It's that time of year again...Spring! Fresh green grass, daffodils, spring robins, and ..Taraxacum officinale...the Common Dandelion. Although dandelions were first brought to this country as a colorful garden plant, and are beloved by children, most adults consider T. officinale to be an eyesore...a blight on an otherwise green expanse of lawn...in short - a weed.

(Does this sound like YOU?)
"As adults we often lose sight of the many small things that intrigued us in the spring of youth. The same children who delighted in the little white balls just a decade ago, can now be found in the lawn and garden stores, searching through containers of deadly toxins for the perfect poison to irradicate what previously had delighted them so." [Quote & Image]
((Do you still love dandelions? Check out The Dandelion Page, by Dr. Weed))

What is a Weed?

Generally, the term weed describes any plant that is unwanted and grows or spreads aggressively. The term exotic weed describes an invasive unwanted non-native plant. The term noxious weed is reserved for particularly aggresive weeds, usually of agrinomical importance. (The USDA keeps a Federal list of noxious weeds.) Weeds are very important agronomically because they reduce the yield of crops in three ways: by competing with the crop for water, light and nutrients, by interfering with crop harvest, and by contaminating harvested products with weed seeds and toxins. Weeds can reduce yield up to 50% and are responsible for millions of dollars in crop losses each year. However, what is considered a weed in one area may not be a weed in another. Closing Thought: A weed is no more than a flower in disguise.[Quote]

Another sign of spring....For those of us who may appreciate the dandelion, yet still want it out of our back (or front) yard: RoundUp

RoundUp (or Glyphosate), one of the world's most popular and 'friendly' herbicides, is a non-selective herbicide used primarily for perennial weed control - in short, killing weeds. Glyphosate was discovered by Dr. John E. Franz of Monsanto in 1971 and released commercially in 1974. Starting in 1983, and every year since, RoundUp became the first pesticide with world-wide sales of over $1 billion. It is known as a "postemergence" herbicide, meaning that is cannot be used until after the weed species has sprouted, or 'emerged'. RoundUp is an extremely effective herbicide, but it is also non-selective herbicide: it will kill almost all species of plants, both wanted (crops, grass) and unwanted plants (weeds). Therefore, Roundup can be used for spot weed control, but not for general application on grass or crops, so users must spray with caution!

RoundUp is considered a 'friendly' herbicide to the environment because:
- It is virtually nontoxic to mammals, birds, fish, and insects
- It exhibits essentially no pre-emergence activity. It won't prevent plants in your garden from germinating.
- It exhibits essentially no residual soil activity even when applied at high rates. Roundup binds tightly to soil particles and doesn't move on or in the soil to affect untreated plants nearby.
- It breaks down quickly into natural materials such as carbon dioxide and nitrogen.
- It does not penetrate the woody stems of trees, shrubs, or grapevines
- Finally, the most important feature, once inside the plant, glyphosate inhibits a key enzyme found only in plants and bacteria – EPSP synthase. [Image]

How does RoundUp work?

RoundUp (glyphosate) inhibits a key enzyme that plants and bacteria use to make amino acids called EPSP synthase. Structurally, glyphosate resembles the chemical structure of the amino acid glycine. Because of its structural similarity to glycine, glyphosate binds the active site of the EPSP synthase enzyme that is critical for the production of aromatic amino acids.

Without a functional EPSP synthase enzyme, the plant can no longer make the amino acids phenylalanine, tyrosine and tryptophan or any other compounds that use this pathway. Since plants must synthesize all of the amino acids that they need for protein production, inhibition of EPSP synthase by glyphosate causes the plant cells to starve for amino acids. All plants and bacteria use EPSP synthase to make aromatic amino acids, so all plants and bacteria are sensitive to RoundUp. [Image of EPSP Synthase]

Glyphosate is quickly absorbed by leaves and shoots of plants. Once absorbed into the leaves, glyphosate cannot be broken down. The glyphosate moves quickly through the plant and accumulates in areas of active growth called meristems. Spraying a plant with RoundUp results in a lack of protein synthesis in that plant. Without amino acids, Plants stop growing. Within a week or so, many plant tissues and parts slowly degrade due to lack of proteins. Death of the weed ultimately results from lack of nutrients and dehydration a week or so later.

Roundup does not affect mammals, birds, fish, and insects because these organisms do not have the enzyme EPSP synthase. However, a recent study has found an increased risk for Non-Hodgkin's Lymphoms (NHL), a type of leukemia, in agricultural workers exposed to large quantities of RoundUp.

Monsanto has developed a type of genetically engineered plant, called RoundUp Ready, that is genetically resistant to RoundUp. Over 70% of all soybeans grown in the USA are RoundUp Ready, and almost 50% of corn and cotton are RoundUp Ready. We will discuss this in detail when we talk about Ag-Biotech on March 27th and April 1st. [Image]

Material from this Good For came from the following sources:
RoundUp.com
Glyphosate, Advanced Topics
Glyphosate: The Acutely Toxic Chemical Element - by Guarding our Earth.com

A final thought from Dr. Marrs on the weed homeowners love to hate...
The Canada Thistle, Cirsium arvense (from the Latin: 'Cursed Thistle')

From the Weed Science Socety of America: "Canada thistle, the best-known thistle species, is native to Europe or temperate Asia. Immigrants brought C. arvense to America in crop seed. It spread so rapidly that control legislation was enacted in Vermont in 1795 and in New York in 1831 (8). It is a noxious perennial weed that has invaded the whole northern half of the United States.

An earlier common name for C. arvense was "cursed thistle". Linnaeus in 1753 wrote of this plant, "It is the greatest pest of our fields" (2). Its roots may penetrate 20 feet deep, and the Canada thistle sends up countless new shoots when its underground parts are broken and are spread by tillage. One plant, propagating by its roots and rhizomes, "can colonize and area several meters in diameter during the first one or two seasons of growth" (5). It infests 27 crops in 37 countries and is worst in temperate regions of the northern hemisphere (5). It readily destroys pastures, as cattle dislike and avoid it. The thistles are characterized by spiny stems, alternate leaves, and terminal discoid flower heads set in spiny bracts. The flower heads vary in color from creamy white to deep purple and are from 1 to 2 inches across, consisting of compact, tubular, perfect flowers that look like a shaving brush."

My Favorite Remedy for the Cursed Thistle:

 

 

Extra Point Questions:
1. How does glyphosate kill plants?
2. Why doesn't glyphosate kill mammals, fish, insects or birds?
3. What does the name Dandelion mean? (See Dr. Weed's section on "What is a Weed?")

The text of this "What is Biology Good For" exercise is copyrighted under the name of Dr. Kathleen A. Marrs, 1999, 2000, 2001, 2003. There are no restrictions on its use by educators or by non-profit institutions as long as its content not modified, proper copyright acknowledgement is retained, and this statement is not removed.