germination and dormancy

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Supported by Also supported by

Improving the identification, handling and storage of ‘difficult’ seeds

Germination and Dormancy

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© Copyright Board of Trustees of the Royal Botanic Gardens, Kew Germination and dormancy

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 egTerminalia

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

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.

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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

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.

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Environmental factors affecting seed germination

• Water• Temperature• Light • Gases

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.

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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

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.

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Imbibition damage: prevented by RH conditioning.

Seeds held above water in sealed box for 1-2 d at 20°C

Problems associated with water uptake (imbibition)

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.

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Effect of high humidity conditioning on germination of dry-stored Lathyrus sphaericusseeds, chipped to remove physical dormancy

The drier the seeds the more susceptible they are to imbibitiondamage

Conditioning at 100% RH for 24 h prevents damage

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.

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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.

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.

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

The effect of temperature on germination

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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 germinateThe 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.

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Variation in temperature range for germination in plants in the same desert

habitat

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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.

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Requirement for alternating temperatures

Seeds near soil surface experience wide diurnal variation in temp

Buried seeds experience constant temperature

SOILD D D DN N N N

Temp

As the diagram illustrates, sensitivity to the amplitude of diurnal (day / night) variation in temperature probably acts as a depth sensingmechanism. 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.

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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.

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.

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Alternating temperatures

• In the lab:– 25/10 (8h/16h) temperate– 35/20 (8h/16h) tropical– illumination during warm phase

Using Alternating Temperatures in the LaboratoryWhen 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.

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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

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.

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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.

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Evidence that some relatively large seeds may be inhibited by light

• Light inhibition has been reported in some Cucurbitaceae

Acanthosicyos naudinianus

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.

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The effect of gases on germination

• Reduced O2 or elevated CO2 usually reduces germination.

• Except some submersed aquatics where germination is stimulated by anaerobic conditions.

• Nitrogen dioxide gas may have potential for dormancy breaking.

Since germination depends on metabolic activity it is not surprising that the gaseous environment surround seeds is important for germination.As a rule, lowering O2 or raising CO2 reduces germination. However, some aquatic species have been reported to be stimulated by low O2 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.

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Seed Dormancy

Definition: failure of viable seeds to germinate under ‘favourable’ conditions

Function: to synchronise germination with environmental conditions suitable for plant growth.

Seed dormancy is the failure of viable seeds to germinate under favourableconditions. 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.

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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 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.

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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 dormancyBUT

• 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

A population of seeds will display a normal distribution of dormancy states. Some seeds may exhibit no dormancy or be very weakly dormant andsome 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.

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Seed dormancy types

• Endogenous(embryo related)

• Exogenous(seed/fruit coat related)

• Physiological• Morphological• Morphophysiological

• Physical• Combinational

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 beunderdeveloped 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 dormancy20.8% possesed physical dormancy1.6% possesed morphophysiological dormancy2.1% 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).

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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

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.

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‘Difficult’ seeds with physiological dormancy

• Physiological dormancy is common in tropical grasses such as Panicum, Eragrostis, Eleusine

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Physiological Dormancy

Physiological dormancy (PD) occurs in all kinds of embryos.

endosperm embryo

Physiological dormancy occurs in all kinds of seeds irrespective of the nature and size of the embryo.

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Physiological dormancy

• PD enables seeds to avoid seasons unsuitable for seedling establishment

• Hence seasonal patterns of emergence in many species

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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

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.

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Seasonal synchronisation:when is germination favoured ?

• Mediterranean and tropical dryland species:– Wet season

• Temperate species: – Spring or Autumn germination

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Understanding natural patterns of germination and emergence

• Seed burial experiments• Seeds recovered at intervals and tested for

germination in the lab

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Jan Dec

Summer Autumn Winter SummerSpring

Seed fall1 Seed fall1

Warm stratification3

Germination stimulant5

After-ripening2 After-ripening2

Germination

Cold stratification4

Dormancy loss Dormancy induction Dormancy loss

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Soil temperature

Soil moisture

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 moiststratification in the winter and that additional triggers (germination stimulants) such as smoke may be required for some cases.

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Embryo

Germ

W W WSp Sp SpSu SuA A

Seasonal changes in temperature

Seeds of some species require several seasons before germination occurs

Cardiocrinum cordatum (Japan): embryo grows in second autumn, visible germination following spring

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.

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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)

germination conditions

( 25 - 30°C)

Tropical grasses such asEleusine indica will probably respond favourably to ‘dry’ after-ripening

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.

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Overcoming physiological dormancy (mimicking the seasons)

Cold, moiststratification (chilling)

5°C, 8-16 weeks

germination conditions

(15 - 20°C)

Some high altitude species experiencing a ‘temperate’ climate may respond to cold stratification.

(Seeds programmed to germinate after the cold season has passed)

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.

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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.

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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

Surgical treatments often have to be performed under a dissecting microscope. There is a significant risk of embryo damage.

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Response of a range of problem collections to surgical treatment

Effect of Surgical Treatment/Chipping

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14 out of 16 grasses tested showed significant positive effect

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.

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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

Physical Dormancy

Cassia sieberianaPelargonium cucullatum

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:WeatheringMicrobial attackImpaction by soil particlesFire (heat)Extreme diurnal temperature variation in the dry seasonFreezing and thawingAcid scarification after ingestion by animalsSome 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

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‘Difficult’ seeds with physical dormancy

• Corchorus (Tiliaceae)• Vigna, Afzelia (Fabaceae)

The earlier stakeholder workshops identified a number of problematic species with physical dormancy.

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Overcoming physical dormancy in the laboratory

• Most reliable method for small samples

• Small portion of testa removed to aid water uptake

• Care taken to avoid damage to embryo

Baskin & Baskin, 2006

Mechanical scarification

Chipping, nipping, filing

The treatments used to overcome PY are straightforward and simple.Examples of practical methods to overcome PY are shown in the following slides.

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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

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• 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

Wet heat, boiling

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• Can be applied to large samples

• Dry seeds exposed to >100°C for minutes

• Treatment mimics exposure to fire in nature


Dry heat (oven)


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• 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


Acid scarification


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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 NO2 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.

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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 100or 110°C for 5 mins

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.

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MSBP study on effects of smoke and dry heat on problem species:

response to aqueous smoke treatment

Effect of Smoke

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18 out of 45 collections showed positive effect of smoke

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.

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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.

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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)

Combinational Dormancy

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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 GeraniaceaeMalvaceae, Rhamnaceae

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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

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 dormancytends to occur in seeds with highly developed embryos with little or no endosperm (non-endospermic seeds).

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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 ?

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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

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Always dissect a few seeds before you start

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.

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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

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Further information cont.

• Probert, R. J. (2000) The Role of Temperature in the Regulation of Seed Dormancy and Germination. InFenner, 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

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