This is a pretty typical view
during the summer
across fields in the Corn Belt of the U.S.
But this is a not so typical view.
From ground level we can see the neat,
orderly rows of corn
separated by soil that's pretty much
completely free of weeds.
How is that?
Well, many large-scale corn
growers in the U.S.
take a little help from an herbicide called
atrazine to get these weed-free fields.
Atrazine is one of the most common herbicides
used in agriculture in the U.S.
An herbicide is a chemical that is toxic to
plants
and is used to prevent or slow growth.
Every year atrazine is applied to over
50 million acres of agricultural land,
mostly corn fields in the Midwestern U.S.
This amounts to about
70 million pounds of atrazine.
It doesn't kill the corn,
but it does kill lots of the common weeds
you would find growing in cornfields.
Let's take a look at what happens to a plant
that's susceptible to the killing effectsof atrazine.
We can set up a little experiment
using wild clover,
a common plant that might
be growing in cornfields.
If we treat the plant with a commercial
herbicide containing atrazine
the results are pretty dramatic.
We went from healthy and green
to shriveled and brown.
So what is the atrazine doing to the plant?
Why is the plant struggling to survive?
Let's compare a healthy plant and
a plant treated with atrazine.
We pretty much see all the
normal pieces and parts.
So it must be doing something at
a cellular or sub-cellular level.
Something we just can't see.
Let's zoom in a little bit.
If we take a look at a leaf from
a healthy and a sick plant.
And we look even closer,
to see cells in those leaves.
And, now we look even
closer at those cells.
That little thing right there is a chloroplast.
That is atrazine's target.
It's poisoning something in that chloroplast.
Now, you probably already know that
the chloroplast is the site of photosynthesis
but we need to dig in a little deeper.
Let's visualize what's happening during
photosynthesis in a healthy chloroplast.
Here we have the major components
involved in photosynthesis:
two photosystems that absorb light energy
from the sun using chlorophyll,
an electron transport chain,
and an ATP synthase.
Photosystem II has this enzyme that can
strip electrons away from water
leaving behind waste oxygen.
Those electrons are energized by light
and passed on to the electron transport chain.
The electron transport chain passes the
electrons on to Photosystem I
where they are re-energized by light
and passed on to the electron carrier NADPH.
So those electrons that started on water
have now ended up being carried by NADPH.
Now, as the electron transport chain
is passing electrons it's also moving protons around.
The ATP synthase exploits that buildup
of protons to make ATP,
the energy carrier molecule in the cell.
Now we can use carbon dioxide from the air,
along with the ATP and NADPH to fuel the Calvin Cycle.
This cycle uses the energy from ATP to transfer
the
electrons from NADPH to carbon dioxide,
building the molecule glyceraldehyde-3-phosphate
(or G3P).
That's used as a building block for sugars
that will be used by the plant
for energy and growth,
basically keeping it alive.
So what happens with the atrazine?
Let's focus on this part of the pathway.
Atrazine works by stealing the electrons
from photosystem II.
That means photosystem II has no electrons
to send to the electron transport chain,
no electrons reach photosystem I and no electrons
are
available to get passed on to produce NADPH.
If no electrons are being passed along the
electron transport chain
then no protons are getting moved around,
without the buildup of protons the ATP
synthase just isn't able to make ATP.
So if we can't make sufficient NADPH or ATP,
then we don't have the electrons or the energy
needed by the Calvin Cycle to make the sugar
that's needed for the plant to survive.
So that's how atrazine poisons plants.
By targeting the chloroplast and stopping
photosynthesis.
By poisoning that first piece in the pathway,
the whole system shuts down.
And that's the premise for how most herbicides
work,
by targeting the critical components in photosynthesis
to shut it down and kill the plant.
