Uncle Ben
Well-Known Member
Just thought some of you may be interested in something we kicked around many years ago. We used to have some hot and heavy botanical discussions in the early and first cannabis growing forums such as cannabis.com, Overgrow and Cannabis World.
lilroach, you asked for it. The following topic "Do clones get more potent with age" was kicked around by a well known old Canadian grower/breeder and myself, Vic High, at a now defunct forum - Overgrow.com. Vic's position was that clones increase in potency with age. I refuted it. It's long, but quite interesting, about 15 years posted. The "GENERAL" section discusses all types or profiles of plant aging. Suggest you read this while straight.
My response to the abstract regarding chronological, physiological aging and such was the following:
Vic, after digesting the symposium several times with the aid of Mr. Webster, I'm still not convinced of the positive relationship between 1st, 2nd, 3rd generation, etc. clones and the claim that latent clone generations increase regarding their degree of potency, in fact, this paper may suggest the opposite. As an aside, the one of the benefits senescence and latent aging has on a plant is the tendency of a plant to give up metabolites to its fruit and flowers. The positive effect of aging is only within the realm of physiological and chronological changes, which occurs relative to the amount of phytochrome increases that eventually trigger a flowering response. This being dependent on the genotype of the cannabis plant material and environmental conditions i.e. afghani/long nights.
This was a very interesting white paper, but in some parts confusing.... such are postulations posed with different plant material (and one mesmerized reader <g>). This paper also did not represent current viewpoints...as if it matters, and no, I did not seek out recent data as I don't have the time right now. Jury's still out IMO.
What we have here is a paper that defines the meaning of ontogenetical, physiological, and chronological aging as defined by 'early' biological communities - how they may group and inter-relate, and how different types of plant material succombs to one or all processes eventually, unless rejuvenation takes place and that's the kicker - rejuvenation. What this paper cannot address (of course) is the fact that no matter what effect these aging processes induce, cannabis potency is generally dependent on trichome density as well as the amount of cannabanoids and their makeup as found within those glands. But, you can have a heavy trichome field in which the stalked resin glands are filled with cannabanoid crap. Looks good, smokes bad. That is the key. If latent clone generations cause an increase in the amount of cannabanoids and the quality thereof.... via a heavier density trichome field and improved resin gland composites, then we may have something here.
After wading thru a sea of definitions regarding what constitutes aging and senescence, the real meat is presented regarding our question - affects of rejuvenation (or aging) via pruning and the loss or enhancement of favorable characteristics.... and I quote ->
"Pruning also induces younger buds or tissue to form normal or adventitious shoots, these
being more juvenile than those removed. This can he seen as a kind of semi-ontogenetical
rejuvenation. Theoretically such a rejuvenation cannot be continued indefinitely, ...... This leads us to the old question of whether clones age or not (Rijhouwer, 1930), including the problem whether all plant cells remain totipotent..... Based on extensive experiments with clones of Lemna minor L. (Vangerminn, 1965) the question regarding the ageing of clones can he answered *negatively*, provided the proper time and plant parts are chosen for propagation. Yet it cannot be denied that a continuous vegetative propagation sometimes leads to a loss of favourable characteristics (Hartmann and Kester, 196
."
Take note - The "ageing of clones can he answered negatively, provided the proper time and plant parts are chosen for propagation".
Another paragraph addresses the dynamics of clones as it relates to juvenile versus aging stages which again supports the theory that indeed cuttings (clones) may in fact be working AGAINST cannabis growers who hold dear to the hypothesis that clones are older and therefore "better", and I quote ->
"A true ontogenetical rejuvenation would mean that adult meristems could reproduce plants
or parts being again completely juvenile."
How that is warranted or presented is not discussed, but I would presume that that would mean taking a cutting (which of course contains meristem tissue) and growing it out either in vitro or sticking the entire cutting in a rockwool cube or a pot and rooting it. In the case of Cattleyas (used as an example in my earlier post), that tissue is located at the foliar terminal tip.
" Some examples are represented by the production of new seedlings after amphimixis or even apomixis, and the spontaneous or artificial regeneration of juvenile plants, either in vitro or in vivo, from all kinds of adult plant parts, such as stems, leaves and flowers (Plerik, 1975). This ontogenetical rejuvenation is much more difficult to explain than the former physiological rejuvenation. It is supposed that an
isolation of a single or at least a limited number of totipotent cells from the surrounding tissue is a prerequisite for a totel rejuvenation (Steward, 1967). Comparable results were obtained by grafting juvenile seedlings onto the adult parts of the plant (ivy, sugarbeet, rubber), or by spraying them regularly with gibberellin (Stoutemyer et al., 1961). It should be noted that opposite effects were mentioned earlier, while discussing the probabilities of an accelerated ontogeny." <-
Alot of factors and considerations here. If I'm interpreting this right, the taking of meristem tissue (cuttings) actually causes a rejuvenation back to a juvenile state of the meristem portion. That doesn't settle well with me per my earlier reflection of taking 8 year old budwood from a Pecan tree, grafting it to young rootstock, planting it, and watching as the new budwood grows and produces nuts very quickly compared to a seedling. The old budwood doesn't know (chronologically) that it is now a one year old sapling, "thinks" it is an 8 year old tree, and bears as soon as it's well established. Cuttings/budwood taken from the top part of a plant are considered botanically more juvenile than those taken at a lower point on the plant...per this paper, as an aside.
