Germination describes the process by which an otherwise quiescent and viable embryo begins, and progresses through, the sequence of events which cause it to develop and establish as a new plant.

 

The aim of the plant propagator is to simulate nature, under controlled conditions, and attempt to convert the seed lot, via an understanding of this process, into a crop of seedlings.

 

The process of germination consists of a series of often complex and interrelated processes which, despite modern scientific and technological advances, is still imperfectly understood, at least, by the layman.

 

In the production of a crop of seedlings the aim of the plant propagator, in dealing with a particular lot of seed, is to induce germination in the most effective, efficient and productive manner feasible – ideally converting all the viable seeds into seedlings. This would normally be achieved by processing and treating the seeds in such a way as to cause germination in the shortest possible time with the closest synchronisation of emergence - together with the development and establishment of the highest proportion of viable seeds – a process which has been sophisticated to an advanced level by the bedding plant and vegetable young plant industries.

 

Germination occurs when a series of required, environmental factors coincide and are satisfactory for the activity to proceed. Viable seeds for which all of the environmental factors, necessary for germination, are provided and which fail to germinate are said to be dormant. Germination therefore can only be achieved productively when all other constraints and encumbrances have been completely eliminated. This will require an understanding of all these limitations and how they are most effectively eliminated with the least loss of viability.

 

For the process of germination to be initiated and develop adequately, the seed will require exposure to an environment in which there is:-

a) provision of a sufficient, non-limiting, and suitable water supply,

b) an adequate and non-limiting atmosphere containing oxygen,

c) a suitable temperature regime and

d) light (or lack of it) if the seed is photosensitive and/or ultimately for photosynthetic activity.

 

Water and the process of Germination

In the process of seed maturation the seed tends to lose moisture and hence increase its dry weight. The extent to which this drying occurs varies according to the species and this in turn is often a reflection of the type of food reserve which is stored in the seed to support the process of germination. The germination sequence begins with the rehydration of the seed. For this activity to proceed the seed needs to be exposed to moisture. This rehydration starts with a process described as imbibition. Imbibition involves the uptake of water by the seed. Initially this uptake of water occurs as a non-biological reaction – an entirely physical process resulting from the colloidal adsorption of water by the matrix of the seed tissue. During this process the desiccated colloidal contents of the cells rehydrate. At this stage the water status is reversible (ie the seed can be redried) without detriment. This phase of the imbibition process occurs quite independently of any biological or metabolic activity. Seeds, as a result, may well swell to more than twice their original size. This reaction will occur even if the embryo is dead – provided that the contents and the structures of the seed have not deteriorated; indicating that this initial swelling is not an indicator of viability. This part of the imbibition phase is a relatively rapid activity - being completed in a matter of hours, if the surrounding moisture status is not limiting.

 

Once the seed has been rehydrated and the tissues have recovered their operational size, structure, shape and condition, the finer structures of the cells will also have become reconstituted or repaired and returned to as close to their functional mode as is feasible.

 

It is at this stage that the enzymatic activity and mobilisation of the food reserves and other resources begins. This is characterised by a continuing but slower uptake of water as a result of the osmotic gradients thus created. This phase of slower water uptake is described as the lag phase. When the effective moisture status of the seed has been restored, this first phase of germination proper – the first, non-reversible, biological activity – follows quickly. It involves the initiation of those metabolic activities which promote cell enlargement and elongation. This happens, in the first instance, in the cells of the radicle and can cause sufficient growth for it to emerge through the seed coverings.

 

However if the seed is subject to an endogenous dormancy control this immediate post imbibition phase is delayed by the need to eliminate the inhibiting agency (or promoting the growth activity) before the processes concerned with cell enlargement and elongation are released. Indeed imbibed dormant seeds can be maintained indefinitely if not chilled or leached to break dormancy.

 

The cell enlargement and elongation phase, once initiated, is followed by the process of cell division and differentiation.

 

The root elongation and development stage follows, usually, after a delay (which varies from species to species), by the development of the epicotyl and further additional root development. During this phase cell division and cell differentiation occurs and is supported by energy and materials mobilised and derived from the food reserves. It is at this stage that the rate of water uptake begins to increase again. Often however the development of the root is extensive before any epicotyl extension occurs (a condition often encountered in species which experience arid soil situations).

 

Eventually the epicotyl (which may include the deployment of the cotyledons) then emerges and during the development of the plumule the leaves are deployed – at this stage the potential for carbon assimilation accrues.

 

The overriding feature in this sequence of events, which initiates and allows the unimpeded continuation of the process of germination, is the unrestricted availability of moisture so that the process is not limited by a lack of water or indeed a water tension. All of the metabolic activities will require water either as a raw material or as the medium for the reactions. Water is thus fundamental to a successful outcome.

