The Capacity to initiate Roots in relation to Parental Age, Rejuvenation and Vegetativeness

 

After the Second World War the major thrust in advancing the efficiency of plant propagation in the Nursery Industry, as part of the plant production process, was to enhance the ability to initiate roots on stem cuttings. This was prompted by the need to reduce the reliance on various forms of grafting which had traditionally been practiced - because of the inability to root stem cuttings of so many plants (eg Rhododendrons) economically.

 

This advance was essentially associated with a) the use of ‘Auxins’ – which eventually came to be known as Plant Growth Regulating Substances, b) the practice of wounding the basal portion of the cutting and c) an understanding of the necessary functions of the rooting medium. A better understanding of all of these factors - and in their various combinations - did have a considerable impact in forwarding success and this is evidenced in the literature of the time.

 

However the practice of plant propagation and any advances in success, at that time, tended to be jealously guarded as ‘trade secrets’ by individual propagators (even the literature tended to avoid useful detail) and it was difficult for the novice to put together efficient regimes. However by the late 1960s attitudes changed dramatically (with the advent of International Plant Propagators’ Society, in the UK, in 1968) and plant propagation quickly became an open book.

 

At this time a large and extensive ‘experiment’ was convened, by the Society, in which several nurseries and institutions conducted a comprehensive investigation in an attempt to determine the importance and relative effects of PGRS, wounding and medium composition on the rooting of particular plants. The results of this were inconsistent and inconclusive, especially so between sites. An exact a repetition as possible was instituted the following season to determine if the results were at least consistent, on an annual basis, on any one site. On at least half the sites, the results were markedly at variance with the previous year – especially so for those contributors who used the same stock plants for their propagating material. They found that this iteration was generally highly successful in terms of the regenerative success, produced only marginal differences between the treatments and thus vastly improved the overall total response – leading to the conclusion that if the material was ‘good’ then the effect of treatments was marginalised.

 

This result then focussed attention on the condition of the propagating material and the stock plant itself as the major influence in successful propagation rather than the treatment of the propagation material after it had been isolated. In turn this led to the consideration of the age of the stock plant from which the propagating material was taken and its treatment – a factor which had already been widely noted in the rooting of stem cuttings of Leyland’s Cypress.

 

Currently, after a period of forty years, the received wisdom is that the major influence controlling the successful vegetative propagation of woody plants from stem cuttings is determined by the inherent level of regenerative capacity present in the material selected for propagation and by inference that this is a function of the physiological status (age) of the plant from which it is selected.

 

PARENTAL AGE

 

This regenerative capacity, ie the ability of the stem to initiate roots, does not occur as a constant function throughout the life of the plant but declines as the plant ages. This context has particular significance for the continuous clonal propagation of a woody plant insofar as the physiological condition of the material is not represented by the immediate age of the individual donor stock but is a function of the actual (and hence physiological) age of the original selection - however many vegetative generations it is removed from the current material (ie the physiological ‘age’ of the donor plant, providing propagation material, is the same as that of the original selection from which it is derived).

 

The relative capacity to regenerate between different families, genera and even species varies considerably but is normally constant – this implies that there is a spectrum of this capacity which, at the extremes,  means that stem cuttings of a taxon may be ‘difficult’ (eg Quercus and Tilia) or ‘easy’ (eg Fuchsia and Forsythia) to root. Thus the inherent capacity varies significantly and is not the same among all woody plants.

 

The Phases of Ageing

When a seed germinates it starts into a life cycle which will consist of four phases – initially the Embryonic – which becomes the Juvenile at germination. This is the stage at which, the vegetative growth produced, builds the individual to a size and condition at which it is volumetrically large enough, sufficiently physiologically developed and properly established to be capable of flowering and producing seeds. This is the primary method by which a species is regenerated. During this phase the ability to regenerate asexually by vegetative means is high. The next phase is described as the Mature phase and is when the sexual mode initiates and dominates and it is at this stage that the plant develops and increases its capacity for flowering and seed production until it reaches its full, mature size. At this stage the ability to regenerate vegetatively declines dramatically at the phase change and continues to decline steadily, through time, until growth ceases and the last phase is reached. Thus the plant moves into the Senile phase when it concentrates all of its reserves and energy into seed production and this is associated with a complete loss of asexual reproductive ability.

 

The term Ontogeny is used to describe the chronological age of this sequence in an individual and represents the periods and phases of development from germination to death – similarly Morphogenesis is also used.

