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compensation point and senescence?

John Skillman jbs11 at psu.edu
Fri Nov 14 13:10:15 EST 1997

Ahh yes I see the source of the confusion--I was thinking more about the
light compensation point when a shaded leaf is no longer a net source of C
to the plant. ...a hastily written note.  However the CO2 compensation
point is still relevant.
The interest for us is on the role of resource supply on leaf longevity and
senescence.  What does starvation mean in a leaf?  High-light leaves have
shorter lifespans compared to low-light leaves.  Likewise, in some studies
it has been shown that leaves on plants grown under CO2 enrichment senesce
early.  This is consistent with various theories on the relationship
between metabolism and senescence for unitary (i.e. non-modular and
non-clonal) animal systems.  Thus there is a theoretical basis to the
observation that reduced caloric intake can extend life in animals.  If
leaves are put in the dark they quickly die.  However, dark induced loss of
function in leaves does not appear to be the same as "normal" senescence.
This isn't too surprising really.  (And likewise keeping all food and water
from an animal will NOT extend its life!) But what happens to leaves under
very low light (i.e., below the light compensation point)?  Or very low
CO2?  The text book explanantion is that they just starve to death.  Is
this true?  Does the leaf start respiring away every suitable and available
carbohydrate and lipid in the leaf until they are all gone?  Is there any
conservation of respiratory substrate for the generation of energy for use
in leaf functions other than C assimialation?  Clonal plants can share
resources. Shading an older ramet while leaving a new daughter ramet in
full sun can reverse the source/sink polarity so that the older ramet
acquires C from the dauhter ramet.  Why doesn't the same thing happen in
leaves?  Or are the textbooks wrong?  Do "starving" leaves still breakdown
in an orderly and regulated way allowing remobilization and export of
nutrients for use in other parts of the plant?  Can "starving" leaves send
hormone or hoirmone-like signals to other parts of the plant in the event
of insect or pathogen attack?  Can "starving" leaves turn on genes and
upregulate enzymes necessary for these regulated processes?

At  5:27 PM 11/13/97 -0500, Janice M. Glime wrote:
>If a plant is in a situation where CO2 concentration is below the CO2
>compensation point, it means the plant will use more CO2 in respiration
>that it will fix in photosynthesis.  Yes, in a sense it does starve.  I am
>having difficulty interpreting your question about becoming a sink.  If it
>has too little CO2, how could it become a sink for CO2?
Reduced carbon--not CO2.
  Help me out with
>your thinking here - perhaps I can help a little more.
> >
>> Here's a question Doug Bielenberg and I have been pondering that might
>> be interesting for the group to consider:
>> What is the mechanism by which a plant recognizes that a leaf is below
>> the CO2 compensation point?  This is often used to explain why shaded
>> leaves begin to senesce.  Why doesn't the leaf just become a sink for
>> carbon?  It doesn't just starve does it?  If the CO2 compensation point
>> or a negative carbon budget is the cue for leaf senescence what is the
>> mechanism?  It may be pertinent to recall that leaves can have functions
>> other than just carboon acquisition; e.g. nutrient stores used for
>> growth elsewhere in the plant, transpirational "sinks" driving nutrient
>> uptake, modifiers of canopy microclimate, etc. etc.
>> Any ideas on this topic?
>> John Skillman

John Skillman
209 Buckhout Laboratory
Pennsylvania State University
University Park, PA 16802
Tel: 814-863-7399
FAX: 814-863-7217
email: jbs11 at psu.edu

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