Autumn leaves don’t Fall (by accident)
Posted on homepage: 9 November 2013 (GMT+10)
In Autumn, deciduous1 trees don’t lose their leaves—they loose them.
It is the final step in a highly ordered and carefully controlled process initiated in preparation for a resting period (winter) in above-ground portions of the tree.
The pre-fabricated ‘AZ’
The place where the leaf2 separates (abscises) from the tree is typically located at the base of the leaf stalk (petiole). It is called the abscission zone (AZ). The AZ is no random fracture point but is actually built-in, “pre-positioned”3 during leaf formation. As a publication from the University of Georgia (USA) puts it: “Leaves are designed to be disposable.”3 (Emphasis added.)
Retrieving valuable resources from the leaf
With the post-summer hint of coolness in the air, before the onset of wintry weather, trees initiate a “senescence sequence”2 to systematically retrieve the re-usable resources from the leaves. As this process begins, and the green chlorophyll pigment and other parts of the light-harvesting (photosynthetic) complex4 are dismantled, the leaf changes colour.
First, the formerly hidden carotenoid pigments (e.g. yellow xanthophylls and orange beta-carotene) are now revealed, turning the leaves an orange-yellow hue, as the normally-dominant green chlorophyll fades.
Then, when about half the chlorophyll has been degraded, and as the level of phosphate in the leaf drops, the production of anthocyanin pigments increases. Anthocyanins tint autumn leaves red-purple, and blend with the carotenoids to create the breathtakingly beautiful deeper orange and fiery red coloration that tourists travel long distances to see and photograph.
Laying aside for the winter
The valuable materials that the tree extracts from the leaves before leaf drop are stored during winter in the tree’s roots, trunk, and branches until next spring when they are ‘recycled’ to re-leaf the tree. This has to be done during the milder weather of autumn, because there can be no retrieval from leaves after ‘Jack Frost’ arrives. Even a single frost or night of freezing air temperatures can quickly damage the living tissues in leaves and petioles. Once leaf cells are killed, the resources in them cannot be retrieved.5 But by recapturing the mobilizable valuables from the leaf during autumn prior to leaf shedding, both the resources and the tree are safely ready for the winter.
The AZ is activated
With the pulling back of resources from the leaf now completed, the abscission zone becomes a hotbed of activity, in three locations:
- a cell wall degradation area;
- a shear force generation area; and
- a tree protection zone.
All of these must be in place for successful leaf shedding and effective tree survival.
Weakening the cell walls
With the abscission process triggered by a raft of chemical signals (including, it is believed, ethylene produced by the internally-gutted leaf), AZ cells start to secrete enzymes.6 These dissolve the ‘glue’7 that holds cells together and degrade the primary wall between cells. The surrounding AZ cells actively produce the necessary abscission materials throughout; i.e., they remain alive and active until abscission is completed.
Forming the fracture line
While the AZ can be 5–40 cells wide, within that zone only a band 1–3 cells wide will disconnect from each other to form the fracture line. The weakening of the walls of those cells, coupled with increasing internal water pressure inside the cells, causes the cells to swell. This expansion generates tremendous shear forces, i.e., pushing and pulling on surrounding weakened cell walls, mechanically opening up fracture lines between cell walls. Wind tugging on the leaves helps these fracture lines to grow, as do gravity, precipitation and animal interference.
Sealing off the fracture line
As cell walls pull apart, opening up the fracture line, the AZ cells on the tree side close off the opening wound by depositing protective materials such as tyloses, suberin and lignin. This strong protective boundary zone seals off the leaf scar, defending the tree from the cold, as well as from diseases and pests.
With the sealing-off process completed, the leaf can now be safely shed.
A genetic cascade behind autumn’s leafy cascade
While there is still much to learn about abscission, we can see that leaf fall doesn’t just happen, but rather is a carefully coordinated series of complex chemical processes—which would be controlled by the plant’s genes. Researchers have now mapped out a genetic pathway, or ‘signalling cascade’, behind abscission in the common laboratory plant Arabidopsis (water cress).8 They have identified that there is a key network of genes that code for proteins in a sequential manner. Each step of the cumulative processes that make up the cascade is dependent on the one before it.
This presents a challenge to the evolutionary paradigm—because if just one of these steps in the signalling cascade is absent, the abscission process doesn’t work. (In addition, how did evolution produce genes that code for enzymes that can digest themselves?)
All-or-nothing genetic cascades9 don’t fit the claimed step-by-step evolution story, but rather fit with the Bible’s account that plants were designed by a super-intelligent creator—God. He designed them to fit the seasons He made too (Genesis 1:14, 8:22; Deuteronomy 11:14). Autumn, and its colourful cascade of leaves, are no accident!
References and notes
- ‘Deciduous’ refers to trees and shrubs that, unlike evergreens, shed their leaves in the Fall (Autumn) and become dormant during the winter. Return to text.
- The process described in this article applies not just to autumn leaves, but also petals, flowers, ripening fruits, and other dead or diseased parts. Return to text.
- Coder, K., Falling tree leaves: leaf abscission, University of Georgia Daniel B. Warnell School of Forestry Resources Extension publication FOR99–025, December 1999. Return to text.
- For more on the wonders of photosynthesis see Sarfati, J., Green power (photosynthesis) God’s solar power plants amaze chemists, Journal of Creation 19(1):14–15, 2005. Return to text.
- As many orchardists know to their cost, a late spring frost can so damage young shoots and opened flower buds that not only is an entire year’s crop lost, but sometimes younger trees can even be killed altogether. Return to text.
- E.g. pectinase and cellulase. Return to text.
- I.e. the middle lamella. Calcium bridges across cell wall materials are also removed. Return to text.
- Cho, S., Larue, C., Chevalier, D., Wang, H., Jinn, T.-L., Zhang, S., and Walker, J., Regulation of floral organ abscission in Arabidopsis thaliana, Proceedings of the National Academy of Sciences USA 105(40):15629–15634, 7 October 2008. Return to text.
- An example from animals and humans is the blood clotting cascade, with over a hundred factors or steps now known to make up the sequence. See: “Irreducibly complex: The clotting cascade”, Creation 33(3):15, 2011. Return to text.