Why do plants make these drugs?

Can someone explain to me what biological function is performed by the drugs
certain plants produce?



For example, I understand that nicotine is a poison produced by certain
plants (like tobacco) to kill predators.

Why does the opium poppy produce opiates/alkaloids in its sap?

What about cocaine? Why does a plant produce what is cultivated as a
stimulate?

Thanks.

Secondary compounds like the ones you mentioned discourage herbivores
from eating the plant.

This webpage provides a good explanation:

[Only registered users see links. ]

David R. Hershey


"Bob" <[Only registered users see links. ]> wrote in message news:<4002414b$0$88848$[Only registered users see links. ].net>...

In article <4002414b$0$88848$[Only registered users see links. ].net>, [Only registered users see links. ] [Bob]
wrote...


They and many others may be a lot like the nicotine example. Plants
generally live in environments full of other organisms, including
parasites like nematodes, plant-feeding insects and large vertebrate
herbivores, and disease-causing organisms like parasitic fungi and
bacteria. Plants are also often quite good at cooking up and storing
chemicals. It makes sense that many plants will be producing
biologically active compounds that tend to affect other organisms in
various ways.

Their possible effects on humans or other large mammals may often not
be what was being selected for, it might just be a side effect of the
plants "trying" to be less tasty to insects or fungi. Still, the
plants you mentioned probably won't be good food for large mammalian
herbivores either.

cheers

David Hershey <[Only registered users see links. ]> schreef
from eating the plant.



+ + +
This indeed appears to be a careful introduction on this extremely complex
topic. Secondary metabolites are not only involved in directly discouraging
herbivory, but can also affect soil (eg hindering growth of competing
plants) or serve as a signal.
PvR

"P van Rijckevorsel" <[Only registered users see links. ]> wrote in message news:<4003bb1f$0$20196$[Only registered users see links. ]>.. .

There are also other functions for secondary compounds. Some plant
roots excrete chelates (phytosiderophores) that make iron more
available.

Plant use of secondary compounds to inhibit growth of competing plants
(allelopathy) seems to have been exagerated by uncritical observations
in natural ecosystems and unrealistically high doses of
allelochemicals in artificial lab or greenhouse studies.

Taiz and Zeiger (1991) state that "Many scientists doubt that
allelopathy is a significant factor in plant-plant interactions
because good evidence for this phenomenon has been hard to obtain."

St. John (1999) stated that "There are many potential alternative
explanations that are rarely or never addressed in allelopathy
experiments."

One famous photo by Muller (1965) that once appeared in many botany
texts stated that allelopathy was responsible for a 2 meter wide bare
soil zone around the a California shrub (Salvia leucophylla). However,
a subsequent study by Bartholomew (1970) found that fencing the shrubs
prevented the bare zone. Mice, rabbits and birds hiding in the shrubs
and feeding on nearby surrounding plants were responsible for the bare
zone, not allelopathy. Secondary compounds excreted by roots often are
inactivated by soil microbes or adsorbed to soil particles so
allelopathy is probably less important in natural systems than often
portrayed.

References

Bartholomew, B. 1970. Bare zone between the California shrub and
grassland community, Science 170 1210-1212.

Muller, C. H. 1965. Inhibitory terpenes volatilized from Salvia
shrubs. Bull. Torrey Bot. Club 92: 38-45.

St. John, T. 1999. Nitrate Immobilization and the Mycorrhizal Network
for Control of Exotic Ruderals. CalEPPC News 7(1): 2-6.

Taiz, L. and Zeiger, E. 1991 Plant Physiology. NY: Benjamin/Cummings.

> "P van Rijckevorsel" <[Only registered users see links. ]> wrote>
complex topic. Secondary metabolites are not only involved in directly
discouraging herbivory, but can also affect soil (eg hindering growth of
competing plants) or serve as a signal.
PvR

roots excrete chelates (phytosiderophores) that make iron more
available.

(allelopathy) seems to have been exagerated by uncritical observations in
natural ecosystems and unrealistically high doses of
allelochemicals in artificial lab or greenhouse studies.

allelopathy is a significant factor in plant-plant interactions
because good evidence for this phenomenon has been hard to obtain."

explanations that are rarely or never addressed in allelopathy
experiments."

texts stated that allelopathy was responsible for a 2 meter wide bare
soil zone around the a California shrub (Salvia leucophylla). However,
a subsequent study by Bartholomew (1970) found that fencing the shrubs
prevented the bare zone. Mice, rabbits and birds hiding in the shrubs
and feeding on nearby surrounding plants were responsible for the bare
zone, not allelopathy. Secondary compounds excreted by roots often are
inactivated by soil microbes or adsorbed to soil particles so
allelopathy is probably less important in natural systems than often
portrayed.


grassland community, Science 170 1210-1212.

shrubs. Bull. Torrey Bot. Club 92: 38-45.

for Control of Exotic Ruderals. CalEPPC News 7(1): 2-6.


+ + +
Caution is always good when making general statements.
Still, it is quite common to see references of walnuts affecting other
plants:

[Only registered users see links. ]

[Only registered users see links. ]

[Only registered users see links. ]

It appears well-accepted that some such effects do exist.
PvR

Another thought--Some chemicals have the effect of selecting for a
particular kind of animal to disperse the fruit.

Example: Capsicum (chile pepper) fruits are loaded with capsaicin,
which renders them unpalatable to most mammals. Birds aren't affected
by capsaicin. They relish the fruits and disperse the seeds to
greater distances than mammals might manage.

M. Reed

Bob wrote:

"P van Rijckevorsel" <[Only registered users see links. ]> wrote in message news:<4005b00a$0$59457$[Only registered users see links. ]>.. .

