When a viable and quiescent seed is subjected to conditions suitable for germination and it does not respond, the seed is said to be dormant. This implies that there is present, in the make up of the seed and/or its coverings, a mechanism for preventing the process of germination from proceeding.

 

Although this situation presents an irksome problem for the propagator, the condition is an evolved strategy which the particular species has developed to enhance survival after germination or it may also reflect the need to spread germination over time. It has evolved to ensure that germination and establishment occurs when the most suitable environmental conditions for the survival of the seedling are available and conversely that germination is prevented when conditions are adverse. It may also delay germination spasmodically over a number of years in order to achieve success when suitable years occur. However it should not be overlooked that some seed coat conditions have evolved to protect the seed from digestion while passing through the gut of a predator/vector.

 

The propagator needs, therefore, to recognise and understand these limitations and, by investigation, create protocols to overcome them artificially and allow germination to proceed, as and when required and without constraint, in order to achieve the best productivity. In all of the ensuing descriptions the seed is understood to include any significant part of the fruit which may be relevant.

 

The evolution of dormancy controls has not been the simple development of a few standard variations which therefore can be treated with a precise and uniform set of treatments but has developed at various times and in a diversity of situations, with the resulting complex of present day scenarios.  Inevitably however the practicalities of operation, in a nursery context, need to be relatively simple and fit into a manageable programme which is predicated on a few standard treatments, however – as has been indicated - the dormancy controls exhibited by individual species do not necessarily conform exactly to such uniform fixed parameters and it is necessary to be flexible when applying treatments at a species level.

 

In broad terms the factors making up the spectrum of dormancy controls, in the seeds of woody plants from temperate climates, can be attributed to a number of different conditions which can be categorised as:-

A) external conditions in which the seed coat or other parts of the fruit may

 a) prevent the uptake of water by the seed,

 b) restrict gaseous exchange or

 c) limit embryo expansion;

 

B) seeds with an immature or rudimentary embryo and

 

C) biochemical conditions which rely on the endogenous physiological inhibition of some essential reaction which controls a phase of the germination process.

 

Dormancy caused by the condition of the seed coat

The seed coat may be categorised as ‘hard’, ‘tough’ or ‘waterproofed’. If seeds are sown and left to their own devices the seed coat will gradually (or in particular cases suddenly) be degraded in the soil until such time as it is possible for water to permeate and enter the seed and, thus, for it to imbibe. The great majority of species exhibiting this type of constraint to germination (dormancy) achieve the objective by preventing the uptake of water from the surrounding soil and so prevent the seed imbibing. There are a number of mechanisms which fall into this category:-

 

Hard seed coats

Many plants produce seeds which develop a hard seed coat – the commonest form of which is the production of a hard stone-like or shell-like condition. The effective method of reducing this to a water permeable condition varies from caustic digestion, through heat treatment, physical and biological degradation, to warm water soaking depending on the degree to which the ‘hardness’ (and hence impermeability to water) has been developed.

 

The reduction of the seed coat to a water permeable condition can be carried out by the digestion of the seed coat by caustic agents such as concentrated acids. However this is a hazardous enterprise even when quasi-laboratory facilities are available and should probably only be contemplated as a last resort. It is not only the potential danger to the operative which can be a problem but that the determination of the end stage is not easy to monitor so that either the seed coat does not become fully digested or conversely it is too fully digested and the acid penetrates the seed so causing the loss of the sample. For the seeds of those species with a less intractable condition, degradation can be achieved by a relatively long term soak in a weak organic acid (eg 1% citric acid).

 

For the seed of those species which have a hard brittle or hard stony seed coat it is feasible to achieve a physical degradation of the seed coat, to a sufficient level for imbibition to occur, by cracking or physically degrading (abrading) the seed coat - this is the safest and probably the most effective method available.

 

Seeds of species, whose survival strategy in dominated by the occurrence of wild fires (the so-called fire ecology), have hard seed coats which protect and preserve the seed in the soil until the passage of fire. The seed coat of many species is hard, brittle and shell-like and will crack once a particular level of temperature, surrounding the seed in the soil, is reached. Other species have developed similar responses which depend on the temperature causing a differential expansion within the seed coat which causes the irreversible splitting of a hilar fissure or the dislodgement of a strophiolar plug. Once this stage has been achieved the seed will imbibe as soon as water becomes available and then proceed to germinate.

