Poison berries?

Ross Koning Koning at ECSUC.CTSTATEU.EDU
Tue Oct 17 13:51:43 EST 1995

At  2:59 AM 10/16/95 +0000, STACY A CIUFO wrote:

>Why then, do some plants go through the biochemical trouble to
>produce POISON fruit (such as berries)?


I am going to speculate, because I know of no
details for specific examples to assist me.

1. Poison fruits might not stay poisonous forever.
There are fruits rejected by birds all fall, but
over winter biochemical changes occur that render
the fruits palatable by spring.  This could be a
strategy to ensure sufficient ripening of fruits
and maturity of seeds before dispersal.  Some Viburnums,
Berberis, Parthenocissus, and Cornus could be some
possible examples (but again, this is speculation).

2. Poisons are not always generic.  It is possible that
a poison for one frugivore, might not be poisonous
to the seed disperser.  I am speculating that evolution
might result in a mammal-toxin that prevents humans
from eating and digesting both fruit and seed, but
which is not a bird-toxin, permitting birds to digest
fruit but pass seeds.  The mammal-toxin thus improves
the chances for seed dispersal via birds.  Possible
examples here might be Toxicodendron and Rhus vernix.

3. "Toxins" might just be adaptive too!  The reason
you don't want to eat Rhamnus cathartica fruits is
a cathartic drug.  The discomfort causes us to list
the fruits as "toxic" but in fact, by including the
cathartic, evolution provides the plant with a way
to ensure that seeds are carried away, but *not* digested
by a mammal.

There may be someone out there who can give details
on specific examples and the toxins used, but maybe
these ideas will get students to think on their own
about other strategies that evolution might have
provided.  Sounds like a thought provoking topic for
cooperative group activities!

Here is a parallel topic:
Try sprouting some lettuce seeds in fruit juices...
interestingly, though moist and warm and provided with
light, the seeds do not sprout!  The germination toxin
is to prevent premature germination in a warm, moist
fruit.  Abscisic acid may be the active agent in these
juices.  Thus tomato seeds in the rotten fruits in your
autumn garden do not sprout until the snow-melt and
cold wash away and degrade the Abscisic acid.  Then the
seeds are germinable in the spring.  Thus toxins can
be adaptive and their content can be reduced by a winter
of weathering and biochemistry.


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 \ Ross Koning                                  \
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