Coordinating Fall Colors

The purples, reds, yellows, and oranges that decorate our deciduous trees in the fall indicate an underlying chemical change within each leaf. Some of these colors are produced as summer turns to fall to protect the leaf while it breaks down (these are the anthocyanins which produce reds and purples) while others are colors that are present throughout the summer, but only revealed as the dominant greens of the chlorophyll break down (revealing carotenoids, the oranges and yellows). Leaf senescence (the degenerative process where leaf components, like macromolecules and organelles are broken down and pumped out of the leaf) isn’t like aging in humans. Instead, the process is very much an active one, initiated by the leaf and controlled by plant growth hormones in response to changing light levels. As the leaf is self destructing, so too is the branch actively preparing to shed its leaves.

Waning daylength seems to be the primary driver that controls when a leaf enters senescence. But how exactly a leaf determines the right timing and communicates this to its cells isn’t totally clear. It does appear to be influenced by the gaseous plant growth hormone, ethylene. Ethylene is mostly associated with ripening of fruit, which you’ve likely experienced if you’ve ever put a green banana in a bag to make it ripen quicker (ethylene released by the peel builds up in the bag and promotes ripening).

For our trees in fall, it’s the leaves that “ripen.” Without a paper bag, they rely on another hormone, abscissic acid (ABA), which tells the stomata to remain closed. Stomata, tiny pores on the surface of a leaf that allow for gas exchange (mostly to take in CO2 and release O2), trap ethylene within the leaf. The build up of ethylene triggers leaf senescence.

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.