 

This latter strategy is a common feature of the seeds of woody plants from Mediterranean climate plant communities which are dominated by a fire ecology eg Mallee in Australia, Chapparal in California, Mattoral in Chile, Fynbos in South Africa and Maquis and Garrigos in Southern Europe.

 

It should be reiterated that because a seed has a hard seed coat that it is not necessarily a barrier to water permeability. Many seeds have developed a hard seed coat for protection of the seed while it is subjected to the digestive processes during passage through the gut but water uptake is still possible. Similarly the seed coat may be reduced by such passage and water permeability enhanced.

 

The practicalities of treatments for mitigating these conditions are dealt with in Appendix 1

 

Tough Seed Coats

The types of seed coat which constitute this category are quite varied in their constitution and construction but all are to some degree ‘leathery’ ie they are – when fully developed - pliable, tough and impermeable to water. They generally have a more or less constant and uniform consistency or sometimes have a reticulate structure. All are intractable - insofar as the removal of seed coat material, artificially, is difficult and often hazardous. Although it is possible to degrade them by digestion with acid or other caustic materials, the operation is usually difficult to administer and monitor adequately - especially those with a reticulate structure as the acid may not act uniformly through the seed coat and may suddenly penetrate through to the embryo. Although slower, the use of a ‘natural’ material which degrades the seed coat biologically is safer and can be operated effectively if not with great accuracy.

 

The practical aspects of artificially overcoming these types of dormancy are described in Appendix 1.

 

‘Waterproofed’ Seed coats

Although not of common occurrence there are several plants in which the seed is maintained in a quiescent state by a surface waterproofing which prevents the ingress of water. Materials such as waxes, fats and oils are the most commonly found agents. Often the condition is only short lived but should it need to be removed - hot water soaking is usually sufficient, if more robust action is required suitable organic solvents or detergents will be effective. 

 

The practical aspects of artificially overcoming these types of dormancy are described in Appendix 1.

 

Immature (Underdeveloped) and Rudimentary Embryos

In many species of woody plants the process of germination is prevented, simply, because the embryo has not developed to a sufficiently mature stage, either morphologically and/or physiologically, for germination to occur when conditions are suitable. For germination to occur the embryo must have reached a state of ‘maturity’ at which it is capable of proceeding into the sequence of phases which constitute the process of germination.

 

This particular condition is seen in two manifestations:-

a) as a rudimentary embryo in which the embryo exists only as an, more or less, undifferentiated mass of cells. This condition is normally found in parasitic plants but is seen occasionally in conventional, terrestrial subjects from Families of Angiosperms – especially those which produce ‘microseeds’ or ‘dwarf seeds’ (eg Ericaceae), but also includes primitive families (eg Aquifoliaceae);

b) as an immature embryo in which the embryo has been partially or fully differentiated but has not reached a sufficient size or physiological status to be able to germinate. This condition is usually found in those Families which appear early in the evolutionary development of both Gymnosperms and Angiosperms, but not exclusively, – and includes the:- Cycadaceae, Ginkgoaceae, Taxaceae, and Podocarpaceae; Adoxaceae, Araliaceae, Daphniphyllaceae, Escalloniaceae, Monimiaceae, Lardizabalaceae, Magnoliaceae, Oleaceae, Pittosporaceae and Winteraceae.

 

The immature embryo condition can, in general, be mitigated by storing the imbibed seed in a suitable environment, at a ‘warm’ temperature, for a sufficient period to allow the maturation process to be completed. However there are examples where the maturation of the embryo occurs, optimally, at ‘cool’ temperatures (c5˚C) eg Sambucus.

 

The practical aspects of this procedure are described in Appendix 4.

 

Physiological Dormancy Conditions

This type of dormancy control occurs within the seed itself and depends on the presence of particular chemicals which prevent or interrupt the normal patterns of metabolic activity and growth at this stage of development – the process of germination. These activities occur and are controlled at the molecular level by genetic keys which can work at various levels of intensity – an understanding of this series of mechanisms is beyond the compass of these notes (and their author!).

