Lecture 13 - Asexual Regeneration Flashcards
Asexual regeneration in plants: stretching the space/time continuum of existence
Living fossils amongst extant plant genera.
This table shows a list of extant plant genera that have existed for millions of years-as far back as the Permian period of the Paleozoic era.
For genera to remian morphologically unchanged implies widespread asexual reproduction.
This temporal range of persistence at the genus level, is not observed in animals.
Plants evaded two major mass extinctions: The Permian and Cretaceous mass extinctions
Unlike animals, plants suffer only minor declines in species number during mass extinction events.
Two reasons for this:
1. Persistent seed bank. Nothing in the animal kingdom that can match this persistence.
- Asexual reproduction. Many animals can regenerate parts but few can do it when 95% of the biomass has been lost. Plants are far simpler than animals; they are mere collections of reiterated parts (roots/root hairs; shoots/leaves), and few types of cells (parenchyma, collenchyma, sclerenchyma). The simplicity of plants makes them antifragile.
Definition of antifragile –something that benefits from disturbance
There is strong evidence that the Wollemi Pine is able to reproduce sexually in the wild.
We have shown that viable seeds are produced and that these seeds may germinate and develop into healthy plants given suitable conditions. Growth of seedlings in the wild appears to be supressed, however, as the current population structure consists only of small seedlings and large trees.
Evidence of seedling recruitement within this area is provided by the existence of several young, essentially single-trunked trees in area that are relatively open and free from interplant competition…
It is interesting to note that no seedlings are found away from the very small area in which the adult trees exist, altough the adjacent area appear ostensibly the same in terms of environment.
There is also some evidence of asexual regeneration of W.nobilis trees through self-coppicing and there is the possibility of re-establishement through root suckering. It is most probable that the adult population structure of the Wollemi Pine consists os a small number of genes that have reproduced vegetatively.
The advantages of sexual regeneration
An asexual stem begins life with:
* an intact, extensive root system that is well stocked with carbohydrates and other plant metabolites. Furthermore, it has a well symbiotic relationship with mycorrhizal fungi.
A seed on the otehr hand has:
* Food reserves in cotyledons and endosperm
The disadvantages of asexual reproduction is the inevitable accumulation in deletrious mutations. But we know very little about this.
Asexual regeneration in plants is under hormonal control
All three modes of asexual regeneration -suckering, sprouting and layering - are controlled, largely, by the ratio of auxin and cytokinin. But exact mechanism and influence of each hormone is still not well understood.
Auxin
Site of synthesis: shoot apical meristem and young leaves
Major function: enhance apical dominance
Cytokinin
Site of synthesis: primarily roots
Major function: modifies apical dominance
Strigolactones
Site of synthesis: produced in roots in response to high auxin flow from shoots
Major function: controls apical dominance
What do we know from plant physiology literature
Root suckers and basal sprouts orginates from bud primoridia formed from the cork cambium of the roots.
Auxin is produced in aboveground tissues (bud and young leaves) and is transported in the phloem to the roots, inhibiting bud initiation and promoting root growth.
Cytokinins, in contrast, are produced in actively growing root tips, exhibit polar movement away from the root tips towards the stem (opposite to auxin), and are known to play an important role in the inititation of shoot devlopment on roots in many plants by counteracting the activity of auxin.
Suckering
Just under the bark of a tree, there are many vegetative buds that are dormant. Studded along a tree’s vast root system are even more dormant buds. Severe damage to crown greatly reduces the auxin flow downward.
When cytokinin to auxin ratio increases, dormant buds are released from apical dominance and develop shoots.
In addition to changes in hormones ratios, the intensity of the suckering response depends on carbohydrates reserves, moisture, light, temperature, ect…
From estimates on seed production (sexual reproduction) to estimates of the below ground bud bank (asexual reproduction).
Fire triggers the highest suckering response. The intensity of the asexual response increases as a function of stand age.
Relative to basal sprouting and layering, suckers spread farther away from the dead trunks.
Suckers can be as far away as the farthest roots: say, 25 meters.
The crowning achievement of aspen (Populus tremuloides) in asexual regeneration and reason for the hyperbole on slide 1:
- Let’s begin with a single aspen stem with a root system that span out 4 meters from the trunk.
- Now imagine that severe fires occur on this stand every 100 years.
- A century later, there is a fire, and this root system from this loner aspen tree expands outward another 4 m, filling up the area with about 16 trees. Each century this idealized clone expands another 4 meters. After 10,000 years it would occupy 50 hectares and boast 40,000 trees.
- The world record as of now is a 43 hectare aspen clone in the mountains of Utah. Undoubtedly, there are larger clones out there but almost nobody is actively looking them.
Applications of strong suckering responses in trees: land reclamation
The strong suckering of aspen makes it ideal for reclamation and restoration projects.
Over the past 30 years, aspen seedlings have been planted on reclamation sites in the boreal forest region.
But this has prompted questions & concerns:
1. Can planted seedling-origin stands regenerate after an above-ground disturbance. If so, will the suckering response be as expected?
- Aspen can easily outcompete with commercially important species.
While some parts of Canada plant aspen, other parts try to suppress suckering of aspen.
The forest sector, particulalry in Canada, views aspen as a competitors to species that are more commercially valuable.
How can we decrease or inhibit aspen regeenration?
By controlling number os stems removed and the timing of the harvest.
* In natural clonal aspen lands, full stem removal results in significant regeneration through root suckering; While the retention of some stems can have a negative impact on regeneration.
- In younger stands, the cutting of stems can also produce stump sprouts which suppress the production of new root suckers from the parent clonal root system.