All in all, this was some great reading Vic, but ya done confused this cowboy once more - ma brain hurts! Time to smoke some Lone Star goodies!
Uncle Ben
Abstract:
An evaluation of literature leads to the conclusion that ageing and senescence can be
related to ontogenctical and physiological causes. Ontogenctical ageing is genetically
programmed, localized in the meristems, not related to exhaustion, and cannot easily be
reversed. This implies important consequences for vegetative multiplication and for the
characteristics of clones obtained. Physiological ageing is correlatively influenced, caused
by an increased disorganization and exhaustion, and is not localized in the meristems.
When not advanced, a reversal is possible. Senescence regards more often the
physiological ageing, but may he of ontogenctical nature.
Introduction:
Terms related to ageing are used in confusion in every language because they are not
clearly defined. This regards, for example, 'ageing', and 'senescing', having a similar or
different meaning. It indicates the need for a better distinction and description of the
ageing processes, of its different phases, the process of 'rise and shine', covering the
juvenile and mature or adult phase, which received more attention than the 'fall',
representing the phase of full senescence. The latter negative aspect of ageing drew
interest more recently and became a new branch of science: phytogerontology.
This paper describes some of the present views with regard to ageing in the sense of
becoming more full grown, as opposed to ageing In the sense of getting more deteriorated.
It distinguishes an ontogenctical and a physiological type of ageing. This may have
implications and consequences for a discussion on 'Juvenility in Plants', the subject of this
symposium.
General
Ageing in plants has three aspects, a chronological, an ontogenetical and a physiological
one. In this order they are or, or less indicated by such opposing terms as: young and old,
juvenile and adult, improving and deteriorating. There may be no contradiction if a plant is
considered 'young and adult' or 'juvenile and senescent'. Strictly taken all three aspects of
ageing start from the formation of the zygote or, more in general, from germination or
probably regeneration and terminate with the natural death.
Chronological ageing only refers to the time which has elapsed e.g. since the plant
germinated, but does not give any information about the ontogenctical phase or
physiological condition reached.
Ontogenetical ageing more specifically refers to the process of passing through different
phases of development, from germination to complete senescence. Ontogeny is that part of
biology which studies the development of a zygote into an adult individual. It Includes
'embryology', the first part of development, and 'morphology', the subsequent change of
shape or metamorphosis. Ontogeny normally represents the positive aspect of ageing,
which can he depicted by the rising of the sigmoid growth curve. As a rule ontogenetical
ageing is Phenotypically visible and often correlated with, and indicated by, characteristics
such as the plant size reached, the number of leaves produced, the phyllotaxis shown, or by
the absence or presence of flowers. One distinguishes, for example, juvenile from mature
or ,vegetative from generative plants. however, these pairs of notions need not be
identical. An adult plant may remain vegetative if it is not induced to flower by required
specific conditions. This situation does not mean that ontogenesis came to an end, because
the terminal meristem then continues to initiate leaves until it receives the proper stimulus
to produce flowers, thus affecting the phenotype of the plant.
Physiological ageing applies primarily to the changes within the plant coherent with the
'coming of age' phase. It represents the negative aspects of ageing such as loss of growth
vigour, the increased susceptibility to adverse conditions, or deterioration in general. This
process of decline can he depicted by the downward course or last part of the sigmoid
growth curve. It is usually indicated by the term 'senescence', and normally precedes the
actual process of dying off. Senescence may cover the whole plant or only parts of it, and
may therefore begin early during ontogenesis. This indicates that ontogenetical and
physiological ageing, while different, should not he regarded as completely independent of
one another. Both may affect, for example, juvenility and rejuvenation, and determine the
life span of the plant or even the durability of its parts when harvested.
Some characteristics of the ontogenctical ageing
Ontogenctical ageing seems to be located in the terminal meristem (Pierik, 1967) while its
degree would be dependent on the activity of the meristem, as indicated by the number of
cell divisions. The stage of ontogenetical ageing reached is carried over onto its lateral
meristems when these are formed (Robinson and Warcing, 1969). This theory is supported
by many older conceptions such as topophysis' and 'cyclophysis' (Seeliger, 1924). Both
ideas mean about the same, and refer to the phenomena that a cutting or graft will develop
depending on, respectively, its former location on the mother plant, or the stage of
development at the time of its separation.
The theory that ontogenctical ageing is located in the meristems also explains the presence
of an 'ageing gradient' in each plant. A seedling does not age ontogenctically as a whole but
it does from the base to the top and from the inner to the outer parts. In plants there is no
continuous replacement of cells, such as occurs in animals. This explains the paradox that
the first-initiated, lowest, and chronologically-eldest part of a seedling is most juvenile,
while its more recently formed periphery is ontogenctically most mature. Full- grown and
old seedlings long retain many characteristics of juvenility at their base or reproduce them
when forming adventitious shoots at this site.
Ontogenetical ageing Is accompanied by anatomical, cytological and even physiological
changes. These were also observed in form, structure and cell properties of the terminal
meristems (Ali and Westwood, 1966), and could be used for determining the termination of
the juvenile phase. Not much is known about the hormonal regulation of the phase change.
It is supposed that the ratio between growth-promoting and growth-retarding substances
and/or the amount of a specific hormone are inductive for flowering only after a specific
ontogenctical stage, known as maturity, has been reached.