 

It is evident that the processes and reactions which are governing the growth and development of the embryo at the post imbibition phase, ie the start of germination, are determined by growth regulating substances – such as gibberellic acid - which engender cell division, cell enlargement and cell differentiation. Initially, after the process of imbibition is complete the activity of the growth regulating substances should begin and the embryo should proceed to develop in a normal fashion. However in the seeds of those plants in which the embryo does not respond in the normal sequence of events – ie when environmental conditions are suitable and it fails to develop further - are described as being endogenously dormant.

 

This facet of seed biology is still imperfectly understood and the actual mechanisms which are creating this situation are not yet completely elucidated – at least as far as the lay person is concerned. There is currently therefore a void in the spectrum of this area of knowledge. Currently, it would appear, that the prevention of germination of the imbibed seed is caused by the presence of inhibitors which the seed has metabolised during the maturation process. The chief recognised agent being abscisic acid. Further activity then depends on the reduction of the level of the inhibitor with a corresponding increase in the production of the growth promoter (usually cited as a giberellic acid).

 

It would appear that, at present this type of dormancy can be mitigated by the exposure of the imbibed seed to a period of cold temperature.

 

Embryo Dormancy overcome by Chilling

An endogenous dormancy condition which requires the imbibed embryo of the seed to be exposed to a period of ‘cold’ temperature is an evolved strategy developed in many woody plants - especially of temperate climates. It operates on the premise of preventing germination during the unsuitable conditions of winter and then promoting germination during the spring when environmental conditions are most suited for germination, the development of the seedling, seedling establishment and survival. Thus the seed is prevented from germinating before the onset of winter by a physiological (and genetic) mechanism, within the seed, which prevents germination - despite environmental conditions being suitable. The passage of winter provides the ‘cold treatment’ which releases this block to germination and thus allows the imbibed seed to germinate, when the environmental conditions are suitable, in the spring.

 

There are a number of practical implications within the process which need to be addressed by the propagator in order to simulate the natural process of chilling under artificial conditions and so achieve an efficient and productive response:-

a) the removal of all other naturally occurring inhibitions or constraints (dormancy conditions) which could prevent germination;

b) that the seed is fully imbibed and therefore primed for the necessary physiological activity;

c) that there are no other constraints within the process - such as any restriction on the availability of oxygen or the presence of inhibitors in other parts of the seed and

d) the provision of a period of ‘cold’.

 

Cold in this context is the sum of two parameters:-

a) a temperature below the threshold at which the process will occur – ie the availability of a critical pathway, within the physiological activity of the seed, becoming available and

b) the maintenance of this low level of temperature for a sufficient duration such that the constraint is eliminated and thus allows germination to occur.

 

The threshold temperature, below which the chilling process is effective, is a function of the environmental conditions under which the species has evolved and is particular and constant to that species.

 

Warm temperate species may well respond to the maintenance of cool temperatures which are below 7º or even 10ºC; the same is true for many subarctic species which are effectively frozen during the winter and have to respond to the thaw period in the spring. Cold temperate species tend to respond in the range below 3ºC to 5ºC.

 

There is no advantage or disadvantage to the chilling process if the maintained temperature is lower than the threshold - as there is no cumulative effect with lower temperature (the degree-day concept). However, because of the practicalities of administering the cold at various thresholds, a constant ‘low’ temperature is operated, in a universal role. This accommodates the chilling threshold for the great majority of species in one facility. The temperature chosen should not be cold enough to cause physical damage to the seed and especially, in this context, temperatures below 0ºC should be avoided. The freezing of intracellular and intercellular water could well be physically damaging as well as effectively pausing the physiological activity by removing the water from the cell by freezing it into ice. In practice the cold temperature facility should be maintained at +/- 3 or 4ºC as this will be below the required threshold for the chilling for most species.

 

The critical factor for the plant propagator in this process, then, is determining the length of time that is required for the chilling process to be completed and so allow the speediest and most synchronised germination to occur.

 

In most woody plants the optimum duration of this process is reasonably constant and does not vary within a species either within the sample or from year to year. Variations in the response times, reported in the literature, are generally created when another dormancy factor(s) has not been fully eliminated. If variations in response time are encountered then the shortest time will be the one which is the closest to being relevant.

