QUESTION

David Hershey dh321 at excite.com
Sat Jul 10 21:12:03 EST 1999


That's a good explanation. Because plants synthesize all the organic
compounds they need from carbon dioxide, water, and essential mineral
nutrients, it is reasonable to expect that plants should not synthesize
toxic organic compounds unless they have ways to deal with them. 

Plants often do have a major problem with toxic elements in their
environment, and species vary widely in their ability to exclude them or
to tolerate them after they have been absorbed. Boron is an essential
element that is toxic at slightly higher levels than needed to prevent a
deficiency. Parallel-veined monocot leaves are efficient in carrying
most of the excess boron to the leaf tip which helps to limit tissue
damage to the tips. Astragalus plants accumulate large amounts of
selenium but detoxify it by forming nontoxic seleno-amino acids that are
not incorporated into proteins. In selenium intolerant species, selenium
replaces sulfur to form the toxic selenomethionine which is incorporated
into proteins. Some plants have adapted to salty soils by developing a
mechanism to excrete salt onto leaf surfaces or into special salt
bladders on the leaf.

If plants do produce organic compounds that are toxic, either as
unintended byproducts or as allelochemicals (those used to inhibit
growth of other plants) they may store them in vacuoles where they will
not cause damage. Besides shedding of plant parts, woody species could
possibly store waste materials in old, nonfunctional xylem although I do
not know if there has been research on that.

David Hershey
dh321 at excite.com

Bill Williams wrote:
> 
> I would love to hear a definitive answer to this question; my
> introductory-biology students ask similar questions frequently.  Here's my
> answer:
> 
> Toxic metabolic products arise because the food that organisms consume
> doesn't precisely match the organisms' needs.  Urea is an excellent
> example:  heterotrophs necessarily consume complete organisms, or at least
> complete cells, but the ratio of calories to nitrogen *in* organisms is
> much, much lower than the ratio of calories to nitrogen *needed* in food.
> Thus, they have a constant excess-nitrogen problem and excrete urea (or
> uric acid, or ammonia, or some other nitrogenous waste).  But the world of
> autotrophs is completely different:  they obtain energy from the sun (which
> creates is own problems and gives rise to numerous metabolic pathways for
> dissipating excess energy) and materials from the air and soil solution.
> In general, such organisms simply don't take in materials that they don't
> need, or at least that they cannot use without poisoning themselves.
> 
> So:  if you don't want to bother with kidneys and livers, don't eat!
> 
> -W2
> 
> At 08:54 -0700 7/9/99, Santosh Baburao Mane wrote:
> > I am pharmacy student , we study about plants ,and use of plant as
> >sources of drugs
> >
> > So my question is," why plants are able to survive with out excretory
> >system that is  kidney or liver
> >like organs ???" They do not need detoxification ?? why metabolic products
> >are not able to give toxicity
> >to plant and why they induced toxicity to animal cells only .
> >  plants cells having any mechanism  for detoxification or is it
> >natural???
> >
> >So pl help me to finding out answer  of this
> >
> >
> >pl mail me ans.
> >
> >Thanks
> >Santosh
> >San_dha at giaspna.vsnl.net.in
> 
> ________________________
> William E. Williams
> Biology Department
> Saint Mary's College of Maryland
> 18952 E. Fisher Rd.
> Saint Mary's City, MD  20686-3001
> (301)862-0365
> 
> Summer:  Botany  Department
>          Aven Nelson Building
>          University of Wyoming
>          PO Box 3165
>          Laramie, WY 82071-3165
>          (307)766-6293



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