A recent story about how some of our Washington state lawmakers hate the sight of dandelions reminded me that it’s time for an encore of this post. Hope you enjoy it….again?
Dandelions – Much Maligned.
Some people become upset when they see dandelions (please see here, for example).
Few plants generate such annoyance among suburban homeowners with immaculate lawnscapes (and even some Washington state legislators) as the common dandelion. (In North America, they are most likely Taraxacum officinale).
What Do Suburban Lawns and the Vietnam War Have in Common?
Answer: The herbicide 2,4-D.
You may be familiar with this herbicide as an active ingredient in “Weed ‘n Feed®”, “Weed B Gon MAX®”, Turf Builder® With Weed Control”, etc..
During the Vietnam War, it was an active ingredient in Agent Orange.
On lawns it’s used to kill the dandelions, but NOT the grass. (Find out how it does this below).
In the Vietnam War the U.S. military used it to defoliate the trees (so that they could more easily spot the Viet Cong).
You’re likely familiar with the term “Agent Orange” because of the controversy regarding the tragic health problems it caused to U. S. soldiers. (For current info re. this issue see here).
The serious health issues to both Americans and Vietnamese caused by Agent Orange are due to contaminants called dioxins produced during its chemical synthesis. (For more info on this see 2,4-D and dioxins and also here.)
How Does 2,4-D Kill Dandelions…?
First produced in the 1940’s, the herbicide 2,4-D is one of many so-called phenoxy herbicides. These herbicides all are both structural and functional analogs of the plant hormone auxin, more precisely, indole-3-acetic acid (IAA). Such synthetic auxins as 2,4-D are not only structurally similar to IAA, but they are also biologically active as auxins in most plants. Although they both look and act like auxins, plants can not metabolize these phenoxy herbicides as they can with IAA, the natural auxin. This turns out to be the key to why phenoxy herbicides such as 2,4 D are able to kill some plants.
Auxin-based herbicides are referred to as “selective” herbicides because they kill so-called “broadleaf” plants (a.k.a., dicots) but not grasses, for example. (Hence, that’s why they’re such popular herbicides with both growers of lawns as well as of wheatfields.)
But how exactly does spraying 2,4-D on susceptible plants kill them?
This turns out to be very poorly understood, and it’s also the subject of much misinformation. For example, I’ve heard people say that such herbicides kill the plant because ” it grows itself to death”, and I’ve read that 2,4-D “…simply confuses the plant to death”.
At the present time nobody really knows precisely how the auxin-like herbicides kill susceptible plants. As with most effects of plant hormones, it probably has a lot to do with the plant species in question.
However, recent findings have provided important clues. And these clues support the idea that plant death may occur as a result of a combination of factors.
Here’s a summary of the story:
First off, one of the well-know effects of excess amounts of auxin on dicots is to cause them to overproduce the plant hormone ethylene. For example, in 1969, Prof. Mary Hallaway and Prof. Daphne J. Osborne first showed that ethylene is a factor in defoliation caused by 2,4-D.
Because plants can’t break down 2,4-D, it’s action persists. This action includes the excess production of ethylene, which may result in a number of plant responses, including epinasty and senescence.
Another effect of excess ethylene production in response to 2,4-D is to stimulate the production of yet another plant hormone, abscisic acid (ABA). The effects of ABA on the plant may contribute to eventual plant death. (For an illustration of the complex effects of auxin-based herbicides on plants, see Figure 1 in Ref. 1 below.)
The most recent review on the subject (see Ref. 3 below) I’ve been able to find doesn’t provide a much clearer picture. Most of the literature on 2,4-D has to do with mechanisms of resistance to this herbicide in plants.
…and why doesn’t 2,4, D Kill the Grass? (and You?)
Perhaps the simplest explanation for both questions has to do with sensitivity to the plant hormone auxin.
In general, grasses are much less sensitive to synthetic auxin herbicides than are dicots. That is, a much higher threshold level of auxin-based herbicide is required to elicit physiological responses in grasses versus the so-called “broadleaf” plants. So, at the doses used to kill dandelions, for example, grasses are largely unaffected. (Higher doses of 2,4-D may kill the grass, too, however.) Grasses may be more resistant to such herbicides because of differences in leaf morphology, translocation of the herbicide inside the plant, and the ability to metabolize (breakdown) synthetic auxins.
Aside from the toxic contaminant dioxin, 2,4-D has no physiological effects on animals at hormonal levels, that is, at the concentrations that affect plants. (Indeed, there is no reputable evidence that any of the five main plant hormones affects animals.)
Is Sex Necessary? – For The Common Dandelion, Apparently Not
Despite efforts to eradicate them using chemical warfare, the dandelions exhibit a remarkable ability to proliferate.
They do so likely because they produce seeds asexually, that is, without the complications of sexual reproduction, such as pollination.
This is because most dandelions reproduce by a process called apomixis.
Unlike other forms of asexual reproduction in plants such as vegetative plant propagation via cuttings, apomixis is asexual reproduction via seeds.
In the case of most dandelions (i.e., Taraxacum officinale), the embryo in the seed forms without meiosis, thus the offsping are genetically identical to the parent.
Hence, most, if not all, of the dandelions in your neighborhood may be clones.
What are the benefits of apomixis?
Well, despite the lack of the evolutionary benefits of sexual reproduction (lack of diversity), apomixis allows for the “mass production” of seeds, which appears to be an effective strategy for dandelion propagation.
By rapidly producing cloned offspring, sex is certainly not necessary for the common dandelion.
Dandelions – Highly Underrated?
Perhaps to the chagrin of suburban “lawnscapers” who spend so much time and effort and money in eradicating dandelions, did you know that dandelions are actually commercially cultivated in many places in the United States? Vineland, New Jersey, may indeed be the “dandelion capital of the world”. See here and here for why.
So, despite the fact that millions of pounds of herbicides are used every year to kill them in lawns throughout the USA, dandelions can be used for food and as herbs, to make wine (“mellow yellow”), and even to make tires (or tyres).
And, There’s More!
If you’d like more information about these plants that are seemingly ubiquitous this time of year because of their bright yellow flowers, here are some online resources:
- And, finally, an Ode to a Dandelion.
1. Grossmann, K. (2007) “Auxin Herbicide Action: Lifting the Veil Step by Step”, Plant Signaling & Behavior 2:421-423. (PDF)
2. Grossmann, K. (2010) “Auxin herbicides: current status of mechanism and mode of action.” Pest Management Science, Vol. 66, pp. 113–120, (Abstract)
3. Song, Y. (2014) “Insight into the mode of action of 2,4-dichlorophenoxyacetic acid (2,4-D) as an herbicide.” Journal of Integrative Plant Biology, Vol. 56, pp. 106-113. (Full Text)
Up Next: The Selfish Plant – Part 3
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