An Analysis of the Process

The overall cycle of production, involved in the vegetative propagation of a crop of ornamental hardy woody plants by stem cuttings, consists of a chronologically determined sequence of phases. All of these phases are significant and are necessary considerations in determining influences on an ultimately successful outcome. However these influences are inevitably inter-related – as each factor comes into play - and should not, prudently, be considered in isolation.

 

The object of this analysis is to inform - so that a series of organised and sequential protocols can be developed, which will result in the production of a high quality end product and the maximisation of productivity from the stock available.

 

The Production Process can be conveniently catalogued and divided into five phases:-

1. The Preliminary phase

2. The Preparatory phase

3. The Regenerative phase

4. The Establishment phase

and

5. The Production phase

The first four phases combine to determine the propagation sequence which will result in a suitable, autotrophic, young plant (a liner) which is capable of, seamlessly, being moved into the production schedule.

 

Although these notes have been allied to the propagation of the crop by stem cuttings – because it is the most widely practiced technique in the industry – the notes can equally well be transposed to apply to any technique of vegetative propagation when suitably modified.

 

This process will allow the factors which affect ‘quality’ to be determined at each phase of production so that only material of the most satisfactory status moves on to the next phase.

 

The assessment of quality in the production of nursery stock (because of the wide diversity of plants produced) is inevitably a nebulous concept and this can only be defined with exactitude and precision on the perceptions determined by an individual operation. Otherwise it is simply a matter of waffling about being ‘well grown’, ‘true to name’ and ‘vigorous’ – whatever that means.

 

The process relies on the inherent ability of a particular part of the plant (in this case stems) to exhibit a capacity to regenerate the requisite missing components (ie roots) and so produce, when these two components have become anatomically and physiologically integrated, a complete new plant capable of a separate autotrophic existence.

 

Inevitably the initial status of the regenerative process will be defined by the inherent capacity for vegetative regeneration exhibited by any particular plant. This is not a constant feature – in some genera the ability is high and these will be deemed ‘easy’ to propagate (eg Fuchsia) while others will only respond (at the extreme) after considerable manipulation (eg Fagaceae) - while the remainder may effectively not respond at all.

 

Why any plant should have this ability is not generally clear and why one species and not another should vary from the other is a similar conundrum. In general, the capacity also varies with the status of the material to be propagated – young (immature) material will regenerate more successfully than the same material as it progresses through the season of growth and becomes more differentiated.

                     

In the commercial nursery industry the process depends on a complete and integrated sequence of events. All phases need to be implemented and are required to produce a crop of new plants (liners) of the same genetic complement as the identified parent and of a designated quality.

 

The main component of quality - especially in wholesale production - will be the consistency of the uniformity of the product. Each phase of the process provides a platform for the next and ideally all the material will move on from phase to phase. However under even the best nursery conditions there will inevitably be some variation from the standard. Thus the numbers started, originally, in the process should take account of an anticipated level of discarded material that is required to maintain a standard of quality and uniformity. This level of discard should decline at each phase - as material discarded late in the sequence constitutes a higher level of financial cost.

 

In the last fifty years production in the Ornamental Hardy Nursery Stock Industry has become increasingly stratified and it is quite feasible that each phase, in the future, could be carried out by individual and separate businesses. Indeed it is partially stratified already

 

The Propagation Process

 

The plant production process thus broadly consists of five phases - of which four constitute the propagation element – the end product of which is a physiologically integrated young plant capable of normal and active growth without further special treatment and of a size which is compatible with the continuing requirement of the production process (a liner); ie the fifth phase – the Production Phase.   

 

1. Preliminary Phase

 

This phase determines the precursors to production but is an essential element in the production process. It covers the introductory processes which govern the fundamental factors which will ultimately influence the outcomes.

 

a) Parental (Stock) selection,

This consists of the determination of:-

Taxonomic identity

Although this feature is apparently obvious it is important that the stock is ‘true to name’ and represents ‘what it says on the tin’. This is particularly significant in today’s global market place.

Clonal selection

The propagation by vegetative means of a cultivar implies the reproduction of a clone – ie individuals of identical genetic complement representing that particular cultivar description; however for those popular plants which have been propagated continuously or extensively, and/or over a long period, it is quite feasible that some genetic variation (mutation) will have occurred. This variation may then have, unwittingly, been multiplied up to provide effectively a different clone. This may be an improvement or otherwise but it is no longer the original. If this has occurred several times then a melange of variants will have been created. A view then needs to be taken on the situation and some form of selection will, at some time, be necessary.

