Plants are primarily categorized as annuals, biennials,or perennials, based on their growth patterns. Annuals are those plants that undergo a complete life cycle from seed to seed in one growing season.
Biennials require two growing seasons to complete a life cycle; during the first year, only vegetative growth takes place. The above ground portion dies through the winter, and in the next growing season the roots send up a reproductive shoot that produces the seeds.
Perennials have the capacity to live through many successive growing seasons. In this group, those plants referred to as deciduous species shed all their leaves at the same time. The evergreen species shed leaves throughout the year, yet never shed the entire complement at any one time.
Preparation and Precursors
Prior to any natural abscission that may take place, leaves (or any other plant organ subject to abscission, such as flowers or fruit) undergo senescence. Senescence can be defined as the deterioration that occurs in conjunction with aging, and it results in the death of an organ or organism.
Senescence can occur throughout the entire plant, as it does in annuals, or in only the above ground portion, as it does in perennial herbs. In woody perennials, however, only the leaves senesce, while the bulk of the stem and roots remain alive.
From the time that leaves begin to grow, biochemical activities such as photosynthesis increase. This increase will continue until the leaves expand to maximum size. Soon after they reach maximum size, senescence begins, and photosynthetic rates begin to decrease. As the photosynthetic ability of the leaves declines, there is an accompanying decrease in other metabolic activities.
Respiration rates begin to subside dramatically, and leaf protein levels drop sharply because of increased proteolytic activity (enzymatic breakdown of proteins). Protein synthesis diminishes, and there is an increase in the enzymatic degradation of ribonucleic acid (RNA). There is also an increase in the hydrolytic breakdown of carbohydrates.
Finally, destruction of the green pigment, chlorophyll, is accompanied by increased visibility of the yellow or orange pigments called carotenoids, which were previously masked by chlorophyll. Most of the protein, carbohydrates, RNA, and chlorophyll degradation products are rapidly transported out of the senescing leaf. The final result is the production of yellowish, dead leaves.
The senescence process is a natural progression of the normal plant life cycle; however, environmental conditions can influence the process. Lack of water will speed the senescence process in most species. Higher-than-normal temperatures also cause an increase in senescence-related reactions. Darkness dramatically hastens senescence: Most leaves will senesce two or three times faster in darkness than if growing under normal light conditions.
Numerous studies strongly suggest that senescence is under hormonal control. Both ethylene and abscisic acid enhance senescence, but ethylene is the more effective of the two.
The gibberellins, cytokinins, and auxins (other types of hormones) have all been shown to delay the process in various plant species. The exact role of each of these hormones in senescence has not yet been determined, but it is apparent that the process involves the interaction of several of these growth-regulating substances.
Onset of Abscission
|Onset of Abscission|
During the early life of a leaf, auxin is produced in relatively high concentrations and is steadily transported out of the leaf through the petiole. As long as the auxin level remains high in the leaf and a sufficient amount of the hormone is transported across the petiole, both senescence and abscission are delayed.
In addition, gibberellins and cytokinins are produced elsewhere in the plant and then sent to the leaves to help retard the destructive processes. As the leaf matures, however, the level of the senescence-retarding hormones, especially auxin, decreases.
With the decrease in auxin levels, the catabolic (breakdown) reactions begin to outnumber the anabolic (synthetic) processes. In conjunction with the increase in catabolic reactions, there is a rise in the levels of abscisic acid and, especially, ethylene. Ethylene is particularly important in producing the abscission layer.
In most species, the abscission layer is formed from one or perhaps several layers of cells across the base of the petiole. In the earlier stages of abscission, there is a noticeable rise in the respiration rates of the cells of the abscission layer closest to the stem (the proximal cells). As the respiration rates increase to supply additional energy, ethylene stimulates one or more of these cell layers nearest the stem to increase in size.
Along with the increase in size of those cells, the cells in the abscission layer farthest from the stem (the distal cells) increase the production of enzymes that break down polysaccharides in the cell walls. With the secretion of these enzymes into the cell walls, digestion of the cell-wall materials begins.
The pressure created by the expansion of the cells in the proximal region of the abscission layer (causing them to grow against the weakened senescing cells of the distal region) results in the two layers breaking apart. Thus, the leaf detaches and falls from the plant.
Abscission, like senescence, is a natural order of progression during the life cycle of most plants. Although the process is closely correlated with regular seasonal changes, variations in environmental conditions can enhance abscission.
Deficiencies in certain nutrients, such as nitrogen, or lack of water can stimulate abscission. These conditions also hasten senescence, and senescence always precedes abscission. Hence, adverse environmental conditions perhaps trigger only the onset of senescence, and abscission occurs as a secondary result of the aging process.
Function of Abscission
Throughout much of the world, plants are subjected to freezing temperatures. The leaves of most plants are unable to withstand the cold weather and face certain death during the winter. If the leaves did not prepare for the onset of cold weather by undergoing senescence, the first freeze would kill the leaves before materials within them could be salvaged.
Without the abscission process to remove the dead leaves, the senescent tissues would shade the new spring growth that appears the following growing season. Hence, senescence and abscission provide a means by which perennials can recycle a major portion of leaf materials as the plants prepare for both the cold weather and the following growing season.
Competition for nutrients from other parts of the plant may initiate the senescence process. The pull of nutrients to another part of the plant such as roots, flowers, or fruit would reduce the amount of these materials bound for the leaves.
The reduced supply of nutrients could very well decrease synthetic rates, and the overall result would be a decline in major leaf macromolecules such as proteins, chlorophyll, and nucleic acids.
Competition alone, however, cannot account for the senescence and abscission phenomena, because even in plants that do not produce fruit, the leaves experience aging and the loss of leaves. In addition, numerous studies have shown that leaf senescence will still occur when flowers are removed from the plant soon after being formed.
Although competition for nutrients may not be the sole cause of the phenomenon, the mobilization of substances such as amino acids and carbohydrates from the leaves to other metabolic sinks, such as the fruit, is definitely linked to the initiation of senescence.
Several of the plant hormones or other factors that stimulate mobilization also hasten senescence. Hence, it is possible that the competition for nutrients triggers the production of some unknown senescence hormone by the fruit or some other competing plant part.
This theoretical substance would be transported to the leaves, where it would initiate mobilization of leaf contents. This mobilization might enhance senescence, which, in turn, might trigger the metabolic reactions that lead to abscission of the leaves.