GETTING ACQUAINTED
By definition the 'meristem' is "plant tissue
in process of formation; vegetable cells in a state of active
division and growth."
There are therefore several meristematic regions in plants. The
region of interest in this paper is the one at the tip (apex)
of the very young shoots which develop from the areas where the
leaf and stem meet (the leaf axis) (See
figure 1). The meristem is enclosed in a tiny terminal dome
which is the actual end of the growing shoot. In the very youngest
state this area of rapid cell division has not yet developed into
elongated growth but is simply an area of incipient growth. Once
elongation is initiated the termnal dome becomes encased in leaves.
These leaves generally extend beyond the meristematic dome making
it appear that the leaves rather than the dome is the tip of the
shoot. However these leaves which enfold the dome may be removed
until the dome is exposed. The closer we approach the dome the
smaller and less identifiable the leaves become. The very youngest
leaves appearing as no more than bumps protruding from the domed
region just slightly back from (below) the extreme tips. These
leaves, still in the incipient state are called leaf primordia.
The section of plant tissue just below the meristematic region,
and the area from which the leaf primordia project is called sub-jacent
tissue.
The true meristem in dahlias is in the order of 50 to 80 micrometers
(microns) in diameter. Dr. Morel and Dr. Martin in their experiments
used sections of tissue approximately 250 microns in size. All
other known operations including the programs being discussed
herein have used comparable sized sections.
There are certain advantages to be derived from the use of smaller
sections, for example, the smaller the section the better the
probability that it will be free of virus. On the other hand the
smaller the section the more difficult it is to work with and
the lower the survival rate in culture. It has been found that
even with the larger sections (ie approximately 250 microns) the
probabilities of obtaining pathogen free explants (the small plant
that grows from the cutting) are acceptable. The sections that
have almost always been used in the past and which will most probably
be used in the future are in the order of 250 microns. Sections
of dahlia shoots of this magnitude will include one or two leaf
primordia, the few rows of cells between the meristem and the
actual tip of the shoot and some sub-jacent tissue.
The recent interest in Meristem Tip Culture for propagation of dahlias has been stimulated by the growing concern over the apparent spread of virus disease in these plants and the reallization that 75 to 90 percent, if not more, of all dahlias being gown at the present time are infected with one or more virus diseases. Although new varieties of dahlias of excellent quality are constantly being developed and introduced, many people believe that a large number of the older varieties are still the best of the class. The results at the annual dahlia shows will usually bear this out, the debilitating effects of virus not withstanding. Unfortunately most of the older varieties are probably 100 percent infected. Brierley(2)put
it this way, "any variety that has been field grown for five
years or more has a very high liklihood of being infected with
some virus disease".
It has become almost axiomatic with dahlia growers that seedling dahlias are free of virus. Many authors mention in their writings that dahlia virus is not transmitted through seed. However they either fail to lay any basis for this statement or simply cite Dr. Brierley's article. It is interesting to note that Dr. Brierley makes no such claim that such is the case. In discussion of Dahlia Mosaic he has only this to say, "the writer has found no evidence suggestive of persistence in soil, and no evidence of seed transmission." His only statement on the subject is, "no mosaic appeared in 44 seedling dahlias grown from seed of Catherine Wilcox. Three types of seedling dahlias grown from commercial seed proved free of mosaic". In summarizing the article he says simply, "Mosaic persists in vegetative parts of affected plants but has not been found to pass through seed." The idea that virus does not pass through seed is not only virtually unfounded but definitely in error as shown in the article 'Dahlia Virus and Seedlings' in the Dahlia Reporter Vol 10, No. 2, 1980(12). That article based on records and experiments over the years 1976 through 1978 established that virus is indeed transmitted through seed. Growing and comparing over 800 seedlings from known diseased varieties and the same varieties that had been recovered through Meristem Tip Culture showed that virus transmission does occur in a surprisingly high percentage of seedlings from diseased dahlias. Distribution according to type of virus was not determined.
Since only a short and limited program was carried out by Dr. Morel and Dr. Martin, much experimentation and refinement of their technique and procedures will be necessary before an acceptable level of success may be expected. An acceptable level of success is envisioned as one which would make the program commercially feasible. In adddition to the Morel and Martin program there have been a few other experiments carried out along the same line.These programs have in general only verified the possibilities of recovery of healthy plant material and have not greatly improved or modified the basic technique.
The scientific and operational requirements of a program to recover
healthy dahlia cultivars from diseased stock are not highly demanding
nor complicated. They do require a reasonable knowledge of organic
chemistry and botany of dahlias as well as a fairly high degree
of manual dexterity. The expense of setting up a facility suitable
for conducting a commercially successful program may prove to
be the limiting factor allthough the program to be described later
has shown that such a program can be performed successfully in
a normal home environment.
In the United States at the present time the dahlia is primarily a hobby crop and not a highly demanded cut flower. The popularity of dahlias as cut flowers seems to be increasing but would appear to still lag the popularity of roses and carnations as example. Therefore the present commercial value of dahlias is mostly as roots, pot roots, or plant starts sold to dahlia hobbyists, for home garden and exhibition purposes. When the dahlia does attain status as a popular cut flower, top exhibition quality will probably not be a requisite. In fact top exhibition quality would not be practical in a cut flower. The cost of producing top quality blooms would price them out of the cut flower market. Many new varieties coming on the market will be suitable for cut flowers. These new varieties will generally be reasonably tolerant of virus for two or three years. Even after becoming infected the general purchaser of cut flowers would not be aware of such infection, nor would they be concerned since they are purchasing blooms to be enjoyed for a few days and thrown away, and have no concern about further infection of other stock. Another prime consideration for a cut flower variety is prolificness. This is generally not an important characteristic of exhibition varieties where number of blooms are held to but a few on a plant anyway in order to attain that final measure of perfect form and size. Thus it appears that if a program to recover healthy cultivars from diseased stock is to be undertaken it would be primarily for the benefit of the dahlia hobbyist and the producer of exhibition variety stock
It has been variously stated that planting of anywhere from one
thousand to ten thousand seedlings is required to have a reasonable
probability of producing one variety equal to those already in
existence. If this ratio could be reduced to one out of ten or
even one out of 20 it would be a great boon to the world of dahlias.
This is in effect the promise of Meristem Tip Culture. Even this
percentage may be greatly improved after experimentation and development
of an ultimate program.
Unlike experimenting with seedlings the quality of the dahlia
obtained through Meristem Tip Culture is known beforehand. If
we obtain a clone of Juanita or Kidd's Climax
or of Golden Heart we know we have a clone of superb quality.
We may be pleasantly surprise at just how high the quality turns
out to be. We are accustomed to seeing blooms of these varieties
with certain characteristics, which we recognize as outstanding.
However these blooms are outstanding in spite of probable debilitation
from a virus disease. When virus free clones of these varieties
are recovered the resulting blooms, unaffected by the debilitating
effect of virus may be better than we were to expect.
