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[APD] Re:plant/algae competition

Roger wrote:
>>I have an article coauthored by one of the most respected limnologists
in which the authors suggest not only that this is a mechanism for algae
growth on leaves, but that it is an evolved-in defense mechanism.  The
"intentionally" promote growth of algae on their leaves so that herbivores
more likely to eat the algae and less likely to eat the leaves.
I won't comment on whether that makes a lot of sense, but that is what they

That's an interesting idea Roger. I think it is likely that different plants
have many different self defense mechanisims. What works for one plant does
not work for another. If promoting leaf algae growth to protect against
herbivores is for self protection, it is also self destruction in the long
run and in the short term for some plants. Most of us have seen how algae
can destroy some plants, at the very least it can block nutrient uptake
through the leaves and interfere with photosynthesis.  Plant leaves have an
unstirred layer of still water which gas and nutrients must pass thru. This
is called the Prandtl boundary, and is about 0.5mm thick. When leaves are
encrusted with sediment or algae, it makes it that much harder for nutrients
and gases to pass thru.

Allelopathy I am sure is a self defense mechanisim to prevent other plants
from crowing out that specie, and/or  in some cases algae as well. Hornwort
has been well documented for retarding algae, but not nessasarily other
plant species. Tropica has the Hornwort study on their WEB site.  But this
does not mean just by sticking one bunch of Hornwort in your tank you will
not have any algae. This is what is misleading. The affects are very small.
I had a 30 gallon tank that had mostly Hygrophila polysperma in it with
moderate light. Without spending much of any time with the tank or tinkering
with it, it constantly had green algae. I decided to see what would happen
if I added some hornwort, but changed nothing else in the routine of the
tank. One bunch grew for 3 months...algae seemed to slow somewhat, but
barely noticeable. Then I removed all of the hygro and had nothing but
hornwort in the tank. Instead of only one bunch, I had a whole tank full of
it. Within two weeks the algae growth slowed down to barely being
noticeable. A month later, some layers of the hornwort began to die, and it
shed much of its leaves in some areas. That seemed to release even more of
the alga toxins, and from then on for the next six months there was never
any sign of algae whatsoever. I tore the tank down at that point.

So, unless you want to grow nothing but Hornwort, don't expect to see a huge
impact. I suspect that any Allelopathic chemicals released by plants will
have the same affect: very miniscual unless you have a large amount of the
Allelopathic plant in proportion to the tank size.

Robert Paul Hudson

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