....whose leaf shall not fade, neither shall the fruit thereof be consumed: it shall
bring forth new fruit according to his months, because their waters they issued
out of the sanctuary: and the fruit thereof shall be for meat, and the leaf
thereof for medicine.
Ezekiel 47:12
The place where the leaf separates (abscises) from the tree is typically located at the base of the leaf stalk (petiole). It is called the abscission zone (AZ). The AZ is no random fracture point but is actually built-in, “pre-positioned” during leaf formation.
With the post-summer hint of coolness in the air, before the onset of wintry weather, trees initiate a “senescence sequence” to systematically retrieve the re-usable resources from the leaves. As this process begins, and the green chlorophyll pigment and other parts of the light-harvesting (photosynthetic) complex are dismantled, the leaf changes colour.
Then, when about half the chlorophyll has been degraded, and as the level of phosphate in the leaf drops, the production of anthocyanin pigments increases.
With the abscission process triggered by a raft of chemical signals (including, it is believed, ethylene produced by the internally-gutted leaf), AZ cells start to secrete enzymes. These dissolve the ‘glue’that holds cells together and degrade the primary wall between cells. The surrounding AZ cells actively produce the necessary abscission materials throughout; i.e., they remain alive and active until abscission is completed.
As cell walls pull apart, opening up the fracture line, the AZ cells on the tree side close off the opening wound by depositing protective materials such as tyloses, suberin and lignin. This strong protective boundary zone seals off the leaf scar, defending the tree from the cold, as well as from diseases and pests.
With the sealing-off process completed, the leaf can now be safely shed.
If any one of these steps in the signalling cascade is absent, the abscission process doesn’t work."
CMI