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How does the herbicide 2/4-D work?

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Michael Davis

on 27 February 2015

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Transcript of How does the herbicide 2/4-D work?

Flowering plants are divided into two main categories: "Monocots" & "Dicots"
Dicots
two seed leaves
can be woody
secondary growth
branched leaf veins
usually taproot system
Monocots
only one seed leaf
not woody
parallel leaf veins
fibrous roots
A
herbicide
is a chemical used to kill plants or stunt their growth
Systemic herbicides are absorbed (through roots and leaves) and transported throughout the plant.
2,4-D is a synthetic
auxin
, which is a class of plant hormones.
2,4-D, or
2,4-Dichlorophenoxyacetic acid
is a
systemic
and
selective
herbicide.
Monocots vs. Dicots
What is a herbicide?
What is 2,4-D?
How does the herbicide 2/4-D work?
How does it work?
Generally, this classification is based on the number of "cotyledons" in the embryo of a given plant species.
As the names suggest, "monocots" have only one "cotyledon" in their embryo, while "dicots" have two.
Cotyledons
essentially the first embryonic leaves of a plant
serve different purposes in monocots and dicots
In Dicots:
similar in function to regular leaves
photosynthetic
In Monocots:
highly modified leaf
forms tissue that absorbs nutrients
forms a protective cap for the precursor to stems and leaves
The crown (or growing point) of the plant is located at soil level.
The crown is located above the soil at the end of the stem.
There are selective herbicides which kill certain plants while leaving others unharmed.
Contact herbicides kill plants by contact & don't need to be absorbed. Uniform spray coverage is essential because contact herbicides only kill the plants they are directly in contact with.
Some herbicides act by interfering with the plant's growth, and are often based on plant hormones.
Others disrupt the process of photosynthesis or the creation of important amino acids.
It was the first selective herbicide, as it attacks dicots while leaving monocots alone.

So it can be sprayed on grasses and will only kill the broadleaf weeds.

Agent Orange
!
2,4-D is absorbed through the leaf pores (stomata) and is translocated to the meristems of the plant.
The synthetic hormone in 2,4-D encourages the broadleaf plant to grow unsustainably, which results in stem curl-over, leaf withering, and eventual plant death.
The auxin increases cell wall plasticity.
These factors cause cells to divide and uncontrollable growth ensues, leading to tissue damage and eventual death of the plant.
Auxins are required to stimulate growth in the plant cells as well as cell division.
Systemic: 2,4-D
Contact: Aim EW
There are also non-selective herbicides, which kill without discrimination. RoundUp is an example of this.
Why aren't monocots affected?
The best explanation is that monocots have a better ability to deal with excess auxins.

While dicots get "clogged" with the effects of the extra hormone, monocots create accessory pathways that prevent the buildup of auxins.
Essentially, the accelerated growth of these cells takes up much of the available room in the plant.
Creates blockages that prevent the transfer of water and nutrients throughout the plant.
Meristem is a tissue in the plant where growth can occur.
Monocots rapidly metabolize these hormones, breaking them down before they reach the meristem.
However..!
Be careful when selecting herbicides that include 2,4-D
and
other active chemicals.

Dicamba
is a herbicide included in
Killex
along with 2,4-D.
Dicamba
has soil activity which will damage bentgrass greens.

Bentgrass greens generally have low cut heights and have low resistance to drought, making it susceptible to environment damage.
Examples:
Synthetic auxins: 2,4-D
Photosystem II inhibitor: Atrazine

A-2, A-10, A-17, A-18, D-11, D-12
A-4, A-15, D-1, D-3, D-5, D-9
A-8, D-7, D-10, D-16, D-19
A-13, D-6, D-14
The End
Full transcript