QUESTION

Jon Greenberg jongreen at BLUEMARBLE.NET
Sat Jul 10 23:50:15 EST 1999


The question and the answers have all been good and intersting to
follow. This is an angle on plant physiology that we don't often
consider. A coupe of other thoughts on this topic:

1) Plants release a lot of organic material (I've heard estimates of up
to 12% or more of total photosynthate) as root exudates. Although I
don't know of any research on this, I would expect that this could be a
good way to get rid of metabolic wastes.

2) I think the fact that plants are autotrophs has allowed the
discussion to dismiss the issue of excretion too easily. Remember that
even microorganisms (presuambly including autotrophs) excrete metabolic
wastes. I'd be surpised if there aren't pathways that produce solid or
soluble metabolc wastes (ie., other than water and gases) in plants.
Think about nucleic acid and protein turnover, for example. In mammmals,
some purines get recycled via a salvage pathway, but others are broken
down and released as uric acid. Then you have the ornithine and urea
cycles, too. Plants also experience protein and nucleic acid turnover.
These mechanisms are not just a luxury that animals can afford; they
also protect against errors in DNA and protein synthesis, as well as
exogenous (parasitic, viral, etc.) proteins and nucleic acids. Just
reusing all amino acids and nucleotides from broken down protein, DNA
and RNA seems risky, as it can keep damaged (chemically modified)
monomers in the system. Surely plants must have a way of dealing with
this. Does anyone know how it works?


Jon Greenberg
David Hershey wrote:
> 
> 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

-- 

Jon Greenberg, Ph.D.            Curriculum Development
Science Education Consultant    Program Evaluation
719-477-0160                    Preservice & Inservice Teacher Education
mailto:jongreen at bluemarble.net



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