Much can be found online about why leaves change colors in the fall…
…but relatively little about the final part of the story, namely leaf drop.
Components of this terminal process, called leaf abscission, are actually put into place at the beginning of the leaf’s life. At the end, a chemical signal from the leaf itself triggers this “end game”, a sequence of events involving the self-digestion of plant cells.
Abscission is not restricted to autumn leaves, however. It is a process plants use to discard spent flowers (see below), ripening fruits, and other dead or diseased parts.
Relevance anyone?: Some of the enzymes involved in this process may have relevance to commercial biofuel production from plant biomass. (Much more about this later on.)
The Pre-Fab Structure:
The Abscission Zone (AZ) consists of several layers of cells typically located at the base of the leaf petiole. The first step in abscission, then, is the formation of the AZ during leaf development. The second step occurs when the AZ gains the ability to respond to the gaseous plant hormone that triggers abscission.
The Plant Hormone:
The third step in this process involves ethylene, a plant hormone long implicated in plant aging (senescence). The generally accepted story is that ethylene is produced in aging leaves, which, in turn, acts as a chemical signal to cells in the AZ to begin the process of leaf abscission.
Ironically, there is another plant hormone called Abscisic Acid (ABA) that originally was thought to trigger abscission in plants. This turned out NOT to be the case, but, alas, the name stuck.
The final stage in the process of leaf abscission is the digestion of the plant cell walls in the AZ. This weakens the petiole in the region of the AZ, thus producing a separation point. Enzymes that digest the cellulose, hemicelluloses, and pectins in the plant cell walls are produced and then secreted by the cells in the AZ.
It’s interesting to realize that plants apparently have genes that code for enzymes that can literally be used to digest themselves.
These genes apparently code for proteins that act in a sequential manner, culminating in abscission.
Bottom line: Abscission is used by plants as a way to jettison aging or diseased parts. Understanding the cellular and genetic mechanisms involved in abscission may have significant economic implications in horticulture (e.g., longer flower life) and agriculture (e.g., simultaneous fruit drop.)
1. Cho, S. K., et al. (2008) “Regulation of floral organ abscission in Arabidopsis thaliana.” Proceedings of the National Academy of Sciences (USA), vol. 105, pp. 15629-15634. (Full Text)
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