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.