 

Seeds will become imbibed by bringing them into contact with moisture, however this raises several practical issues which relate to the efficiency of its uptake and this in turn will usually be related to the degree of contact with the water carrying medium. If the seed is immersed in soil or a similarly artificially constituted germination medium, the uptake of water is primarily across the points of contact between the seed and the particles of the medium and secondarily from the humid atmosphere. This emphasises the significance of the texture, particle size and structure of the materials used to create the medium and the importance of maintaining the medium at field capacity.

 

Imbibition can be achieved artificially by placing the seeds in, or on, moist paper or cloth or by exposing the seed to a saturated atmosphere, or by soaking (immersing in water). This provides a more controlled process and an opportunity to regulate the temperature more efficiently. However when soaking the seed there are a number of issues to be addressed, the most obvious is that of the time of exposure – in the first (colloidal uptake) phase of the imbibition process the activity is completely physical and the fact that access for oxygen is impaired is not significant – but once metabolic activity begins the requirement for oxygen will rise dramatically – hence soaking should be limited to the few hours necessary for this phase to be completed. Extended soaking may also cause flooding of the intracellular spaces and so impair oxygen access – potentially inducing anaerobic respiration; it may also significantly leach out various important solutes from the cell sap. Usually the first phase will be completed within twelve hours – assuming that the seeds have a completely water permeable seed covering. Once the second phase is initiated seeds should not be immersed but a suitable medium, which provides adequate moisture, aeration and effective contact should be provided.

 

Temperature – its implications and influences in Germination

Germination, however, although principally about water, is also a function of other environmental factors – most notably temperature.

 

Once the initial phase of imbibition has been completed (and it would appear that the rate of this activity is not temperature sensitive to any significant degree), the rate of germination becomes temperature responsive. Once the process moves into the biological phases the rate is determined by metabolic activity and development patterns. In general increasing temperature therefore is reflected in an increased rate of the process of germination.

 

Germination proper, assuming that the seed has imbibed, will then begin once a threshold temperature is reached and then as temperature increases above this base level so will the rate of reaction. However although the base (threshold) temperature is not a fixed or constant feature to all species of a particular provenance it is more or less constant within a species. Indeed this threshold temperature for any one species will reflect part of the overall strategy for seedling development and survival. It will thus restrict germination to a particular window of opportunity within the overall ecological and environmental pattern of its normal habitat.

 

The rate of germination is then subsequently temperature driven once the threshold has been reached. Any increase in the rate of the process will then be a function of increasing temperature, within the normal biological limits - while somewhere in the intermediate range will be the optimal band which provides the greatest economic productivity. The response is typically that of a sigmoid curve.

 

The initial phase of imbibition and the rate at which it occurs, despite not being temperature sensitive, has nevertheless temperature related implications. The rehydration process at ‘low’ temperatures appears to cause injury to (or, at least, an impaired reconstitution of) certain cellular structures and membranes and this is later reflected in a reduced level of growth and performance of the seedling. At ‘warm’ temperatures the various membranes and structures are able to reconstitute and repair more efficiently and more completely and so exhibit a lower level of damage – this in turn permits a more constant and successful pattern of development and activity.

 

The determination of ‘warm’ in this context will be a variable parameter and will reflect the temperatures experienced in the natural environment of a particular subject and the various adaptations and responses of the species itself to ecologically developed strategies.

 

This concept has at least one important practical implication for the plant propagator who seeks to achieve maximum productivity. The temperature at which the seed is imbibed will be more effective and less damaging to future performance if conducted at a warm temperature, even if the seeds are subsequently to be subjected to a chilling treatment. For once the membranes and structures have been successfully reconstituted by hydration they will remain effective and undamaged even when subjected to chilling temperatures.

 

Atmosphere – the influence of gases in germination

Germination occurs most effectively if, in the consumption of the food reserves of the seed, energy production is achieved as a result of aerobic respiration. Most complete use of the food reserves and the liberation of the greatest amount of energy is obtained as a result of aerobic respiration. This infers the necessity for a third environmental parameter for most effective germination – oxygen.

 

The maximum effective, respiratory activity therefore requires the unimpeded availability of oxygen. Any limitation to the availability of oxygen could, at the extreme, limit the efficiency of the process by initiating the far less efficient system of anaerobic respiration. This system liberates much less energy and results in the production and liberation of toxic end products such as alcohol. Although untreated seed coverings may play some part in affecting the development of partial pressure of the oxygen atmosphere within the seed, the most likely impediment to effective gaseous interchange will be waterlogging.