Consequences of ontogenetical ageing
The local differences in a seedling, regarding ontogenctical ageing, may have decisive
consequences for the selection of plant parts for vegetative propagation. It may result in
big differences in the offspring of a single plant. This regards, for example, the rooting
capacity, growth vigour, willingness to flower and even such phenotypic traits of the new
plant as leaf shape, branching, stem colour and the property of keeping or shedding the
leaves after senescence. It is generally known that adult cuttings from the top are more
difficult to root, grow less vigorously and flower sooner than juvenile cuttings from the base
of a seedling. Budding or grafting presents a solution when the capability to root is
completely absent. This practice, and the exclusive use of adult plant parts, may then lead
to the loss of the existence of juvenile tissue. This could explain why some plants, known
from older publications as easily to be propagated, cannot be brought on their own roots
nowadays (Hartmann and Kester, 196. It can be questioned however, why one takes into
account the proper choice of plant material for vegetative propagation in some crops but
not in others. The system of in vitro culture may present answers to some of these
questions. Influencing ontogenetical ageing may be of commercial interest in many cases,
such as shortening or extending the juvenile period.
Regulation of ontogenctical ageing
Ontagenetical aging can be accelerated by improving growth conditions, shortening
endogenous dormancy and avoiding imposed rest. As a rule this leads to accelerated and
prolonged activity of the terminal moristem, which increases apical dominance, reducing
the branching and its requirement for assimilates, which in itself again leads to an
additional stimulation of growth of the terminal bud. In this way the juvenile period and
time to flowering can be shortened to a considerable extent, particularly in perennials
(Doorenbos, 1965; Zimmerman, 1972). The approach, however, normally does not change
the developmental stage, as indicated by the size, stem length or leaf number of the plant
at which it starts flowering.
Flowering at an earlier stage of development was induced in some plants almost
exclusively by chemicals, gibberellins in particular, or by grafting onto adult plants. An
extreme case is the induction of flowering with gibberellins in 8 to 12 months old Sequoia
seedlings, of which the juvenile period normally lasts 20 to 70 years (Pharis and Morf,
1969). With gibberellins, as well as with grafting, contradictory results have been published
(Zimmerman, 1972). To further complicate matters, most publications do not indicate the
stage of development at which the earlier flowering was obtained and what happens when
the chemical treatment is stopped. This makes it difficult to judge whether only the juvenile
period, or also the juvenile phase, was shortened. In the latter case, a continuation of
flowering without a special treatment, and excluding the possibility of an auto-catalytic
formation of a hormone for flowering (Wellonsick, 1966), could indicate that ageing was
really shortened ontogenctically. This does not seem very likely.
Because the ability to regenerate new organs diminishes with ontogenetical ageing, its
retardation may be desired, e.g. for the production of cuttings able to root. This can he
achieved by keeping the growth conditions marginal and by regular pruning of the roots
when no cuttings are needed. The possibilities of rejuvenation will be discussed later.
Some characteristics of physiological ageing
Physiological ageing can be defined as 'growing old', indicating the loss of vitality ending
with death. Senescence is the word commonly used to indicate this process. Initially it may
affect only parts of the plant, but inevitably, the whole plant will succumb. Senescence
accompanies ontogenetical aging and terminates it at last. It affects the whole plant
contrary to the more specifically located ontogenetical aging. Juvenility will not prevent a
leaf from aging physiologically. Senescence of a plant may proceed gradually or may be
more sudden, affecting all parts simultaneously. At the same time it may be acropetally or
basipetally directed. This brings us to the question whether two types of physiological
aging must be distinguished, i.e. one independent and another dependent on the stage of
ontogenetical aging reached by the plant.
Gradual senescence normally starts at the base of the plant and is acropetally directed. It
is commonly accompanied by a mobilization of metabolites from the ageing organs by the
active parts of the plant, and is obviously a correlative phenomenon. This is supported by
the observation that cotyledons or first leaves will age much later if higher located parts of
the plant are removed. Moreover, if these parts are rooted, they can reach a much larger
size and greater age than would have been possible if they were left on the plant. This
so-called compensatory growth is due to increased cell enlargement and DNA content, but
senescence cannot he escaped, whether or not accompanied by the occurrence of
endomitosis or endoploidy (Butterfass, 1970).
Gradual senescence can also be observed in very old trees or monocarpic plants kept
vegetative. This deterioration or dying back, however, has a clear basipetal direction and is
not accompanied by a remobilization of reserves. It is a consequence of starvation or
exhaustion of the most distant parts and of a disorganization in general. The internal
transport path gradually becomes too long, the balance between dissimilation and
assimilation and between root and shoot activity is less favourable, while a decreased
supply of water, nutrients and minerals limits a continual growth. The only remedy is an
extensive pruning back which at the same time induces growth of ontogenetically younger
meristems.
Whatever the direction of physiological ageing may be, it is not controlled by the terminal
meristems as is the case in the ontogenetical aging. This is demonstrated by the fact that
an aging top may resume its normal growth after a separation in time and rooting, or in
other cases, grafting on a vital plant. Top shoots of old vegetative sunflowers produced
more internodes and resumed rapid growth when grafted onto a younger plant (Warcing
and Phillips, 1970).. Here senescence completely lacks all characteristics of a
preprogrammed process.
One of the oldest living organisms known is a Pinus aristata Engelm. in California. It has a
measured age of 4600 years which illustrates that meristems may have a long lifespan.
However, it was discovered that mitosis in very old meristems of a number of plants was
not normal. This results in a deviating number of chromosomes and in decreased cell
division, an indication that physiological ageing may finally affect the meristems
themselves.
