Much like a ripe apple is easily plucked from the branch, so too are these “ripe” leave easily pulled from the branch by a gentle breeze as fall progresses. Rather than being actively shed, it’s probably better to think of leaves as actively maintaining their grip on the twig during the growing season. While the leaves deconstruct, decreasing day length inhibits the production of a third hormone: auxin. Throughout the summer, leaves produce copious amounts of auxin, which generally stimulates cell growth and elongation. Auxin gets piped back down the leaf through the petiole to the twig and other parts of the plant. At the place where the petiole connects to the twig, auxin keeps the connective cells, which have a propensity towards self-destruction from, well, self-destructing. Under stress, as in the fall when light levels drop, the leaves produce less auxin and these cells slowly begin to kill themselves off. Cells on the branch side of this abscission zone secrete a waxy cuticle that eventually corks off twig and protects it from pathogens and parasites (this is zone is the leaf scar).
It is then a race for the plant to retrieve as much of the nutrients and sugars from the leaf as possible before the leaf is shed. Generally, leaves run a pretty balanced race and are good at coordinating the timing of leaf senescence and leaf shed. As I mentioned in the last newsletter, chlorophyll breaks down much quicker than carotenoids and anthocyanins so the leaves on the forest floor are often shades of yellow, orange, or brown. In the next newsletter we’ll look at insects that hijack this process.