leaf senescence

[John Skillman]

I am interested in the mechanism of senescence in leaves of plants and the more general question of 
how senescence in clonal organisms compares to that of unitary (non-clonal) organisms.  Here is a 
question I put to the photosynthesis newsgroup.  Perhaps the followers of this group will also have 
some thoughts on this...
> 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