Simultaneous senescence. More dramatic than general senescence is the annual
occurrence of a simultaneous and total senescence in monocarpic plants, of the
over-ground parts in perennial plants, and of a total leaf shedding in deciduous plants.
Normally, this sudden senescence is also accompanied by a redistribution of metabolites
from the dying plant parts to the surviving organs, such as seeds, bulbs, corms, rhizomes
and buds. For this senescence Molisch (193 already introduced the conception of
'Erschopfungstod.' The idea that exhaustion could be the cause of senescence and death
was supported by the observation that a timely removal of the reproductive organs could
postpone senescence or prevent death for long. The best known example is Aqave
mnericana L., which in its normal habitat flowers in about 8 years and dies subsequently.
Death of this monocotyledonous plant is normally induced by the formation of an
inflorescence, and therefore is almost ontogenetical by its cause. In climates where no
flower induction occurs the plant may become older than a century, but it will not
indefinitely escape physiological ageing. A similar type of physiological ageing terminating
ontogenesis occurs in maturing crops such as peas. A tip of an aged pea plant does not
recover after grafting onto a young plant, while a tip of a young plant grows normally when
grafted onto an aged plant (1ockhart and Gottschall, 1961). In monocarpic plants like
Agave and pea the sudden and simultaneous senescence seems to have been induced by
ontogeny itself. Many investigations demonstrated that exhaustion of metabolites is rather
a result than a cause of senescence, beginning normally after most of the fruits have
developed. In dioecious plants the males may start senescence long before the females are
fruiting.
The periodic leaf shedding of deciduous plants occurs in the juvenile as well as in the adult
phase, and is therefore clearly independent of the ontogenetical stage reached. Moreover,
it cannot be related to an exhaustion by other plant parts. The sudden and simultaneous
occurrence of senescence, however, indicates also in this case a hormonal induction and
regulation of physiological ageing.
The examples presented support the supposed differences in the triggering of physiological
ageing. The final total plant senescence cannot be regarded as a correlative phenomenon
but seems to be governed by the meristems similar to that of ontogenesis itself.
The possibilities of rejuvenation
Rejuvenation is the opposite of ageing and as a consequence of the foregoing discussion,
we should distinguish now a physiological and an ontogenetical type of rejuvenation.
Each plant, even a Pinus aristata, cannot escape a final total senescence and death.
Prevention of flowering or fruiting does not produce any rejuvenation. In polvcarpic plants
it will favour vegetative growth and by that only advance the time of total senescence
contrary to the delay of it in monocarpic plants. A severe pruning of the branches or stems
is much more effective. It shortens the internal transport system and improves the supply
of the periphery with water and nutrients. This can he regarded as a physiological
rejuvenation.
Pruning also induces younger buds or tissue to form normal or adventitious shoots, these
being more juvenile than those removed. This can he seen as a kind of semi-ontogenctical
rejuvenation. Theoretically such a rejuvenation cannot be continued indefinitely, because
each pruning activates the meristems present, stimulating their ontogenetical ageing.
Better possibilities for the preservation of a plant or tree are nresented by its timely
vegetative propagation, as is the case naturally in many perennials. This leads us to the old
question of whether clones age or not (Rijhouwer, 1930), including the problem whether all
plant cells remain totipotent. In one-cell cultures it appears that the number of non-dividing
and dying cells increases with time, notwithstanding a regular dilution of the culture and the
presence of sufficient food. Based on extensive experiments with clones of Lemna minor
L. (1Vangerminn, 1965) the question regarding the ageing of clones can he answered
negatively, provided the proper time and plant parts are chosen for propagation. Yet it
cannot he denied that a continuous vegetative propagation sometimes leads to a loss of
favourable characteristics (Hartmann and Kester, 196.
A true ontogenetical rejuvenation would mean that adult meristems could reproduce plants
or parts being again completely juvenile. Some examples are nresented by the production
of new seedlings after amphimixis or even apomixis, and the spontaneous or artificial
regeneration of juvenile plants, either in vitro or in vivo, from all kinds of adult plant parts,
such as stems, leaves and flowers (Plerik, 1975). This ontogenetical rejuvenation is much
more difficult to explain than the former physiological rejuvenation. It is supposed that an
isolation of a single or at least a limited number of totipotent cells from the surrounding
tissue is a prerequisite for a totel rejuvenation (Steward, 1967). Comparable results were
obtained by grafting juvenile seedlings onto the adult parts of the plant (ivy, sugarbeet,
rubber), or by spraying them regularly with gibberellin (Stoutemyer et al., 1961). It should
be noted that opposite effects were mentioned earlier, while discussing the probabilities of
an accelerated ontopeny.
Summary and conclusion
Juvenility is an important phase in the development of plants. it is depending on, and
affected by, a process of ontogenetical as well as physiological ageing and rejuvenation..
Ontogenetical ageing can be considered as genetically programmed, localized in the
meristems, accelerated by improved growth conditions and difficult to reverse. It is
accompanied or terminated by physiological ageing or senescence. This senescence is
correlatively influenced, caused by an increased internal disorganization, and normally not
localized in the meristems. If not advanced, its reversal is possible. Only the final and total
plant senescence can be considered to be of an ontogenctical nature, located in the
meristems, and as such irreversible. These conceptions may imply important consequences
for vegetative multiplication, both in vitro and in vivo, and for the characteristics of the
clones obtained. It probably supports the necessity of further research on the effects of
ontogenetical and physiological ageing of plants in the culturing of their meristems and
tissues.
lilroach, you asked for it. The following topic "Do clones get more potent with age" was kicked around by a well known old Canadian grower/breeder and myself, Vic High, at a now defunct forum - Overgrow.com. Vic's position was that clones increase in potency with age. I refuted it. It's long, but quite interesting, about 15 years posted. The "GENERAL" section discusses all types or profiles of plant aging. Suggest you read this while straight.