 

It should be emphasised that the great majority of woody plants do not reproduce asexually unless there is a specific stimulus to cause them to do so – normally the ability to reproduce asexually by vegetative means is latent and although many plants even have stems which have developed preformed root initials, no further development occurs unless the stem becomes separated from the parent.

 

A woody plant, thus, progresses through three phases during its life cycle after germination – Juvenility, Maturity and Senility. The potential asexual regenerative capacity during this cycle declines in the expected (and normal) sigmoid curve fashion. The highest level of this regenerative capacity occurs during the juvenile phase (declining only slightly as time progresses); at the onset of sexual maturity, however, the capacity declines markedly and continues to decline steadily until the senile phase is reached when this potential is virtually lost. At this late stage in life the normal reaction of the plant before dying is to flower profusely and produce a large quantity of seed. This pattern leads to the conclusion that vegetative regeneration is only significant in the plant when it is in a non-flowering condition.

 

In practice the habit and the abilities of the plant in its phases can be catalogued as below:-

Juvenility is characterised by:-

a relatively fast growth rate, absence of the physiological ability to flower and produce seed, a high level of vegetative regenerative capacity and often exhibiting distinct morphological characteristics to those of the mature phase. This is exemplified by:-

Malus in which the juvenile stems are often spiny, and the stems and leaves are hairy;

Hedera in which the juvenile phase is climbing in habit and the mature is shrubby;

Passiflora has small cordate leaves as compared with the highly divided leaves of the adult phase;

Plagianthus has roundish leaves as compared with lanceolate and a wiry twiggy habit as opposed to the mature condition of a single trunked tree and many conifers – most noticeable that in the juvenile phase the foliage is distinct – in many Cupressaceae it is ‘feathery’ and very different - giving rise to the ‘genus’ Retinospora in Victorian times.

 

The duration of the juvenile phase will also have some relevance and according to genus or species this can be quite extensive – tree Magnolia species in excess of fifteen years, in Wisteria around a period of seven years but in Cornus capitata it may be as little as three years.

 

As the plant reaches sexual maturity there is inevitably a physiological change and the duration of this change can vary so that, in some tree species, it may extend over several years (as many as fifteen years) – this is known as the Transitional phase and while it adjusts the reproductive abilities reflect the condition – that although the plant may produce flowers (probably not seeds) it still retains a degree (although continually declining) of capacity to regenerate vegetatively.

 

Maturity is characterised by:-

the onset of flowering and the production of seed, however in the early stages of maturity the plant is still putting the preponderance of its food reserves into continued growth and an increase in size and hence leaf area – it is not until the plant has reached towards its full mature size and status that it maximises flower and seed production by diverting major food reserves to this activity.

 

As this maturing process increases steadily so does the capacity for vegetative regeneration decline.

 

Senility is characterised by:-

a virtual cessation of the ability to regenerate vegetatively and as the maximum size is reached it switches into a substantial flower and thence seed production mode giving up its food reserves in the process.

 

The significance then of this pattern has particular relevance in the vegetative propagation of clonal material (normally cultivars) as the physiological age of the plant is not regressed in any way as a result of a new generation being vegetatively propagated from stem cuttings – the physiological age is still that of (or what would have been) the original selected parent. This probably accounts for the demise of, or the extreme difficulty in rooting cuttings of, many cultivars from earlier times.

 

An example of this status is provided by the original varieties of Ghent Azaleas, which are now 200+ years old and (despite generations of macropropagation) they have, over the years, become progressively difficult to regenerate from stem cuttings. Subsequent deciduous Azalea groups such as Mollis (1880s), Knaphill (inter War years), and Ilam (1960/70s) are progressively easier to root. This factor is true even when the presence of viruses is allowed for.

 

Observations, conducted on Ribes sanguineum cultivars, have demonstrated a similar decline in regenerative capacity with age. In the 1960s ‘Pulborough Scarlet’ (c1930s) was regarded as ‘easy’ and ‘King Edward VII’ (c1890s) less so. Nowadays the former is regarded as relatively ‘difficult’ and the latter as still more difficult. Newer cultivars such as ‘White Icicle’ (1970s) however are seen as easy.

 

In all these instances the current donor (stock) plant, however ‘young’, is far removed from the originally selected plant but it will necessarily still be the same age physiologically. It is therefore relevant to determine whether it is possible to recover any of the advantages of improved vegetative multiplication that is associated with juvenility

 

REJUVENATION

 

The most productive rooting of stem cuttings in the mature phase therefore appears (logically?) to be associated with the non-flowering condition (ie the juvenile phase) and is borne out by the fact that regenerative capacity declines steadily, with ageing, through the mature phase as the capacity to flower increases. Hence it would appear that the ability to regenerate asexually declines with an increasing ability to regenerate sexually.