 Health

It will be obvious that the multiplication of material infected with fungal or bacterial diseases would be counterproductive. Many plants are subject to virus infection and in most cases this is to the detriment of the vigour of the subject and such material should be avoided or discarded and virus free material selected or created. However in some cases the virus is part of the accepted characteristics of the cultivar – some variegated plants and some woody plants in which the particular shape of the plant (eg Sato-Zakura Cherries) is affected.

 

Such screening and assessment will enable and provide the selection, establishment and production of reliable parental stock providing known characteristics and identifiable nomenclature.

 

b) Parental Age

When propagating any particular cultivar there are two influences which relate to the age of the stock plant and which can dramatically affect the potential capacity of the propagules to regenerate.

 i) the actual age of the cultivar itself will affect the potential capacity to regenerate ie its real chronological age – the date at which it germinated as a seedling will thus  determine its current age (this is not the date of introduction). A plant progresses through three phases during its life – Juvenility, Maturity and Senility and through this duration, the vegetative material declines in its capacity to regenerate (by the accepted sigmoid curve phenomenon). Although it is well known that capacity is highest in the juvenile phase it is (usually) not until the plant reaches maturity that particular attributes deem it suitable for cloning. Through the mature phase capacity continues to decline until at senility it is virtually lost. Hence regardless of the age of any individual stock plant it is physiologically the same age notwithstanding how many generations it is separated from the original individual seedling. Thus cultivars (clones) do age and as they get older their ability to regenerate declines until it virtually disappears.

ii) the age of the individual stock (donor) plant – as determined by its own propagation date – is also a factor insofar as there is much reported evidence that the ‘younger’ and more vigorous (see later) is the plant then the higher is the regenerative capacity of its propagules. Hence there is a requirement for a programme of renewal of the stock plants in order to maintain an enhanced and acceptable level of capacity.

 

c) Plant Husbandry of the Parental Stock

This is simply a function of maintaining a suitable and satisfactory mode of husbandry to keep the stock plant growing properly and without constraint. It will involve attention to:-

   Nutrition

   Irrigation

   Light

   Competition - weed and pest and disease prevention and control

 

Where the regenerative capacity of a crop plant is naturally high and the subject is regarded as easy to propagate - then propagules are, more often than not, taken from a growing crop in the process of pruning and shaping. However the same assessments and practicalities still apply.

 

2. Preparatory Phase

 

When a plant is sufficiently difficult to propagate or it is not possible to remove cuttings satisfactorily from the growing crop or there is some particular treatment required to enhance the capacity to regenerate - then it is necessary to establish stock plants which can be prepared or manipulated to provide the required amount of the desired propagules with an acceptable level of capacity.

 

a) Plant manipulation

The phase involves the manipulation (ie the preparation and the treatment) of the parent plant to develop the most productive system for the production of potential stem cuttings both in terms of numbers and regenerative capacity. Primarily this is an understanding of the development of material with the greatest potential capacity to regenerate. This will relate to:-

i) The age of the parent material - and its relation to the potential regenerative capacity - has been alluded to above and this phase concerns the treatment of the plant in order to enhance the inherent regenerative capacity where this is necessary.

 

The regenerative capacity of a particular cultivar has so far been shown to be a function of the inherent status of the species and its physiological age (ie how far it has progressed down the decline in capacity during the mature phase). Evidence also shows that the capacity can be enhanced if the donor plant can be manipulated and so induced to develop a high degree of ‘vegetativeness’ – initially this was associated, physiologically, with the non-flowering condition or juvenile phase – but it is effectively a response to the speed of growth of the stem, upright growth patterns and thus also the potential attenuation of the pericyclic sheath.

 

Practically this is achieved by relatively severe annual pruning so that considerable regrowth is engendered and this will necessarily be fast in order to correct the balance of the root/shoot ratio. Over the course of the growing season this will be speediest at bud break and will progressively slow throughout the season until it reaches balance when approaching dormancy. The highest regeneration will be obtained, therefore, in cuttings which incorporate the swollen base of the annual growth - ie when the bud burst and the stem grew fastest.

 

However it should not be ignored that the increased capacity associated with the juvenile phase is not just associated with seed germinated material but can also be derived from material developing from adventitious bud production. Away from the mainstream of nursery production there has been some attention to increasing capacity using these techniques but as an industrial application they are unlikely to be very high on the agenda.

 

There are examples in the nursery industry of particularly sophisticated treatments of which one example (which was popular for a while) was Etiolation. This involved excluding light from the base of the new season’s shoot as soon as it was feasible; it involved wrapping the base of the current season’s shoot in an opaque material in order to encourage the initiation of roots but its application is time consuming and like other techniques it is not widely practiced.

