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The engineering process begins with a protein called abscisic acid, or ABA, that helps plants acclimate to stressful changes in the environment.

Not abscisic acid, the one that “abscission zone” would seem to imply.

Evergreens don’t lose their foliage, but they do crank up production of abscisic acid, or ABA, the hormone that triggers leaf fall in deciduous trees.

It is produced in an extracellular space between the plasma membrane and cell wall called the apoplast, in response to a range of factors, including stressors, plant hormones such as abscisic acid, and physical or chemical changes outside the cell1.

The older trees had lower levels of a growth hormone called indole-3-acetic acid and higher levels of a growth-inhibiting hormone called abscisic acid.