Essay on Culls from Tissue Culture (continued}
Dear Hosta-Open Forum Group,
My essay continues in the form of a series of questions and answers. You
may wonder why I am taking the trouble to write this essay. The answer
is.....I am a retired Horticulturist ( Horticulture is the science of
applied Botany) with 60 years of work experience in various fields of
plant science including Plant Physiology, Plant Anatomy, Genetics,
Biochemistry, Soil Science and Systematic Botany among others. I am a
perpetual student intrigued by the mysteries of Nature. I spend
virtually all of my time in retirement in Hosta gardening and related
activities. In inclement weather or out of season I spend my time in
growing seedlings, tissue culturing, reading, studying and writing about
hosta related science. Scientific inquiry remains one of my keen
interests as a carryover from my professional life. My motivations are
driven by my curiosity and interest in finding out all I can by reading,
studing, simple trials, observation, analysis and re-presentation of
ideas in writing. The intellectual challenge and my search for answers
is a purposeful endeavor as I grow older...comparable to jogging as
practiced by some people. Surely readers will be indulgent of these
hosta related idiosyncracies. I constantly remind myself that I have
already achieved all possible accomplishments programmed for achievement
in my lifetime. And that any further efforts on my part are for my own
With that expose out of the way, other questions arise.Why is
information on culls from tissue culture important to know? I am frankly
surprised to know that they are so important to certain growers. I buy
tissue culture liners just as others do and I do not see the drastic
problem in numbers of culls that appear in flats of liners that are
reported by some people. A growing number of hosta cultivars are being
propagated by tissue culture technology. Hostas are fast becoming very
popular with shade gardeners and a mass market demand has opened up
in recent years. Existing TC Labs are rapidly expanding their capacity
to produce more and more tc plantlets for this mass market. In addition
there is keen interest in new labs coming into existence for meeting
these same opportunities for expansion. To prove this point of view, I
refer you to the large numbers of tissue culture technical people ( a
list of some 1200 individuals) who are on-line in a large network of
communicators exchanging information on the internet on TC technology,
some of which is on Hostas. There is no estimate of how many exist who
are NOT on-line, but the numbers must be staggering. I am one of four
people in the hosta community of friends that I know who on-line in this
TC technology line-server. It represents just one indicator of the
importance of tissue cultue as a technology driving the hosta related
industry of micropropagation of thousands of new plants by TC. This
engine can not be stopped or even slowed....not by complaints, name
calling, expression of discontent or any other means I can think of. TC
is now an integral part of the Hosta business and will continue to grow
rapidly. ( I wish I were younger so that I could be a part of it).
What is a cull...this thing that seems so important and that which some
complain may be causing serious problems in the industry? I suppose that
a good definition may be any plant product that a given lab decides
within its management policy to be an aberrant form which has little or
no market value. A cull could be an aberrant of a monocolored green or
yellow form, or an aberrant of variegated forms as well. It represents
off-types, odd forms, different colors, different patterns of
variegation, wierd growths (mutants), distinctive, highly prized
streaked or other types of variegated sports, plants that are diseased
or insect infected, those which lack vigor and fail to grow, those which
die within their cell-pack from unknown causes, those which are obvious
mechanical mixtures from the rest of the batch with a given label or
empty cell packs within a flat. An aberrant is any deviation from the
normal excepted description of the plant being propagated for sale or
distribution. Its description is limited only by the readers'
imagination of examples of what COULD go wrong in a TC Lab and its
related growing-on operations.
Are there any failures in maintaining good quality control to avoid a
high level of poor quality plants? Of course there are occasional
failures in some operations as there are in all operations.
Competition, which provide opportunities to select alternative sources
of plants with better prices, quality and selection at most appropriate
times, will come into play as factors controlling the marketplace.
Discontented customers at both the wholesale and retail level will have
options to make choices which will avoid their discontent.
What are some of the causes of culls from tissue culture? Using the
narrow definition of a cull as being an aberrant such as a sport, let's
look at such causes. I have indicated already in my recently posted
message re: " Origin of Sports" that the technical causes are several.
They include any change such as:
a mutation of genes in chromosomes
mutations of DNA in plastids
variable cell differentiation patterns as leaves grow
shifts of tissue layers (L1 and L2) within the leaves
overgrowth of one tissue layer over another
crossingover , fracture and rejoining of chromosomes during
somatic cell division creating new combinations of genes on affected
mutations of DNA in mitochondria within cytoplasm of cells.
Readers, please be advised that these changes occur at the molecular,
cellular and tissue level within the plant.They are beyond the control
of tissue culture lab operators. The only possible intervention is that
of rogueing aberrant types when timing is most appropriate. It is an
integral part of tissue culture proceedures.
What about frequency? How often do mutations occur? Does tissue culture
itself influence frequency? Do mutations occur in the garden with the
same frequency? My understanding is that within a population of
approximatelly 100,000 plants, one mutation may occur with a year's
time. I have heard several references to this approximate figure.
Whether it is accurate is not too critical. In summary, mutations are
rare but they occur. Even though the phenomenon of sports is more
frequent than a specific mutation, even sports are rare. I have about
10,000 plants growing in my hosta garden. In recent years I have
experienced about 10 sports each year, so this estimate may be somewhat
I have already commented on the chance occurence of sports occurring
EARLY or LATE in the tissue culture process and the influence of this
chance occurence in time as affecting the possibilities of numbers of
sports(see several E-mail Messages on TC and OS).
A final comment is necessary on what type of plant material is being
propagated. If unstable, variegated cultivars are being propagated, the
incidence of sports will be dramatically higher than if green or yellow
relatively stable form are being propagated. This is an important
consideration which has been explained by the classical research work of
Jones on the origin of chimeras as early as 1934 in England.
Several bibliographical references are included from inquiring minds.
Additional information which they provide will be valuable for serious
Haig Dermen, "Origin of Plant Sports", Amer. Hort. Mag.,39:123-173. 1960
Jim Hawes,"Sports Term Forum", E-mail Message to Hosta-l, March 28, 1997
Jim Hawes,"Origin of Sports", E-mail Message to Hosta-l, March 30, 1997
Jim Hawes,"Stability in Hostas", Article to be Published in BHHS, June,
W.N. Jones,"Plant Chimeras and Graft Hybrids", 1934,Methuen and Co.
Kevin Vaughn,"Chloroplast Mutants in Hostas", AHS Bulletin 11, 1780.
Ben Zonneveld,"Mutations, Recombinations,Sports and Chimerae", The Hosta
Journal, Vol.27,No 1.
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