My response to the abstract regarding chronological, physiological aging and such was the following:
Vic, after digesting the symposium several times with the aid of Mr. Webster, I'm still not convinced of the positive relationship between 1st, 2nd, 3rd generation, etc. clones and the claim that latent clone generations increase regarding their degree of potency, in fact, this paper may suggest the opposite. As an aside, the one of the benefits senescence and latent aging has on a plant is the tendency of a plant to give up metabolites to its fruit and flowers. The positive effect of aging is only within the realm of physiological and chronological changes, which occurs relative to the amount of phytochrome increases that eventually trigger a flowering response. This being dependent on the genotype of the cannabis plant material and environmental conditions i.e. afghani/long nights.
This was a very interesting white paper, but in some parts confusing.... such are postulations posed with different plant material (and one mesmerized reader <g>). This paper also did not represent current viewpoints...as if it matters, and no, I did not seek out recent data as I don't have the time right now. Jury's still out IMO.
What we have here is a paper that defines the meaning of ontogenetical, physiological, and chronological aging as defined by 'early' biological communities - how they may group and inter-relate, and how different types of plant material succombs to one or all processes eventually, unless rejuvenation takes place and that's the kicker - rejuvenation. What this paper cannot address (of course) is the fact that no matter what effect these aging processes induce, cannabis potency is generally dependent on trichome density as well as the amount of cannabanoids and their makeup as found within those glands. But, you can have a heavy trichome field in which the stalked resin glands are filled with cannabanoid crap. Looks good, smokes bad. That is the key. If latent clone generations cause an increase in the amount of cannabanoids and the quality thereof.... via a heavier density trichome field and improved resin gland composites, then we may have something here.
After wading thru a sea of definitions regarding what constitutes aging and senescence, the real meat is presented regarding our question - affects of rejuvenation (or aging) via pruning and the loss or enhancement of favorable characteristics.... and I quote ->
"Pruning also induces younger buds or tissue to form normal or adventitious shoots, these
being more juvenile than those removed. This can he seen as a kind of semi-ontogenetical
rejuvenation. Theoretically such a rejuvenation cannot be continued indefinitely, ...... This leads us to the old question of whether clones age or not (Rijhouwer, 1930), including the problem whether all plant cells remain totipotent..... Based on extensive experiments with clones of Lemna minor L. (Vangerminn, 1965) the question regarding the ageing of clones can he answered *negatively*, provided the proper time and plant parts are chosen for propagation. Yet it cannot be denied that a continuous vegetative propagation sometimes leads to a loss of favourable characteristics (Hartmann and Kester, 196
."Take note - The "ageing of clones can he answered negatively, provided the proper time and plant parts are chosen for propagation".
Another paragraph addresses the dynamics of clones as it relates to juvenile versus aging stages which again supports the theory that indeed cuttings (clones) may in fact be working AGAINST cannabis growers who hold dear to the hypothesis that clones are older and therefore "better", and I quote ->
"A true ontogenetical rejuvenation would mean that adult meristems could reproduce plants
or parts being again completely juvenile."
How that is warranted or presented is not discussed, but I would presume that that would mean taking a cutting (which of course contains meristem tissue) and growing it out either in vitro or sticking the entire cutting in a rockwool cube or a pot and rooting it. In the case of Cattleyas (used as an example in my earlier post), that tissue is located at the foliar terminal tip.
" Some examples are represented by the production of new seedlings after amphimixis or even apomixis, and the spontaneous or artificial regeneration of juvenile plants, either in vitro or in vivo, from all kinds of adult plant parts, such as stems, leaves and flowers (Plerik, 1975). This ontogenetical rejuvenation is much more difficult to explain than the former physiological rejuvenation. It is supposed that an
isolation of a single or at least a limited number of totipotent cells from the surrounding tissue is a prerequisite for a totel rejuvenation (Steward, 1967). Comparable results were obtained by grafting juvenile seedlings onto the adult parts of the plant (ivy, sugarbeet, rubber), or by spraying them regularly with gibberellin (Stoutemyer et al., 1961). It should be noted that opposite effects were mentioned earlier, while discussing the probabilities of an accelerated ontogeny." <-
Alot of factors and considerations here. If I'm interpreting this right, the taking of meristem tissue (cuttings) actually causes a rejuvenation back to a juvenile state of the meristem portion. That doesn't settle well with me per my earlier reflection of taking 8 year old budwood from a Pecan tree, grafting it to young rootstock, planting it, and watching as the new budwood grows and produces nuts very quickly compared to a seedling. The old budwood doesn't know (chronologically) that it is now a one year old sapling, "thinks" it is an 8 year old tree, and bears as soon as it's well established. Cuttings/budwood taken from the top part of a plant are considered botanically more juvenile than those taken at a lower point on the plant...per this paper, as an aside.
All in all, this was some great reading Vic, but ya done confused this cowboy once more - ma brain hurts! Time to smoke some Lone Star goodies!
