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compensation point and senescence

John Skillman jbs11 at psu.edu
Tue Nov 18 11:23:53 EST 1997

We appreciate all the responses to this query and I realize that I have alot of reading to do!  But 
let me continue on with this line of questioning...what are the mechanisms?  Senescence is a careful 
sequential dismantling of the leaf while it continues to perform essential functions--gene 
expression, protolysis, respiration, DNA repair, scavenging of oxidants, maintenance of 
compartmentation etc. etc.  Does this happen during "starvation"?  Does it happen equally under low 
CO2 and under very low light?  If so how does the leaf/cell know that it is carbon limited and that 
it is time to engage the senescence program?  If leaves below the compensation point do engage the 
"normal" senescence program then it runs counter to the model that some measure of cumulative 
photosynthesis is what turns on the senescence genes.  
By the way, I am hearing reports that mature leaves can continue to be sinks or the source/sink 
polarity can be reversed.  I am told that this has been shown in experimentally shaded poplar leaves 
in studies on wound signal transmission and it has been shown in galled leaves.  Now of course this 
can't be too common of a phenomena or the dogma of "once a source always a source" would not be so 
deeply held and, as Mark points out, trees would eventually "shade" themselves to death. 

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