If you read what I said, I did use caution, e.g. "allelopathy is
probably less important in natural systems than often portrayed." and
"Plant use of secondary compounds to inhibit growth of competing
plants (allelopathy) seems to have been exagerated by uncritical
observations in natural ecosystems and unrealistically high doses of
allelochemicals in artificial lab or greenhouse studies."

Juglone from walnut does affect some plants but if you read the list
of plants on the websites you cited, it seems more of an agricultural
phenomenon because the affected plants are often cultivated species
such as tomato, potato, bean and corn. Are tomato, potato, bean and
corn really strong competitors in the natural habitat of black walnut?

You really have to read the actual research literature and see how the
researchers got data for the "allelopathic effects." The websites you
list cite no research literature and say their lists of
juglone-susceptible plants are preliminary and not based on research.
Some of the vague statements made indicate the webpage authors(s) have
probably never read any of the research literature on juglone and
allelopathy, e.g. "At the physiological level, juglone interrupts the
metabolic processes of susceptible plants and causes their demise."

I have read several allelopathy research articles and often
allelopathy research is based on artificial experiments involving
unnaturally high doses of crude extracts sprayed on seedlings in a
lab. That is very unnatural. Such research also rarely considers other
effects such as toxic levels of mineral nutrients that can be present
when plant tissue containing allelopathic chemicals is ground up and
sprayed on other plants.

My parents have two large black walnut trees in their yard and all
sorts of flowering perennials thrive under them. The second website
you listed has an extensive list of "Plants That Do Not Grow Within 50
Feet of Drip Line of Black Walnut." However, some of the plants on the
list, such as lilac and privet, thrive under the black walnuts in my
parents yard so that list is not completely accurate in the real
world.

Lots of plants won't thrive under black walnuts because of the shade
or competition from walnut roots for water and mineral nutrients.
Those competitive effects are very difficult to separate from any
allelopathic effects in a real situation. That is why so much
allelopathic research is based on artificial lab experiments that do
not always translate well into the real world.

Purdue University [Only registered users see links. ]
indicates the poor state of knowledge on juglone sensitivity of
plants, i.e.

"Few plants have been experimentally tested for tolerance or
sensitivity to juglone."

I agree with you that it is "well-accepted that some such
[allelopathic] effects do exist."

However, many things are "well-accepted" even when they are untrue,
e.g. the supposed allelopathy of Salvia leucophylla, which was really
caused by feeding of rabbits, mice and birds.


David R. Hershey

David Hershey <[Only registered users see links. ]> schreef

+ + +
What I wrote was "Caution is always good ...", which I certainly did not
mean to imply you were not cautious.
+ + +

of plants on the websites you cited, it seems more of an agricultural
phenomenon because the affected plants are often cultivated species
such as tomato, potato, bean and corn. Are tomato, potato, bean and
corn really strong competitors in the natural habitat of black walnut?

researchers got data for the "allelopathic effects."

+ + +
This is all relative.
When dealing with a statement in the popular press it is almost imperative
to go back to the literature. When dealing with widely used manuals one can
always hope that these have their statements right. Going back to the
literature has dangers of its own (How widely to read? Are there different
schools of thought? Some publications have since been irrevocably shown to
be wrong, but which ones? Etc)
+ + +

list cite no research literature and say their lists of
juglone-susceptible plants are preliminary and not based on research.
Some of the vague statements made indicate the webpage authors(s) have
probably never read any of the research literature on juglone and
allelopathy, e.g. "At the physiological level, juglone interrupts the
metabolic processes of susceptible plants and causes their demise."

+ + +
Yes, that is a really beautiful sentence.
+ + +






+ + +
That site appears to add little (beyond the quite novel fact that black
walnut was known to the Romans), being a summary, repeating what is stated
elsewhere (including that lilac and privet do not grow under black walnut).
The sentence you quote is the only statement of doubt.

I rather like this one better (from the third site originally listed):
[Only registered users see links. ]
which incidentally includes lilac as a walnut-tolerant plant.
+ + +




+ + +
I hope we can agree that this is a complex topic, not lending itself to
sweeping statements, but that almost certainly allelopathic effects do
exist.
PvR

"P van Rijckevorsel" <[Only registered users see links. ]> wrote in message news:<4007b9d3$0$95833$[Only registered users see links. ]>.. .

No, I don't think it is all relative as you say. Given that
allelopathy is a controversial area, a botanist needs to read the
original research literature. The botanist also needs to use logic and
scientific skepticism to determine if the experimental data actually
support a supposed case of allelopathy. Not all of the scientific
literature is correct. That is why a botanist has to carefully analyze
it. You don't necessarily have to read every article or all the very
old literature. Provide just one literature citation on allelopathy
research, and I'd be glad to analyze it and discuss it.

You said "The sentence you quote is the only statement of doubt." The
quote was

"Few plants have been experimentally tested for tolerance or
sensitivity to juglone."
<http://www.agcom.purdue.edu/AgCom/Pubs/HO/HO-193.pdf>

That is a very big statement of doubt!

Where did the webpages and reference books get the information on
juglone sensitivity if it was not based on experiments? The
information appears to be anecdotal. I'm sure if I talked to enough
gardeners, I could come up with a long list of plant species that died
when they were planted under Norway maple trees. That doesn't mean
Norway maple produces allelopathic chemicals.

Here's an interesting discussion by gardeners about "The Black Walnut
Problem." Many contributors disagree with the lists of juglone
susceptible plants.
<http://forums.gardenweb.com/forums/load/woodland/msg050035022096.html>

David R. Hershey