Uncle Ben
Abstract:
An evaluation of literature leads to the conclusion that ageing and senescence can be
related to ontogenctical and physiological causes. Ontogenctical ageing is genetically
programmed, localized in the meristems, not related to exhaustion, and cannot easily be
reversed. This implies important consequences for vegetative multiplication and for the
characteristics of clones obtained. Physiological ageing is correlatively influenced, caused
by an increased disorganization and exhaustion, and is not localized in the meristems.
When not advanced, a reversal is possible. Senescence regards more often the
physiological ageing, but may he of ontogenctical nature.
Introduction:
Terms related to ageing are used in confusion in every language because they are not
clearly defined. This regards, for example, 'ageing', and 'senescing', having a similar or
different meaning. It indicates the need for a better distinction and description of the
ageing processes, of its different phases, the process of 'rise and shine', covering the
juvenile and mature or adult phase, which received more attention than the 'fall',
representing the phase of full senescence. The latter negative aspect of ageing drew
interest more recently and became a new branch of science: phytogerontology.
This paper describes some of the present views with regard to ageing in the sense of
becoming more full grown, as opposed to ageing In the sense of getting more deteriorated.
It distinguishes an ontogenctical and a physiological type of ageing. This may have
implications and consequences for a discussion on 'Juvenility in Plants', the subject of this
symposium.
General
Ageing in plants has three aspects, a chronological, an ontogenetical and a physiological
one. In this order they are or, or less indicated by such opposing terms as: young and old,
juvenile and adult, improving and deteriorating. There may be no contradiction if a plant is
considered 'young and adult' or 'juvenile and senescent'. Strictly taken all three aspects of
ageing start from the formation of the zygote or, more in general, from germination or
probably regeneration and terminate with the natural death.
Chronological ageing only refers to the time which has elapsed e.g. since the plant
germinated, but does not give any information about the ontogenctical phase or
physiological condition reached.
Ontogenetical ageing more specifically refers to the process of passing through different
phases of development, from germination to complete senescence. Ontogeny is that part of
biology which studies the development of a zygote into an adult individual. It Includes
'embryology', the first part of development, and 'morphology', the subsequent change of
shape or metamorphosis. Ontogeny normally represents the positive aspect of ageing,
which can he depicted by the rising of the sigmoid growth curve. As a rule ontogenetical
ageing is Phenotypically visible and often correlated with, and indicated by, characteristics
such as the plant size reached, the number of leaves produced, the phyllotaxis shown, or by
the absence or presence of flowers. One distinguishes, for example, juvenile from mature
or ,vegetative from generative plants. however, these pairs of notions need not be
identical. An adult plant may remain vegetative if it is not induced to flower by required
specific conditions. This situation does not mean that ontogenesis came to an end, because
the terminal meristem then continues to initiate leaves until it receives the proper stimulus
to produce flowers, thus affecting the phenotype of the plant.
Physiological ageing applies primarily to the changes within the plant coherent with the
'coming of age' phase. It represents the negative aspects of ageing such as loss of growth
vigour, the increased susceptibility to adverse conditions, or deterioration in general. This
process of decline can he depicted by the downward course or last part of the sigmoid
growth curve. It is usually indicated by the term 'senescence', and normally precedes the
actual process of dying off. Senescence may cover the whole plant or only parts of it, and
may therefore begin early during ontogenesis. This indicates that ontogenetical and
physiological ageing, while different, should not he regarded as completely independent of
one another. Both may affect, for example, juvenility and rejuvenation, and determine the
life span of the plant or even the durability of its parts when harvested.
Some characteristics of the ontogenctical ageing
Ontogenctical ageing seems to be located in the terminal meristem (Pierik, 1967) while its
degree would be dependent on the activity of the meristem, as indicated by the number of
cell divisions. The stage of ontogenetical ageing reached is carried over onto its lateral
meristems when these are formed (Robinson and Warcing, 1969). This theory is supported
by many older conceptions such as topophysis' and 'cyclophysis' (Seeliger, 1924). Both
ideas mean about the same, and refer to the phenomena that a cutting or graft will develop
depending on, respectively, its former location on the mother plant, or the stage of
development at the time of its separation.
The theory that ontogenctical ageing is located in the meristems also explains the presence
of an 'ageing gradient' in each plant. A seedling does not age ontogenctically as a whole but
it does from the base to the top and from the inner to the outer parts. In plants there is no
continuous replacement of cells, such as occurs in animals. This explains the paradox that
the first-initiated, lowest, and chronologically-eldest part of a seedling is most juvenile,
while its more recently formed periphery is ontogenctically most mature. Full- grown and
old seedlings long retain many characteristics of juvenility at their base or reproduce them
when forming adventitious shoots at this site.
Ontogenetical ageing Is accompanied by anatomical, cytological and even physiological
changes. These were also observed in form, structure and cell properties of the terminal
meristems (Ali and Westwood, 1966), and could be used for determining the termination of
the juvenile phase. Not much is known about the hormonal regulation of the phase change.
It is supposed that the ratio between growth-promoting and growth-retarding substances
and/or the amount of a specific hormone are inductive for flowering only after a specific
ontogenctical stage, known as maturity, has been reached.
