds, may provide 

important clues to germination requirements. 

52

Further information 

• Baskin, C. C. and Baskin, J. M. (1998) Seeds Ecology, 

Biogeography and Evolution of Dormancy and 

Germination. Academic Press. ISBN 0-12-080260-0 

• Baskin, J. M. and Baskin, C. C. (2003) New 

Approaches to the Study of the Evolution of 

Physical and Physiological Dormancy, the Two Most 

Common Classes of Seed Dormancy on Earth. In: 

Nicolás, G., Bradford, K. J., Côme and Pritchard, H. 

W. (Eds.) (2003) The Biology of Seeds: Recent 

Research Advances, CAB International, chapter 40, 

pp 371- 380 



dormancy 

53

Further information cont. 

• Probert, R. J. (2000) The Role of Temperature in the 

Regulation of Seed Dormancy and Germination. In 

Fenner, M. (Ed.) Seeds The Ecology of 

Regeneration in Plant Communities, 2nd Ed. CABI 

Publishing. ISBN 0-85199-432-6 

Web resources: 

• MSB Seed Information Database: 

http://www.kew.org/data/sid/sidsearch.html 



dormancy 

54

Germination and Dormancy - Royal Botanic Gardens, Kew

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