 

The period of chilling that is required to eliminate the dormancy control is not concluded suddenly with the achievement of a complete process. When suitable conditions for germination are provided there is, once a minimum threshold chilling period has been achieved, a slow and erratic response if exposed to suitable conditions for germination. Then, with incremental additions of chilling, the process gradually unifies although germination in the early stages is still slow, erratic and incomplete. As the period of chilling increases the response time becomes gradually quicker, with a more uniform emergence and a greater proportion of the seeds responding - until the optimum period is reached when the block to germination has been completely removed. At this stage the response, during the process of germination, is quickest, with the closest synchronisation of emergence and with the maximum number of seedlings being produced - and further enhancement of the chilling period does not produce any improvement. It is at this stage that the period of chilling is designated and incorporated into a production protocol.

 

There is also some evidence to suggest that before the optimum period is achieved that germination only occurs in a much narrower range of temperature – ie in that period before the chilling process is completed.

 

Thus once these factors have been addressed and resolved it is then possible to derive a protocol to overcome this type of dormancy in any particular species.

 

In general terms species from warm temperate climates exhibiting this type of dormancy have high threshold temperatures and require only a short period of exposure to achieve the response. Species from cold temperate climates, as might be expected, have lower threshold temperatures and require longer periods of exposure to eliminate the condition. If this period is an unknown then it is helpful to have some knowledge of the type of the native climatic environment of the particular species so that pointers to likely outcomes could be proposed.

 

In line with many other biological processes concerned with germination the concept of chilling and its evolution, as a strategy in the germination and survival process, is not always as straight forward as it would appear – as there are varying levels at which the depth of the effect operates. In practice however those seeds which have an obligate chilling requirement are dealt with as above but there are some species in which the chilling merely enhances the speed and synchronicity of emergence compared with non-chilled seed which will emerge more erratically, more slowly and generally at lower levels of productivity.

 

The practical aspects of achieving the chilling process effectively, productively and efficiently are outlined in Appendix 2.

 

An Eccentric Dormancy Condition

There are inevitably areas of knowledge, which are concerned with ‘dormant’ embryos, which are unclear and often confusing for the layman to understand – especially in relation to traditional perceived wisdom and practice. One such concerns the perception that hard endocarps prevent imbibition and hence germination. Modern knowledge however indicates that more often than not (outside particular Families – eg Fabaceae and Anacardiaceae) the hard condition does not indicate impermeability and that the associated practice of warm stratification does not reduce the hard seed coat to create permeability – it is in fact always permeable. This period of warmth is required to ‘mature’ a physiological condition within the seed (the embryo of which otherwise is morphologically fully developed) which is blocking germination, once it is mitigated the already imbibed embryo can be successfully chilled or treated.

 

With this insight it is thus possible to understand the difficulties which have been prevalent in determining the conditions required to create germination in certain taxa. Classic in this respect is probably the uncertain nature of the recommendations available for the germination in Prunus.

 

Chemical Inhibition

The prevention of germination induced by the presence of a chemical inhibitor(s) (probably abscisic acid) is a more widespread phenomenon than is generally appreciated.

 

This form of dormancy is fairly common in woody plants from desert and other arid communities where rainfall does not necessarily provide an adequate reservoir of water in the soil to allow germination and establishment of the seedling. Under such conditions the water soluble inhibitor acts as a gauging mechanism to determine rainfall (and/or flooding) and so measure the accrual of a sufficient reservoir of water in the soil to allow germination and establishment of the seedling to a survival threshold.

 

This response depends on the water solubility of the chemical and the passage of water through the seed coat to remove, by leaching, the chemical – such that when the chemical has been fully leached there should be a sufficient reservoir of water in the soil to support the process of germination through to establishment and hence potential survival.

 

Other chemical inhibiton occurs in many woody subjects of cold temperate climates, once again involving the presence of abscissic acid in the pericarp or fruit. Leaching of the seed is also effective in these instances. It is also possible that external abscisic acid is also found in conjunction with endogenous abscisic acid induced dormancy requiring mitigation by a period of chilling and when not eliminated fully can create confusion in the length of the chilling process.

