Reduction Treatments for Seed Coat Conditions

 

Dormancy in seeds with intractable seed coats (which prevent the seed from imbibing) creates a problem for the woody plant propagator. Seeds with such a condition will require one or more of the following procedures to prepare the seed to a stage at which the seed coat is sufficiently degraded that it will become permeable to water and so imbibe. It can then proceed into the next relevant stage of the germination process – which may be germination itself or may be the mitigation of an endogenous dormancy control or the maturation of an immature embryo.

 

These seed coat conditions have evolved as a strategy to prevent the seed from germinating and establishing as a seedling under adverse environmental conditions or to protect the seed in its passage through a digestive tract of an animal while it is dispersed or to spread germination over a number of years by the variability in the rate of decomposition. The procedures, which are indicated here, are for the treatment of the mature, fully ripened seed.

 

It may be possible to avoid the conditions by an analytical understanding of the maturation process and then collecting the seed when it is fully mature (ie when the embryo and food reserves are complete) but before the seed coat begins the maturation (hardening) process.

 

Not all plants which produce seeds, appearing to have a hard seed coat, actually have intractable conditions which prevent the uptake of water – although dry storage may well subsequently induce an impermeable state – a secondary dormancy. Many subjects have developed a ‘hard’ seed coat simply to protect the seed proper, as part of a fruit, in its passage through the digestive tract of an animal (eg the drupes of Prunus, the pomes of Crataegus, the hips of Rosa, the arils of Taxus and Podocarpus etc). The strategy is to entice the animal to eat the fruit with its valuable food and hence energy rich content in order to achieve dispersal of the seed; similarly the fat-rich outer, seed coat of the various Magnolia species achieves the same end. In all of these seeds the coat protects against the action of the digestive juices during the fairly rapid passage of the seed through the gut but does not subsequently generally prevent imbibition – in many cases the fruit has laxative properties which aid this process by easing the activity.

 

Thus in some instances this hard seed coat is part of a strategy where an animal eats the fruit, digests its ‘reward’ and the passage through the gut actually degrades the hard seed coat condition sufficiently for imbibition to follow quickly once it is liberated and finds itself in damp soil (eg Daphne).

 

The objective for the plant propagator is to develop a protocol for the reduction of the intractable seed coat to a stage at which it will readily imbibe. This needs to be achieved in such a fashion that a uniform response is engendered in the sample and that there is no damage internally to the seed itself.

 

Types of seed coat condition

It is not easy to categorise seed coat conditions with any accuracy as each plant has evolved for its own particular condition and the response, structure and mechanism is unique to that species (or probably genus). However many plants, especially when they are closely related or live in similar environments, have obviously evolved very similar protections and can therefore be treated in very similar fashion. Nevertheless there is no universal basis to this mechanism so that mitigation strategies have to be tailored individually.

 

Intractable seed coat conditions can be described as ‘hard’ - in which the protection is shell-like or stone-like; ‘tough’ – in which the coat is described as fibrous, leathery etc; or the condition may be intermediate or possibly more than one type is encountered in layers surrounding the seed (eg Tilia, Hamamelis).

 

Additionally there are a number of species in which the seeds have what is basically a permeable seed coat but this is waterproofed by the presence on, or in, the surface of the seed of a water repellent material such as lipids.

 

In order to overcome such seed coat conditions a variety of techniques are available – depending on the nature of the individual problem. The various techniques can be classified quite simply as physical, chemical or biological.

 

Methods to overcome seed coat conditions

a) Physical methods

1. Hot water soak

In the very simplest condition the seed coat exists as a structure that will eventually allow the passage of water if it is soaked in water for long enough. The seeds of many species develop seed coats, which are marginally resistant to water uptake but once they are dried they are substantially resistant and so prevent water access to the embryo for imbibition.  However once they are exposed to continuously damp conditions the seed coat gradually softens and water will access the seed. An artificially provided warm water soak, which starts with hot water and the maintenance of warm conditions, and gradually cooling over a period of several days, will normally be sufficient to achieve the required level of penetration. The time necessary to achieve this will be a reflection of how dry the seed case has been allowed to become. The treatment will be most usefully conducted with about four volumes of hot water being poured onto one volume of seed.

 

Where possible it would be prudent to collect the seed as early as is feasible so that the development of the ‘hardening’ is kept to a minimum - so that the period of soaking is also reduced to a minimum.

 

2.Scarification

This technique is most effective for the degradation of those seed coats which are hard and shell-like or stone-like. This usually occurs when the seed is fully mature and dry, and has become fairly brittle. The process depends on reducing the seed coat until it is breached in one place to a level at which water can permeate - it is not necessary to reduce the whole of the seed coat.

 

If the seed is big enough to handle individually (eg Gymnocladus dioicus) it is quite possible to chip the coat of the seed – preferably without cutting into the seed itself.

 

If the seed coat is brittle it may be possible to break or crack the seed coat sufficiently by pressing the seed between boards. For those seeds which are of regular shape and in which the sample is relatively uniform in size it would be possible to achieve the same outcome by passing the seed through critically set rollers.

 

If these methods are not practical because of size, quantity or structure then seed coat reduction using sand or other abrasive materials is feasible. The use of two sandpaper boards and rubbing the seeds between them or lining a jar with sandpaper and shaking until a sufficient reduction is achieved will eventually succeed, although this can be tedious to work.

