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[Screaming Eagle]

The Essence of Senescence


This is the 10th and final installment in a series of articles on basic botany for bonsai. The information is adapted from a basic college text published as The Nature of Life written by John Postlethwait and Janet Hopson in 1995 and published by McGraw-Hill. The primary purpose of this series has been to further acquaint us with some basic biology concepts related to plant anatomy and physiology in an effort to help us to better understand how plants function. The idea is very simply that if we understand more about plant structure and function, we can have healthier and more productive bonsai.
Up to this point we have delved into detail about plant evolution, structure, function and adaptation to various environments in an effort to more fully understand their growth patterns and why they function the way they do. This article will explore the phenomenon of dormancy, aging and wound healing.
Plants as a whole have adapted to synchronize their life cycles with seasonal variation and demands. They seem to have the natural inclination to seek a period of quiet rest in order to restore themselves. Humans would do well to follow their lead. These cycles can be thought of in a year to year pattern called abscission that removes mature fruits and dying leaves as well as senescence which is the process of aging and dying. These events are both observed each fall and result in leaves changing color and falling from trees. These events are triggered largely by external factors (cold temperatures, dryness, diminishing light sources) that trigger mass senescence. Flowering is a key to this process in annual plants when apices of shoots convert into flower buds and the growth potential of that plant ends.
Perennial plants have a different process that involves the plant conserving resources over the cold dry winter and the deciduous growth habit is observed. Leaves lose their photosynthetic capabilities in winter and also provide an avenue for transpiration and water loss so they become a burden rather than an asset. Apparently, they also store wastes which are excreted when the leaves fall from the plant. Chlorophyll from the leaves feeds the trunk and roots and is one of the last molecules to be broken down in fall. When chlorophyll breaks down, other pigments remain (red and yellow anthocyanins; yellow xanthophylls; red and orange carotenes) and can show through which brings us the familiar colors of fall. In our discussion of hormones, we mentioned cytokinin which helps lateral buds to grow and works in opposition to auxin. The presence of cytokinin will cause green spots to remain on some leaves that are otherwise changing color. For this reason, scientists suspect that autumn conditions may influence levels of hormones and that the hormones actually determine when leaves will senesce. They believe that the balance of ABA (abscisic acid), gibberellin, and cytokinin is influenced by the external factors to set the process in motion. ABA is probably the hormone that causes senescence and works in opposition to cytokinin.
Abscission (scissoring) of leaves is ruled by environmental cues and ratios of hormones also. During summer, leaves produce auxin which inhibits the formation of an abscission zone in the stem of leaves. The abscission zone is an area of weak cells at the base of the stem where the leaf will eventually separate from the branch naturally. As auxin levels drop in fall, enzymes begin to break down cell walls in the abscission zone and a strong puff of wind or water will cause the leaf to fall from the tree.
After leaves have fallen, the plant survives the winter in a state of reduced metabolic activity. This dormancy may take several forms depending on the type plant involved. For our purposes, it's best to recognize that this state of dormancy is marked by a greatly reduced use of energy, slowed protein synthesis (healing), cell division and maintenance. Light cycles (mediated by phytochromes) are related to the metabolic slowdown and hormones essentially relay that information to the cells of the plant and help to maintain and enforce the state of dormancy.
Plant defenses

Plants have some very effective ways of defending themselves from attacks by predators as well as bacteria. Features such as thorns, leaf hairs and sticky sap serve as barriers for protection. Some plants are able to generate toxic compounds to protect themselves and almost all plants are able to wall off injured areas to minimize damage.
Plants produce both primary and secondary compounds that are necessary for growth and regulation as well as repelling, killing or interfering with normal activities of plant-eating organisms. Many trees produce as much as twice the amount of vegetative matter necessary for survival in order to insulate themselves from predators.
Some of the toxic compounds produced by plants solely to protect themselves are substances that have become familiar in our lives. Cyanide, camphor, tannin, nicotine, caffeine and cocaine are only a few of the 10,000 or more chemicals produced by plants that are toxic to animals. Some discourage plants from eating leaves and others may discourage animals from laying eggs on them. Some (like nicotine) act as poisons that kill various predators (not to mention other critters) and cyanide deters snails from eating leaves. There is some evidence that substances in cell walls of microbes or injured plant cells can stimulate plants to make antibiotics that kill attacking bacteria. Others may generate compounds that mimic insect hormones which disrupts insect metamorphosis.
Many trees respond to physical insults by increasing secondary compounds that can account for 10 percent by weight of fruits or vegetables from those plants. Just to show how potent these secondary defensive compounds can be, it's best not to eat damaged celery, apples, peanuts and other produce. These substances can harm people as well as insects. The other side of the coin is that these substances are the source of a large number of medicines that are used by humans. Aspirin comes from the bark of willow trees, digitalis is a heart medicine that comes from the foxglove plant, chemicals used in cancer treatment come from periwinkle and the Pacific yew, morphine comes from poppy flowers and oral contraceptive ingredients come from yams to name just a few. We've probably all heard about the expected potential of medicines that may come from tropical rain forests that are severely threatened.
Anyone who has made hard cuts on collected trees that are followed by further hard cuts has probably witnessed the change in color of damaged wood that has been walled off within the plant. Some of the color of grainy wood that is so intriguing and highly valued is there because large branches were once removed causing an injury that the tree had to protect itself from. An injury through the bark and cambium results in a small area of deadwood that decays. Some of the sapwood becomes infected by microorganisms and the tree forms a barrier zone of toxins within the sapwood that compartmentalizes the infected area. This is part of the growth response that protects them from viruses, bacteria, fungi and animals (not to mention concave cutters). Recall that plants have rigid cell walls that cannot migrate so they wall off damaged areas to prevent invading organisms from gaining access to healthy tissue. This compartmentalization involves the production of toxic chemicals (like tannins) in the damaged area. Xylem and phloem tubes in the area are plugged by sap or resins that prevent bacteria from spreading to other parts of the plant. Once the injury is walled off, the tree can resume growth around the damaged area.