The Plant Propagator will be concerned with the germination of seed lots at various times, from varying sources and for particular intensive or extensive production systems and circumstances. On any one occasion and for any particular sample, however, it will be necessary to be aware of the potential productivity, how this is influenced and how it is maximised, over time, in production practice. When determining the sowing rates for a sample of seed, in order to achieve a predetermined density of seedling production, it is an essential part of the calculation to know what proportion of the sample is viable and thus potentially capable of developing into a seedling.

 

Viability

 

In assessing the value of a seed sample, the quality of the seed lot will not only be determined on the basis of genetic considerations and the size of the individual seeds (which reflects both the size of the embryo and the level of food reserve) but also on the fraction of live seeds present and the vigour that is expressed by these live seeds in the germination process.

 

What will chiefly concern the plant propagator, who seeks to produce a crop of seedlings, from a particular seed sample, will be the actual potential of the lot to produce seedlings not only in terms of the quantity but also of their quality. This potential will not only be a reflection of the number of apparently live seeds within the sample but will also reflect the condition of the structures and processes within the seed which determine the vigour of the process. The ideal is characterised by the presence of a normal, live embryo and a sufficient and available food reserve which can be readily mobilised to support the duration of the germination process until autonomy is achieved.

 

The term Viability is used to represent that portion of the seed sample (at the time that it is measured) which is alive and capable of producing a seedling - ie the number of viable seeds.

 

A viable seed is therefore one which has the potential to germinate and establish as a seedling, given the provision of suitable conditions; and is thus one which is capable of producing both a sufficient root and shoot system and developing a sufficient photosynthetic apparatus to ensure continuing autonomy.

 

There will be many causes for the lack of viability in seeds – even those which, externally, appear to be sound.

 

The lack of viability of a seed may be attributable to:-

a) the absence of an embryo,

b) the presence of a malformed embryo,

c) the lack of a sufficient food reserve,

d) an inability to mobilise and/or use the food reserves,

e) physical damage engendered by the collection, extraction and cleaning treatments,

f) an attack by biological agencies causing degradation of the various structures and/or processes,

g) the seed may have ‘aged’ to an irreversible state of deterioration which effectively does not allow the process of germination to proceed with sufficient vigour to be completed.

 

The potential viability of a seed lot is usually expressed as a percentage of the number of seeds present and reflects the proportion of the sample which is capable of a satisfactory germination. This figure however only represents the potential at the time that it is measured and is limited by the particular method of assessment.

 

The viability of a sample does not represent a constant state. It will inevitably be in a continuous state of decline – if for no other reason than the effects of the natural ageing processes. As is usual in the great majority of biological systems this process of decline – the losses due to the ageing processes and general biological losses due to other causes – does not occur at a constant rate, even given an optimal and stable environment, but follows the well documented pattern of a sigmoid curve.

 

As the viability of a seed lot is naturally declining over time there will inevitably be an interest in the rate of the time scale of this deterioration. However it is influenced by so many environmental factors which can accelerate or delay the rate that any assessment can be little more than a relative comparison.

 

 

Longevity

 

The term Longevity is used, in the context of seed viability, to describe the ‘length of life’ of a seed sample during which some useful capacity for germination is retained. It is however rarely defined in a measured way and is, in general, used in a relative or comparative fashion to determine the effect of storage conditions on maintaining viability.

 

In practice this period can vary from a few days (Acer rubrum, Acer saccharinum, many Willows and Poplars) to many years (many of the hard seeded Leguminosae).

 

Vigour

 

The measure of viability provides the overall picture of the live status of a seed lot but experience will indicate the potential this level will have on the rate and strength of seedling emergence during the germination process. This aspect is described as the Vigour of the seed lot.

 

When a seed is fresh and ready for dispersal there is (as would be anticipated) normally no deterioration in either the structure or the physiological processes - the maximum number of embryos are ‘live’ and the support systems (metabolic processes) are at maximum efficiency. When the germination process is delayed this causes, with the progress of time and subject to whatever the environmental conditions prevail, damage and deterioration of structures and processes.

 

Observations of the internal structures of a particular seed lot, over time, will reveal areas of necrotic tissue developing and eventually as these areas coalesce it will be evident that this must give rise to some level of impairment. However the crucial aspect of such deterioration, in relation to viability and vigour, is dependent on the exact location of this necrosis – if this condition is in the embryo itself the effects will usually be markedly deleterious whereas if they are present within the food reserve the effect will largely be as restrictions to the availability of the food reserve and hence the general vigour of the germination process.

 

Any deterioration in these parts and processes will slow, in some degree, embryo enlargement, seedling emergence, rate of development and seedling establishment. This will be seen, as time elapses, as a depression in the rate of germination, an erratic emergence and a reduced capacity to establish as a seedling – consequently this, in turn, reflects the number of seedlings which survive or conversely the number of seedlings which germinate but fail to establish.

 

The accurate assessment of both viability and vigour of a seed lot for the determination of seedling establishment, for use under field conditions, is not an easy task. Germination tests measure the rates and levels of potential emergence by assessing the response under controlled and suitable environmental conditions but are only practicable once any limitations to germination have been removed. Artificial tests, using staining techniques or excised embryos have the advantage that any germination inhibition present is not relevant but it does require a considerable experience in the practical interpretation of the responses - in order to give a measure which is useful under field conditions. Obviously the more controlled is the proposed environment for germination, in the production process, then the more useful and applicable are the results from such artificial assessments and are more useful for use under practical nursery conditions.