 

It is possible to ascertain that the seed coat reduction has been achieved by immersing a sample of the seed in water for a few hours when the seeds will visibly swell as they imbibe.

 

Seeds which have a hard seed coat - but which water can access when a strophiolar plug is dislodged can be treated simply by bouncing the seeds vigorously in a solid container until the plug is disengaged.

 

3. Heat

Many species, especially from Mediterranean climate biomes, exist in a fire ecology system. These species depend for their survival on being able to germinate and compete in an area which has had the vegetation destroyed by the passage of wild fire. Seeds which have lain dormant, over many years, in the soil are induced to germinate by rains following this pattern. The protective coat of the seed (which allows survival for many years in the soil and is able to resist the normal decomposing agencies) is modified by exposure to heat at a sufficient level that it causes the coat to crack or for a strophiolar plug to be sprung (by differential expansion) or by the splitting of a hilar fissure (which is irreversibly opened by seed coat expansion). In nature only a fraction of the seed present in the soil gets the right temperature - seeds near the surface may be overheated and killed then there is a layer in which viability is maintained and the necessary action is achieved and below this the level of heat penetration is not sufficient to cause any activity. Thus whatever the intensity of the fire some seeds will receive the correct treatment.

 

Artificially the temperature treatment of seed samples can be carried out by pouring hot (boiling) water onto the seeds, however it is quite feasible that the duration of the requisite temperature is not maintained for long enough and no result ensues.

 

The alternative is to use dry heat which can be controlled more satisfactorily both in terms of level and duration. The seed is extended in about four times the volume of dry sand and placed in a suitably sized metal tray, it is then transferred to a preheated oven at 100ºC (+/-10ºC), normally a period of four to five minutes will achieve the desired result, however a longer period will not usually harm the seed and so considerable leeway is available. 

 

b) Chemical methods

1. Digestion by caustic inorganic substances

The use of inorganic caustic materials, such as concentrated acids and strong alkalis, to degrade seed coats can be effective but, in general, it is a dangerous procedure and requires expert knowledge in the handling of the materials, access to the proper equipment and facilities - and in these days of ‘Health and Safety’ an unwelcome bureaucratic paper trail! It should only be contemplated in the last resort.

 

2. Digestion using organic acids

For those species where seed coats which will respond to less drastic action than the foregoing – less caustic materials such as organic acid will degrade seed coats albeit as a slower response. The best documented material, and the one used reasonably extensively is Citric acid; it is used at an aqueous concentration of 1% (10,000ppm) as a soak for (usually) between four and six days. The seed is soaked in about four times the volume of the dilute acid.

 

c) Biological methods

1. Digestion

There are many species of woody plants which develop intractable seed coat conditions which are described as ‘tough’ – as opposed to ‘hard’. The texture of the outer coat of these seeds develops a structure which is normally, more or less, peculiar to a species or genus and is described variously as amorphous or fibrous or leathery or reticulate etc. These seeds do not respond easily to treatment with caustic materials as often the outer surface chars and prevents further action or the structure is not uniform and the action penetrates ‘thinner’ areas quickly and destroys the embryo.

 

Tough seed coats can therefore be dealt with by using a biological digestion which approximates to what happens under natural conditions. For quickest action it as well to collect the seeds while they are still green, soak them for a few hours to top up their water status and then commence the treatment immediately - as this will arrest the further development of the seed coat.

 

A measured quantity (by number or weight) of seed is mixed with about four times the volume of a damp, organic substrate such as peat or a suitable substitute with a satisfactory air filled porosity. This is then inoculated with a nominal amount of proprietary compost accelerator (which will induce a quicker decomposition of the organic seed coat material). This mixture is then placed in suitably sized polythene bag (zip lock type) and incubated in a warm environment (30ºC). The bags should be monitored on a weekly basis, shaken to maintain an even atmosphere and moisture content – if the material appears to be drying then add moisture to maintain the water status. The seeds will imbibe and swell once the treatment has been completed. This type of treatment is necessarily less exact than some but experience will allow a reasonable estimate to be made of the likely periods needed for a successful outcome and a standardised procedure with reasonably accurate parameters can be developed.

 

Other Impermeable conditions

A short term protection against the uptake of water by the seed is provided by a water repellent material on, or in, the surface of the seed coat. These are usually waxes, fats or oils and under natural conditions are relatively quickly removed in the soil, by natural agencies, so that imbibition does not occur until dictated by the plant’s strategy for successful germination.

 

In many of the species however it is just as likely that the protection is provided to maintain the water status of the seed itself until such time as it gets into the soil.

 

Lipids of these types are readily extracted by using a hot water soak and flushing two or three times or by organic solvents such as alcohols, acetone etc.

 

In some species associated with a fire ecology the seed is surrounded by a thick, dense layer of waxy material which protects the seed until the passage of fire when effectively the material melts and frees the way for water to permeate when it is available.

 

In California it is often the practice when small seeds, such as Dendromecon and Romneya, are to be treated that the seed is sown by mixing with a dry compost and then placing in an earthenware container, covering with excelsior and igniting – thus simulating natural conditions – however only the seeds in the stratum of compost receiving the right temperature will be successfully treated (see above). The seed can then be left in the container and, when watered, will quickly germinate.