No Plant Says “Christmas” Like….
If there’s a plant most associated with Christmas, then, of course, it’s the Christmas tree.
What would be in second place?
Did you think poinsettia?
If so, I suspect most people would agree with you.
If you’re looking for a comprehensive website regarding poinsettias (e.g., history, fun facts, care, etc.), then I recommend the University of Illinois Extension’s Poinsettia Page.
If, however, you’d like some information regarding the plant physiology of poinsettias, then read on….
The Long (Tall) And Short Of It
As you probably already know, poinsettias are native to Mexico, and, in Mexico, the poinsettia is a perennial shrub that will grow 10-15 feet tall, way taller than the poinsettias we see all around at Christmas time.
“The modern era of poinsettia culture began in 1923 with the introduction of the seedling cultivar selected and developed mainly by Paul Ecke of Encinitas, California. This cultivar was shorter in stature and produced a more attractive branching plant.” (From Ref. 1 below)
Unbeknownst to other horticulturists at the time, the secret to Ecke’s small, bushy poinsettias was grafting.
“They produced a fuller, more compact plant by grafting two varieties of poinsettia together. A poinsettia left to grow on its own will naturally take an open, somewhat weedy look. The Eckes’ technique made it possible to get every seedling to branch, resulting in a bushier plant.” (from Wikipedia)
It wasn’t until around 1990 that the secret to Eckes’ success was revealed by horticultural graduate student, John Dole, and his advisor, Dr. Harold Wilkins, at the University of Minnesota (see Ref. 2 below)
Briefly, they reported that some sort of a “graft-transmissible agent” was responsible for the compact, so-called “free-branching” cultivar of poinsettia.
But what was this “agent”?
Many suspected it was a virus. But this theory wasn’t supported by subsequent experiments.
It wasn’t until 1997 that the “graft-transmissible agent” was found to be a phytoplasma. (See Ref. 3 below)
This finding was consistent with known symptoms of phytoplasma infection. “Many phytoplasma-infected plants develop a bushy or “witch’s broom” appearance due to changes in their normal growth patterns. Most plants exhibit apical dominance but infection can trigger the proliferation of auxiliary (side) shoots and a reduction in internode size. Such symptoms are actually useful in the commercial production of poinsettias. Infection triggers more axillary shoot production; the poinsettia plants thus produce more than a single flower. (from Wikipedia; also please see a previous post about apical dominance.)
To date, it’s not precisely known how phytoplasmas induce such symptoms in plants, but it likely involves plant hormone metabolism, action, or both (e.g., see Refs. 4-6 below).
To inhibit shoot elongation in poinsettias not infected with phytoplasmas, commercial growers resort to (1) pinching off shoot apical meristems, (2) treating plants with PGR (plant growth retardants) such as flurprimidol that inhibits the biosynthesis of the plant hormone gibberellin (GA), (3) adjusting the photoperiod, and (4) withholding water.
Just The Bracts, Ma’am
You likely already know that the showy parts – typically bright red – of a poinsettia are not flowers, but modified leaves called bracts.
“Bracts are specialized plant structures that serve varied functions such as attracting pollinators and protecting inflorescences (flower structures). Often leaflike, bracts range from the inconspicuous to the wildly showy. Perhaps the best example of the latter is poinsettia (Euphorbia pulcherrima), whose bright red bracts surround the inconspicuous yellow-green flowers and are often mistaken for petals. And, in fact, the bracts serve the same purpose that petals often do: attracting pollinators.” (from: Bract Facts by Jon Hetman)
So, even though they are not part of the poinsettia flowers, the bracts are associated with poinsettia flowering, which, by the way, is photoperiodic.
A poinsettia “…needs long, daily dark periods to initiate flower buds and the coloring of the upper leaves or bracts. Many commercial growers use black-out cloths to cover benches or whole greenhouses of plants to assure that the plants get the required number of hours of dark for the required number of weeks to get their poinsettias ready to sell at the right time.” (from: Are you photoperiodic? Poinsettias are.)
So, there you have it: Briefly, “just the facts, ma’am” (at least as far as I know them) regarding the physiology of poinsettias.
1. Lee, I.-M. (2000) “Phytoplasma Casts a Magic Spell that Turns the Fair Poinsettia into a Christmas Showpiece.” DOI:10.1094/PHP-2000-0914-01-RV (Full Text)
2. Dole, J. M. and H. F. Wilkins (1991) “Vegetative and Reproductive Characteristics of Poinsettia Altered by a Graft-transmissible Agent.” Journal of the American Society for Horticultural Science, Vol. 116, pp. 307-311. (Full Text)
3. Lee, I.-M., et al. (1997) “Phytoplasma induced free-branching in commercial poinsettia cultivars.” Nature Biotechnology, Vol. 15, pp. 178-182. (Abstract)
4. Nicolaisen, M. and D. P. Horvath (2008) “A Branch‐inducing Phytoplasma in Euphorbia pulcherrima is Associated with Changes in Expression of Host Genes.” Journal of Phytopathology, Vol. 156, pp. 403-407. DOI: 10.1111/j.1439-0434.2007.01372.x (Abstract)
5. Maejima, K., K. Oshima and S. Namba (2013) “Exploring the phytoplasmas, plant pathogenic bacteria.” Journal of General Plant Pathology, Vol. 80, pp. 210–221. (Full Text)
6. Ashraful Islam, M., et al. (2013) “Overexpression of the AtSHI gene in Poinsettia, Euphorbia pulcherrima, results in compact plants.” PLOS One, Published: January 7, 2013. DOI: 10.1371/journal.pone.0053377 (Full Text)
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