 

The definition of Juvenility as previously indicated, in this context, is that period from the onset of germination to the point of sexual maturity.

 

This characterisation however is complicated by the fact that the same morphological and thus presumably physiological characteristics will also arise when a stem is regenerated from an adventitious bud – as from a root or from an epicormic origin. This might then provide an opportunity to exploit this condition – which is of course theoretically possible but if quantities of plants can be produced by this route why bother to produce stock plants to produce stem cuttings?

 

Although it is evident that a stem with apparently juvenile characteristics is produced from an adventitious bud and that this stem will exhibit the increased capacity to initiate roots it is not clear what the duration of the effect will be.

 

Speculation once suggested that the rapid propagation of alternative generations of root cutting and stem cuttings (where it was possible) could push the juvenility factor back up the scale and ease propagation by a massive increase in regenerative capacity. However it may adversely affect the development of the desired characteristics of the mature phase and there was some doubt as to how long the effect might last.

 

When sexual maturity is reached there is a marked decline in regenerative capacity (ie at the shoulder of the sigmoid curve). The problem, however, is that it is not until the mature phase has been reached that the plant will be selected for its particular desirable characteristics and by which time it has lost the greater proportion of its regenerative capacity.

 

VEGETATIVENESS

 

The issue then, which affects the propagator, is to determine whether any of this regenerative capacity can be recovered in the mature phase by the manipulation of the parent plant.

 

Observation on the patterns of growth during the mature phase reveals that heavy pruning (by whatever means) causes the production of stems which do not flower and that as a corollary they also recover a considerable degree of regenerative capacity – for want of a better term this condition can be referred to as ‘Vegetativeness’.

 

Further investigations will demonstrate that the greater is the proportion of stem material that is removed, the faster will be the replacement growth as the plant seeks to replace the stem tissue removed and restore the root/shoot ratio. The faster the growth, achieved by this process, the higher is the level of regenerative capacity regained.

 

Effectively therefore it is the speed of growth which is the critical factor in determining regenerative capacity and morphologically this is represented by upright growth, anatomically by an attenuation of the sclerenchyma sheath surrounding the conducting tissue and physiologically by a non-flowering condition. Not surprisingly the rate of growth of this replacement tissue is not constant but again the pattern seems to respond to that of the sigmoid curve.

 

This effect can be demonstrated by establishing a vigorous, relatively easily rooting, plant such as Salix daphnoides and then stooling it in the late winter. The consequent growth during the following growing season will be considerable and virtually upright shoots around 4+m long will be developed. In the autumn, just after leaf fall, these shoots can be severed flush with the stool (incorporating the basal swelling) and then from the base making successive 15cm hardwood cuttings. These can be inserted in a suitable soil bed and at the end of the following growing season they can be assessed on the basis of root and shoot production. The results will show that the quality of the new plant is greatest when derived from the basal cutting – with, progressively, a declining quality to the tip of the cutting. The advantage of incorporating the basal swelling into the basal cutting can similarly be demonstrated by removing it in some of the basal cuttings to make a comparison.

 

The importance of the incorporation of the basal swelling, in the ultimate success, can also be readily demonstrated with early season, softwood cuttings of the large flowered cultivars of Lilac (Syringa).

 

Thus in the propagation of woody plants from stem cuttings the significant pointers to maximum success – especially with those subjects which are designated as being difficult to root – are that the basal swelling is incorporated into the cutting and that only this particular cutting is likely to be successful. This basal swelling which develops as a result of the initial surge of growth from the dormant bud represents the fastest rate of growth – ie the top of the sigmoid curve. This phenomenon is not a new concept – it was ably demonstrated at East Malling, in the late 1950s and 1960s, in the recommendations for using the Garner Bin technique of rooting hardwood cuttings of fruit tree rootstocks and was also applied at that time to a variety of ornamental subjects.

 

The effect of this particular manipulation is relatively transient and fortunately is effectively lost by the second season after propagation so that it does not impair the development of the mature plant characteristics for which the clone had been selected.

 

The practical significance, to the propagator, of this particular status will therefore be related a) to the age of the ‘variety’ (bear in mind that date of introduction is not an absolute and that the plant must have been germinated well before - as it is the adult features which recommend it) in determining its potential (and relative) regenerative capacity and b) the ability to recover regenerative capacity by manipulation of the parent plant.