Consequences of ontogenetical ageing
The local differences in a seedling, regarding ontogenctical ageing, may have decisive
consequences for the selection of plant parts for vegetative propagation. It may result in
big differences in the offspring of a single plant. This regards, for example, the rooting
capacity, growth vigour, willingness to flower and even such phenotypic traits of the new
plant as leaf shape, branching, stem colour and the property of keeping or shedding the
leaves after senescence. It is generally known that adult cuttings from the top are more
difficult to root, grow less vigorously and flower sooner than juvenile cuttings from the base
of a seedling. Budding or grafting presents a solution when the capability to root is
completely absent. This practice, and the exclusive use of adult plant parts, may then lead
to the loss of the existence of juvenile tissue. This could explain why some plants, known
from older publications as easily to be propagated, cannot be brought on their own roots
nowadays (Hartmann and Kester, 196
. It can be questioned however, why one takes intoaccount the proper choice of plant material for vegetative propagation in some crops but
not in others. The system of in vitro culture may present answers to some of these
questions. Influencing ontogenetical ageing may be of commercial interest in many cases,
such as shortening or extending the juvenile period.
Regulation of ontogenctical ageing
Ontagenetical aging can be accelerated by improving growth conditions, shortening
endogenous dormancy and avoiding imposed rest. As a rule this leads to accelerated and
prolonged activity of the terminal moristem, which increases apical dominance, reducing
the branching and its requirement for assimilates, which in itself again leads to an
additional stimulation of growth of the terminal bud. In this way the juvenile period and
time to flowering can be shortened to a considerable extent, particularly in perennials
(Doorenbos, 1965; Zimmerman, 1972). The approach, however, normally does not change
the developmental stage, as indicated by the size, stem length or leaf number of the plant
at which it starts flowering.
Flowering at an earlier stage of development was induced in some plants almost
exclusively by chemicals, gibberellins in particular, or by grafting onto adult plants. An
extreme case is the induction of flowering with gibberellins in 8 to 12 months old Sequoia
seedlings, of which the juvenile period normally lasts 20 to 70 years (Pharis and Morf,
1969). With gibberellins, as well as with grafting, contradictory results have been published
(Zimmerman, 1972). To further complicate matters, most publications do not indicate the
stage of development at which the earlier flowering was obtained and what happens when
the chemical treatment is stopped. This makes it difficult to judge whether only the juvenile
period, or also the juvenile phase, was shortened. In the latter case, a continuation of
flowering without a special treatment, and excluding the possibility of an auto-catalytic
formation of a hormone for flowering (Wellonsick, 1966), could indicate that ageing was
really shortened ontogenctically. This does not seem very likely.
Because the ability to regenerate new organs diminishes with ontogenetical ageing, its
retardation may be desired, e.g. for the production of cuttings able to root. This can he
achieved by keeping the growth conditions marginal and by regular pruning of the roots
when no cuttings are needed. The possibilities of rejuvenation will be discussed later.
Some characteristics of physiological ageing
Physiological ageing can be defined as 'growing old', indicating the loss of vitality ending
with death. Senescence is the word commonly used to indicate this process. Initially it may
affect only parts of the plant, but inevitably, the whole plant will succumb. Senescence
accompanies ontogenetical aging and terminates it at last. It affects the whole plant
contrary to the more specifically located ontogenetical aging. Juvenility will not prevent a
leaf from aging physiologically. Senescence of a plant may proceed gradually or may be
more sudden, affecting all parts simultaneously. At the same time it may be acropetally or
basipetally directed. This brings us to the question whether two types of physiological
aging must be distinguished, i.e. one independent and another dependent on the stage of
ontogenetical aging reached by the plant.
Gradual senescence normally starts at the base of the plant and is acropetally directed. It
is commonly accompanied by a mobilization of metabolites from the ageing organs by the
active parts of the plant, and is obviously a correlative phenomenon. This is supported by
the observation that cotyledons or first leaves will age much later if higher located parts of
the plant are removed. Moreover, if these parts are rooted, they can reach a much larger
size and greater age than would have been possible if they were left on the plant. This
so-called compensatory growth is due to increased cell enlargement and DNA content, but
senescence cannot he escaped, whether or not accompanied by the occurrence of
endomitosis or endoploidy (Butterfass, 1970).
Gradual senescence can also be observed in very old trees or monocarpic plants kept
vegetative. This deterioration or dying back, however, has a clear basipetal direction and is
not accompanied by a remobilization of reserves. It is a consequence of starvation or
exhaustion of the most distant parts and of a disorganization in general. The internal
transport path gradually becomes too long, the balance between dissimilation and
assimilation and between root and shoot activity is less favourable, while a decreased
supply of water, nutrients and minerals limits a continual growth. The only remedy is an
extensive pruning back which at the same time induces growth of ontogenetically younger
meristems.
Whatever the direction of physiological ageing may be, it is not controlled by the terminal
meristems as is the case in the ontogenetical aging. This is demonstrated by the fact that
an aging top may resume its normal growth after a separation in time and rooting, or in
other cases, grafting on a vital plant. Top shoots of old vegetative sunflowers produced
more internodes and resumed rapid growth when grafted onto a younger plant (Warcing
and Phillips, 1970).. Here senescence completely lacks all characteristics of a
preprogrammed process.
One of the oldest living organisms known is a Pinus aristata Engelm. in California. It has a
measured age of 4600 years which illustrates that meristems may have a long lifespan.
However, it was discovered that mitosis in very old meristems of a number of plants was
not normal. This results in a deviating number of chromosomes and in decreased cell
division, an indication that physiological ageing may finally affect the meristems
themselves.