 

The practical arrangements to achieve satisfactory leaching is dealt with in Appendix 3.

 

 

Chemical enhancements for promoting germination

There are many species which are native to fire ecologies in which germination is promoted by particular chemical components produced in the process of burning the natural organic matter. These may be present in ‘smoke’ or may be the result of leaching from the residual charred material (‘Charate’). The actual active components creating this effect are far from perfectly understood.

 

The treatment of seeds with other chemicals including potassium nitrate, giberellic acids and kinetins is also used to promote germination in particular circumstances.

 

These situations are dealt with in Appendix 5.

 

Epicotyl Dormancy

The condition known as epicotyl (double) dormancy, although not uncommon in herbaceous plants – especially the Liliaceae, is not common among woody plants in general. It is typified by the emergence of the radical, from the imbibed seed, during the first warm period after dispersal; however the plumule requires a period of chilling before it will germinate when conditions are again suitable. This usually means that the seedbed is occupied for two growing seasons before the seedling is produced. Examples among woody plants that have adopted this strategy include:- Chionanthus, Davidia and some North American species of Viburnum. In some warm temperate and tropical species the plumule is not ‘mature’ and requires a period of warmth for maturation to occur and then proceed into the germination phase – the temperature for these two processes is not necessarily the same.

 

Multiple dormancy

It is not uncommon to find, among woody plants from temperate climates, the development, as a result of evolved strategies for survival, of more than one type of dormancy. This more complex picture is certainly more irksome for the plant propagator but it can be dealt with by eliminating the conditions in sequence in order to achieve germination. In nature this usually combines the need to spread germination over more than one year with avoiding the vagaries of winter survival.

 

In some species this entails, at the extreme, all three modes of dormancy and involves in order of elimination – a seed coat condition, an immature embryo and a chilling requirement. The classic exponent of this state is the Common Ash of the British Isles - Fraxinus excelsior.

 

Dormancy packages

In several genera of woody plants – especially those which contain species which fulfil a pioneer role in plant succession – the seeds are often dispersed regularly or intermittently over the winter period. These seeds are often able to respond to the environment and time scale which is available by having, inherently, more than one dormancy control and only one of which needs to be satisfied and so allow germination at a suitable time (eg Betula).

 

Avoiding Dormancy

Dormancy control caused by intransigent, seed coat conditions can pose a number of vexing problems to the raiser of woody plants from seed. These conditions are conventionally overcome by the degradation of the seed coat by (more or less dangerous) caustic agents for swift results or quasi-natural methods which are relatively long term ad difficult to develop accurately. Ideally the most successful outcome would be to be able to collect the seed before any of these conditions have developed in the maturation process of the seed/fruit.

 

Although it is currently not feasible to avoid endogenous dormancy conditions, it is however possible to avoid those dormancy conditions which are controlled by the condition of the seed coat. It is only in relatively recent years that it has become accepted that it is possible to harvest the seed as soon as it is complete, mature and before the seed coat condition has developed.

 

In the development of the seed, the cells of the fertilised embryo begin to divide to produce a food store and as this reaches adequate proportions it is used to provide the basic materials for the development of the embryo – which grows at its expense. Subsequently the conventional sites for the food store are differentiated and the seed is completed by the development of these food reserves. At this stage the endogenous dormancy controls are implemented and the seed begins to reduce its water content. As this phase reaches its conclusion the embryo finally matures and is isolated by the maturation of the surrounding membranes and encapsulating tissues. It is then that the seedcase and/or fruit begins its development and eventually develops the condition which protects the seed against imbibition by providing an impermeable barrier to the ingress of water.

 

Even thirty years ago it was barely recognised and accepted that the collection of the seed, at the point of maturation and before the development of the impermeable seed coat, was feasible and could be undertaken without any potential damage. However the concept was not new and as long ago as 1945 van der Graf in the Netherlands published on this topic in relation to Daphne mezereum. Plants in as widely divergent genera and species as Daphne mezereum, Acer campestre, Acer truncatum, Hamamelis mollis, Carpinus betulus and Viburnum lantana have all been shown to respond to an adaptation of this technique.