 

ii) The maximisation of the number of propagules produced is also a significant economic factor and the pruning (and perhaps the pinching) of the plant – depending on its growth habit – will govern this factor; eg if Rhododenrons are cut back the regrowth tends to produce a single thick shoot but if this is pinched it will develop an ‘umbrella’ of thin shoots – which also, coincidentally, generally show an improved capacity to root (probably this is  a function of the attenuation of the pericyclic sheath).

 

3. Regeneration Phase

 

 This phase considers the factors influencing the performance of the material – ie the actual physical expression of the potential capacity (root production) once the cutting has been removed from the parent.

 

a) Seasonal influence

This relates to the variation in the expression of any regenerative capacity as a function of the influences of the seasonal condition of the plant; hence the timing of operations when capacity is at its most potent. An example being that hardwood cuttings respond most satisfactorily if taken at leaf fall (leaf abscission).

 

b) Treatment

The degree to which this capacity is expressed may be improved by the physical or chemical treatment of the propagules in order to enhance the inherent regenerative capacity, especially if it is sub-optimal.

  Chemical treatment

Just before the Second World War it was discovered that plants (like animals) produced ‘hormones’ and that one particular group of these ‘Auxins’ could encourage the initiation of roots on isolated stems. The most effective were NAA, IAA, IBA and IPA - at relevant concentrations – and for a period in the 1950s to the mid-1970s were regarded as a panacea, however once the effect of vegetativeness became more widely accepted it became apparent that their value was limited to sub-optimal material or to enhance the capacity of subjects with low inherent response. In recent time these materials have been referred to as ‘Plant Growth Regulating Substances’. The placing of such agents when treating is significant as it is evident that the concentrations of Plant Growth Regulating Substances which will initiate regeneration are higher than those necessary for root growth.

  Physical treatments

One identified constraint to the initiation and/or emergence of roots, in certain species, is the structural presence of the pericyclic sheath which may prevent the external application of Plant Growth Regulating Substances getting to the required sites for root initiation or conversely it may impair their development and emergence. This can be overcome by applying a longitudinal wound to the base of the cutting either as a slice or a score – so damaging the sheath.

An obvious sine qua non in the regenerative process is the maintenance of polarity – Plant Growth Regulating Substances generally move in the stem in response to gravity and it is therefore important to keep the material ‘the right way up’ – this may seem too obvious to mention and with leafy cuttings is not usually a problem – however it is not difficult to envisage hardwood cuttings being inserted upside down!

 

c) Environment

The provision and/or adaptation of environmental conditions to:-

a) ensure the survival of the propagule – chiefly the maintenance of water status and a suitable temperature and

b) encourage and optimise the expression of the inherent capacity to regenerate the relevant ‘missing parts’ – usually a function of temperature, nutrition and water availability.

 

This will be a function of the assessment and optimisation of the following elements of the environment as they affect the survival of the cutting in ‘good’ condition (ie the cutting is not under stress from any of these parameters) and the enhancement of the regeneration process:-

   temperature,

   water status/stress,

   light,

   atmosphere,

   hygiene,

   protection

   and

   the structure and condition of the rooting medium.

 

The provision of particular environmental conditions will necessarily be adapted to the type and condition of cutting material that is to be propagated.

 

4. Establishment Phase

 

Once regeneration has occurred and become established, the aim is to achieve the continuing development of the material to produce a physiologically integrated young plant capable of an autotrophic existence. The end product is a suitably prepared and established ‘liner’ plant. This represents the transformation of the regenerated propagule to a physiologically integrated young plant.

 

Although most plants will regenerate roots and develop to a physiologically integrated condition in the available normal growing season, there are some plants which do not and if they progress into dormancy before integration is achieved there is a very real possibility that they will be unable to respond to stimulation when suitable growing conditions are resumed. Hence the provision of those conditions and/or the timing of the operations required to ensure the development of physiological integrity and the subsequent development of normal growth patterns will be significant. This may involve extending the active growing season either by advancing the season of availability of propagules or by manipulating the environment (usually temperature, daylength and nutrition) to extend the season.

 

The integrated condition can be determined by the development of stem extension growth – albeit minimal. At this stage the material can be allowed to become dormant with every expectation that the plant has become physiologically integrated and that it will respond to environmental conditions, exit from dormancy and develop normally the following spring.

 

5. (Production Phase)

Further development of the liner plant to larger sizes.