Simultaneous senescence. More dramatic than general senescence is the annual
occurrence of a simultaneous and total senescence in monocarpic plants, of the
over-ground parts in perennial plants, and of a total leaf shedding in deciduous plants.
Normally, this sudden senescence is also accompanied by a redistribution of metabolites
from the dying plant parts to the surviving organs, such as seeds, bulbs, corms, rhizomes
and buds. For this senescence Molisch (193
already introduced the conception of'Erschopfungstod.' The idea that exhaustion could be the cause of senescence and death
was supported by the observation that a timely removal of the reproductive organs could
postpone senescence or prevent death for long. The best known example is Aqave
mnericana L., which in its normal habitat flowers in about 8 years and dies subsequently.
Death of this monocotyledonous plant is normally induced by the formation of an
inflorescence, and therefore is almost ontogenetical by its cause. In climates where no
flower induction occurs the plant may become older than a century, but it will not
indefinitely escape physiological ageing. A similar type of physiological ageing terminating
ontogenesis occurs in maturing crops such as peas. A tip of an aged pea plant does not
recover after grafting onto a young plant, while a tip of a young plant grows normally when
grafted onto an aged plant (1ockhart and Gottschall, 1961). In monocarpic plants like
Agave and pea the sudden and simultaneous senescence seems to have been induced by
ontogeny itself. Many investigations demonstrated that exhaustion of metabolites is rather
a result than a cause of senescence, beginning normally after most of the fruits have
developed. In dioecious plants the males may start senescence long before the females are
fruiting.
The periodic leaf shedding of deciduous plants occurs in the juvenile as well as in the adult
phase, and is therefore clearly independent of the ontogenetical stage reached. Moreover,
it cannot be related to an exhaustion by other plant parts. The sudden and simultaneous
occurrence of senescence, however, indicates also in this case a hormonal induction and
regulation of physiological ageing.
The examples presented support the supposed differences in the triggering of physiological
ageing. The final total plant senescence cannot be regarded as a correlative phenomenon
but seems to be governed by the meristems similar to that of ontogenesis itself.
The possibilities of rejuvenation
Rejuvenation is the opposite of ageing and as a consequence of the foregoing discussion,
we should distinguish now a physiological and an ontogenetical type of rejuvenation.
Each plant, even a Pinus aristata, cannot escape a final total senescence and death.
Prevention of flowering or fruiting does not produce any rejuvenation. In polvcarpic plants
it will favour vegetative growth and by that only advance the time of total senescence
contrary to the delay of it in monocarpic plants. A severe pruning of the branches or stems
is much more effective. It shortens the internal transport system and improves the supply
of the periphery with water and nutrients. This can he regarded as a physiological
rejuvenation.
Pruning also induces younger buds or tissue to form normal or adventitious shoots, these
being more juvenile than those removed. This can he seen as a kind of semi-ontogenctical
rejuvenation. Theoretically such a rejuvenation cannot be continued indefinitely, because
each pruning activates the meristems present, stimulating their ontogenetical ageing.
Better possibilities for the preservation of a plant or tree are nresented by its timely
vegetative propagation, as is the case naturally in many perennials. This leads us to the old
question of whether clones age or not (Rijhouwer, 1930), including the problem whether all
plant cells remain totipotent. In one-cell cultures it appears that the number of non-dividing
and dying cells increases with time, notwithstanding a regular dilution of the culture and the
presence of sufficient food. Based on extensive experiments with clones of Lemna minor
L. (1Vangerminn, 1965) the question regarding the ageing of clones can he answered
negatively, provided the proper time and plant parts are chosen for propagation. Yet it
cannot he denied that a continuous vegetative propagation sometimes leads to a loss of
favourable characteristics (Hartmann and Kester, 196
.A true ontogenetical rejuvenation would mean that adult meristems could reproduce plants
or parts being again completely juvenile. Some examples are nresented by the production
of new seedlings after amphimixis or even apomixis, and the spontaneous or artificial
regeneration of juvenile plants, either in vitro or in vivo, from all kinds of adult plant parts,
such as stems, leaves and flowers (Plerik, 1975). This ontogenetical rejuvenation is much
more difficult to explain than the former physiological rejuvenation. It is supposed that an
isolation of a single or at least a limited number of totipotent cells from the surrounding
tissue is a prerequisite for a totel rejuvenation (Steward, 1967). Comparable results were
obtained by grafting juvenile seedlings onto the adult parts of the plant (ivy, sugarbeet,
rubber), or by spraying them regularly with gibberellin (Stoutemyer et al., 1961). It should
be noted that opposite effects were mentioned earlier, while discussing the probabilities of
an accelerated ontopeny.
Summary and conclusion
Juvenility is an important phase in the development of plants. it is depending on, and
affected by, a process of ontogenetical as well as physiological ageing and rejuvenation..
Ontogenetical ageing can be considered as genetically programmed, localized in the
meristems, accelerated by improved growth conditions and difficult to reverse. It is
accompanied or terminated by physiological ageing or senescence. This senescence is
correlatively influenced, caused by an increased internal disorganization, and normally not
localized in the meristems. If not advanced, its reversal is possible. Only the final and total
plant senescence can be considered to be of an ontogenctical nature, located in the
meristems, and as such irreversible. These conceptions may imply important consequences
for vegetative multiplication, both in vitro and in vivo, and for the characteristics of the
clones obtained. It probably supports the necessity of further research on the effects of
ontogenetical and physiological ageing of plants in the culturing of their meristems and
tissues.
