How does a cockleburr know when to flower?
By Chelsie Vandaveer
May 9, 2002
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The cockleburr (Xanthium strumarium Linnaeus) [zan' the um strew mar' ee um] is a hitchhiker. The seed pod is a burr with hooked spines. It tangles the fur of animals unfortunate enough to brush against the dying plant.
The cockleburr is a short-day plant. It will not flower until the days have 16 hours or less light. It does not matter when the seed germinated. One plant can be three months old and another six weeks old, when the day length declines in summer,
the plants begin to flower.
A short-day plant is a misnomer. Actually, it is the length of uninterrupted darkness that brings on the flowering. The plants would flower when the nights were eight or more hours long. The exact number of hours required was dependent upon the latitude where they were growing.
In 1938, James Bonner and Karl Hamner discovered something strange about the flowering of cockleburrs. If all of the leaves were removed, they failed to flower. But, if only one-eighth of one leaf was left on the plants and they received eight hours of dark, they would flower.
If the plants were exposed to as little as one minute of light during the eight nighttime hours, they failed to flower. But, if the plants received any periods of darkness during the daylight hours, they would flower. These facts led to the discovery of the phytochromes, pigments that convert to their counterparts in the presence or absence of light.
Phytochrome R (PR) absorbs red light at 660 nanometers and converts to Phytochrome FR (PFR). PFR converts back to PR during the dark. It takes at least eight hours for the conversion. The presence of all PR and no PFR in the morning is the biological clock that releases the hormones for flowering.
The Digital Libraries of Texas A & M University has posted a photograph of the cockleburr taken by James Manhart. To view the photograph, click on the link:
http://www.csdl.tamu.edu/FLORA/LCP/LCP131.JPG
killerPlants Tendrils: ~~1~~2~~3~~4~~5~~
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