Auxin and Ethylene in Leaf Abscission Auxin is synthesized in shoot apical meristem. Synthesized auxin is transported from there. Young leaves can also synthesize auxin. When leaves are young, the auxin produced by leaves is in balance with the auxin produced by the shoot apical meristem.

As leaves get aged, its ability of auxin synthesis declines. Therefore, a difference in auxin concentration between shoot and leaf ((  [auxin]) is established.

C2H2C2H2 When the  [auxin] is too big between leaf and shoot, ethylene is produced. Ethylene makes leaf and the cells of the abscission layer undergo programmed cell death (apoptosis).

C2H2C2H2 Because these cells have very thin walls and there is no fiber cells around the vascular tissue in this region, after their death the leaf can be easily separated from the shoot simply by the weight of the leaf.