 

The Practical Assessment of Viability

 

Effectively all the practicable methods and techniques used in the assessment of viability have some real disadvantage in either the accuracy of the process itself or in the evaluation of the results. This often reflects, in general, more the ability of the operator to interpret the outcomes than the process itself. Experience, as ever, is a very real necessity. However each technique has a niche in the overall spectrum of resource and has a place for particular purpose or context. The significant factor in all of these assessments is to be able to correct the apparent results to the actual figure that will be achieved under suitable conditions for germination – this measurement is sometimes described as the level of Germinability. This measurement eliminates seeds with apparently germinable embryos but which, for various reasons, do not produce any or suitable (deformed) seedlings.

 

The various techniques for the assessment of viability under nursery conditions can be categorised as:-

a) Cutting Tests,

b) Germination Tests,

c) Excised Embryo test and

d) Staining Techniques.

 

The Cutting Test is the simplest operation to carry out, as it relies on exposing the internal tissues of the seed to observation – and from this, by a visual assessment, the status and condition of the embryo and food reserves can be determined.

 

The operation is carried out on a randomly selected, relevant sized sample of the seed lot. The individual seeds are then cut in half in such a way that a representative cross section of the structures within the seed are exposed and it is possible to assess the status, presence and condition of the various parts of the seed. Although this operation can be carried out with the aid of low powered magnifying equipment, it is really only a satisfactory technique for use on seeds which are large enough to be viewed satisfactorily in this context – arbitrarily - it is difficult to be conclusive with seeds less than 2mm in diameter or length. This measure inevitably depends on the experience of the operator in determining the size and position of the areas of necrosis and then having the experience and ability to translate this into a practical application under field conditions.

 

The Germination Test provides a relatively more accurate assessment of the potential for germination which might be achieved under operational conditions. Seeds which are otherwise unencumbered can be subjected to reasonably constant and reproducible environmental conditions which are suitable for initiating and sustaining the process of germination. The results as measured by the seeds responses and these can then be evaluated.

 

This technique can be operated at varying levels of sophistication – from sowing the seeds in a suitable container using a relevant compost and placing it in a practical environment to the placement of the seed on a damp pad in a Petri dish and placing it in an incubator under definite and constant controlled conditions. The responses measured from such observations not only provide a picture of the proportion of seeds which actually germinate but also allow an assessment to be made of the rate and uniformity of seedling emergence. This gives an indication of the vigour of the sample as well as the potential number available.

 

Inevitably this technique is only valid for seeds which are not dormant or have any other encumbrance to germination.

 

These observations give useful and real indications but their application will only be relevant if a correlation   to the relative environmental sophistication of the actual level of the practical process can be determined.

 

The Excised Embryo Test is necessarily a more complex test to operate and requires the availability of some limited laboratory resource and the development of some expertise in being able to dissect out the embryo from within the seed without damage. This technique involves the removal of the embryo from its surrounding tissues and seed coats and its transference to a moist situation in a Petri dish or similar receptacle, where it can be suitably incubated and any responses assessed.

 

This operation allows seeds with an imposed dormancy due to seed coat condition to be assessed quite simply on the fact that embryo development will begin with the growth of the radicle.

 

For those seeds which have an endogenous embryo dormancy the radicle however will not develop but viability is potentially indicated by the greening of the cotyledons if light is available.

 

The use of Staining Techniques in determining the presence and pattern of living tissue within the seed (and conversely the presence and pattern of necrotic tissue) can be achieved using a range of vital staining agents. There are a number of such chemicals which will achieve useful results and which are assessed on a visual determination. 

 

The most universally implemented of the staining techniques involves the use of 2,3,5 triphenyl tetrazolium chloride (TTC). This material will stain living tissue and thus allow an assessment of the status of the seed in terms of viability and vigour without the use of a great deal of sophisticated resource. However it will require the availability of simple laboratory facilities. This material will stain the ‘living’ tissue of the seed a bright pink and leave the necrotic tissue colourless.

 

This quick biochemical viability test can be applied to a seed sample and will give, with ongoing experience of the test, results which are comparable with the Germination Test. The test involves the use of this soluble, diffusable chemical which will stain living tissue deep pink and will leave necrotic (dead) tissue uncoloured.

 

The process involves imbibing the seed, cutting the seeds in half and then immersing them for a period of time in a solution of the chemical. The time needed for the take up of the stain varies with the species and will vary from a few hours to a couple of days. The seeds are then assessed under a low magnification, by dissection, to determine the presence and distribution of the necrotic areas. This in turn, with experience will give an indication of the viability and vigour of the sample.

 

An advantage of such a test for viability is that it allows an observation to be made on endogenously dormant seeds which cannot, otherwise, be measured by a Germination Test.

 

The assessment of viability, using staining tests such as this, is necessarily subjective and almost always suggests a higher viability potential than is actually achieved in practice using a germination test. An allowance therefore needs to made to correct this discrepancy.

 

Some knowledge of the viability of a seed lot is however fundamental to any study concerned with the germination (and pre-treatment) of seeds – and is ultimately the significant basis for controlling the quality of seedling production.