 

Variability in germination performance, within an otherwise satisfactorily treated seed lot could therefore be attributed to the variations between individual seeds, in their ability to continue an effective gaseous exchange between the seed and the atmosphere. Some seeds may well be affected more than others, having developed a greater degree of waterlogging in the same sample - as a result of soaking. This is often a reflection of the practical difficulties which are encountered under field conditions and which are less likely to be a problem with a more sophisticated application of technique and greater artificiality (control) within an integrated and monitored system.

  

 

Light and its relation to Germination.

Light is a much researched response activity, in relation to the control mechanisms of the germination of seeds, as there is a recognised and obvious vehicle for the response activity – the presence of phytochrome molecules within the seed. A positive response to light is part of an ecological strategy in providing the seed with an indication and a measure of available light thus giving a measure of the potential for seedling development and survival. Species with positively photosensitive seeds are not common among woody plants of temperate climates but knowledge of the potential is significant in the germination of the seeds of those particular species (eg Betula and the Ericaceae).

 

Some species of plants demonstrate a negative response to light – being inhibited by the presence of light and may be a mechanism to ensure that the seed is sufficiently deep in the soil for a satisfactory germination and establishment.

 

In practical terms the presence or absence of light is not a significant environmental feature for the germination of the majority of temperate woody plants. However for those species which are forest dwellers, especially, it can virtually act, for some species, as a dormancy control - insofar as the germination is inhibited until a clearing in the canopy lets through enough light for germination to proceed and so allow seedling development and establishment.

 

Light also has a significant role - post germination - once the photosynthetic apparatus is deployed and carbon assimilation is possible.

 

The pattern of epicotyl deployment in germination

In the physical process of embryo expansion and growth, there are potentially two patterns of response in relation to that part of the epicotyl which emerges and undertakes the first positive role in carbon assimilation. This characteristic describes the position adopted by the cotyledons at this time. In those plants in which the hypocotyl extends to promote the cotyledons above the ground to form, in effect, the first leaves of the plant, the sequence is described as epigeal. In those plants in which the cotyledons remain below ground and there is no extension of the hypocotyl, the cotyledons gradually give up their food store to promote the first true leaves above the ground so that these are the first photosynthetic organs of the plant – this sequence is described as being hypogeal.

 

Ancilliary considerations associated with germination

Many plants depend, for their successful establishment and survival, on strategies associated with the development of symbiotic relationships. In temperate woody plants this is usually associated with the engagement of mycorrhizal associations or relationships with nitrogen fixing agents.

 

Mycorrhizal associations are of various types and complexity and exhibit various degrees of obligation. They are chiefly concerned with the efficiency of water and mineral absorption from the soil, by the roots, in which the fungal component receives elaborated foodstuffs from the plant in exchange.

 

Nitrogen fixing agents, which are usually either particular species of bacteria or blue-green algae, effectively act in the same symbiotic fashion.

 

As far as the plant propagator is concerned it is useful to have some knowledge of these relationships and to know how soon in the life cycle of the plant this relationship becomes an essential for realistic development rates.

 

When propagation is taking place in traditional open ground seedbeds this issue is rarely significant as substrate inoculation will occur from ‘spore rain’. However when germination and seedling establishment is taking place under more controlled and sophisticated conditions then it may be necessary to inoculate the germination substrate with suitable agencies to ensure proper development.

 

Other considerations

The sequence of processes described here lead up to seedling establishment – the stage at which the leaves have become deployed and the new plant becomes a self supporting

(autotrophic) individual and no longer needs to rely on the food reserves of the seed. Germination can be said to be complete when the seed has finally exhausted its food reserves.

 

Many seeds contain, not insignificant, quantities of nitrogen containing compounds in the form of amines, cyano-genic glycosides and alkaloids which in many cases represent an important available source of nitrogen in the metabolic synthesis of protein. However these toxins may also represent a chemical defence system; which means by taste, appearance or poison; deters likely predators and enhances the survival of some of the seed production. It is well documented however that specific predators can overcome these mechanisms by the development of a modified metabolism which is capable of detoxifying the deterrent chemical.

 

Practicalities

As the seeds of ornamental woody plants are normally sown on a tray containing a compost as opposed to sowing directly into a plugs or other individual units – because of the potentially variable or even unreliable levels of germination success – unless the practically non viable or non-responsive seed is able to be separated and extracted – then there is a need to be able to separate the seedlings, soon after emergence, from the compost readily and without damage.

 

A compost composed of a high proportion of relevantly sized sharp grit and a suitably structured organic matter component – which will easily fall apart and separate from the seedling root system will allow a swift and easy transfer to a liner unit. This phase benefits from carrying out this particular operation before there is any substantial development of lateral roots – the presence of which will slow the transfer operation and potentially cause damage to the roots.