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Maybe you’ve noticed that those tomatoes you planted in your backyard aren’t doing so great.
If you planted them anywhere near a black walnut tree,
then you can take comfort in knowing that this time,
your inadequate gardening skills aren’t to blame.
You and your tomatoes could be the victim of sabotage.
But not the jealous neighbor kind, the botanical kind.
It’s called allelopathy, and it’s kind of like the plant version of Round-Up.
You see, some plants produce chemicals that can suppress or even kill certain other plants.
These are called allelochemicals.
They give the plants that produce them an advantage
when competing with other species for limited resources.
In other words, they help the plant bushwhack the competition.
One of the most well-known examples is the aforementioned black walnut tree.
The Roman naturalist Pliny the Elder was the first to observe that black walnut trees tend
not to have many leafy green neighbors, but now we know why they’re such lonely trees.
The tissues of the black walnut tree contain a chemical called hydrojuglone.
Hydrojuglone is found in the leaves and stems,
but it’s more concentrated in the walnut hulls, buds, and roots.
Now, hydrojuglone on its own is harmless, but when it’s exposed to air,
it becomes a toxic chemical called juglone.
Scientists are still figuring out exactly how juglone works,
and why it’s so toxic to the plants that are sensitive to it.
One of the ways juglone is thought to work is by getting into the mitochondria
in the roots of certain plants, which can prevent them from taking in oxygen.
Because the roots of the tree release juglone, juglone-sensitive plants
can’t flourish in the soil around the root system.
But the tree also drops leaves and nuts, and as these decompose,
they add even more juglone to the soils surface, which means the ground
under a black walnut tree's canopy is a
particularly hostile place if you are sensitive to juglone.
Now Black walnut trees aren’t the only plants to do this,
and juglone is not the only toxin that plants have evolved to make.
A cereal grain called sorghum has taken its sabotage underground.
Sorghum is farmed for livestock feed in the southern United States,
and is also used to make sweetener and ethanol.
But, sorghum exudes an allelochemical called sorgoleone from its root hairs.
Sorgoleone reduces the shoot growth of competing plants in a couple of different ways.
It’s thought to interfere with photosynthesis by binding to an important protein complex
called photo system II or PSII,
which inhibits the transportation of electrons that are necessary for photosynthesis.
Hank makes a PS5 Joke here, should we leave it?
It might also block an enzyme the plant use to make carotenoids,
compounds that collect energy from light.
And it seems to impede water uptake in the roots which is, you know, important.
Another plant vying for the title of “least friendly neighbor” is musk thistle.
This large, invasive plant is native to parts of Europe, Asia, and Africa
and now grows in pastures and along roadsides in much of North and South America,
as well as New Zealand and Australia.
The plant grows a rosette of leaves at the ground level in its first year,
then the following year it produces a stalk from which the plant’s purple flower grows.
These plants die after flowering once, and when the musk thistle rosette decomposes,
chemicals within the rosette impede nitrogen fixation
and upset the long-term nitrogen cycle in that area .
This inhibits the growth of certain pasture plants, and it’s especially harmful to legumes,
the group of plants that includes things like beans and peas.
This can pose a problem for farmers since we like
to grow and eat those things, so we want them to do well.
Another thistle-like plant called spotted knapweed
has roots that exude a type of allelochemical called catechin,
which prevents seed germination in competing species.
Catechin is so good at preventing germination that it even inhibits germination of spotted knapweed,
which scientists think might be a way for the plant to control its own population a
nd prevent competition over limited resources in a given area.
But, Allelopathy isn’t always foolproof.
Some plants are immune to allelochemicals.
We know that while tomatoes, cabbage, and eggplants are especially sensitive to juglone,
other foods like onions, parsnips, and snap beans are not affected by it at all.
What’s more interesting is that exposure
to certain allelochemicals may even help some kinds of plants.
Juglone, for example, has been shown to increase
the growth of young muskmelon plants and the germination of white oak trees.
For muskmelon, scientists think this might have something
to do with the cells that transport water and nutrients from the roots of the plant.
The radius of those cells seems to increase when
juglone is applied to the seeds before germination.
Juglone also seems to be associated with an increase in the protein content of the seedlings.
What’s really interesting is that these experiments
also tested the effects of juglone on cucumbers,
which are close relatives of muskmelons.
And the juglone had the opposite effect in the cucumbers.
Which is to say, it did what juglone usually does,
and suppressed their growth.
The researchers don’t have a clear idea why these two plants react so differently to juglone,
which is yet another reason we’re so interested in these allelochemicals.
Besides being a fascinating look into the cutthroat world of plant evolution,
allelopathy also has huge implications for horticulture.
Recent studies have suggested that commonly used herbicides like glyphosate
could be harmful to the health of humans and other animals.
But herbicides have helped us grow our crops efficiently for decades,
so to get rid of them completely would harm our food supply chain.
We need more herbicides and safer herbicides, and as it turns out, they’re right there in nature.
Scientists are still figuring out exactly how allelochemicals work,
so it might be a while before you’ll be able to pick up an
allelochemical herbicide at your local garden store.
In the meantime, make sure to plant your tomatoes a long, long way
from that black walnut tree.
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