Abscission

Abscission is the process by which a plant sheds or separates a part from itself, such as a leaf, flower, or fruit. It is a natural process of plant growth and development, and is necessary for the plant’s survival and reproduction.

Abscission is the act of cutting off a part of a plant that is no longer essential, which is a naturally occurring phenomenon. It involves the separation of fruits, flowers or leaves from plants at specialized separation layers, and is the normal shedding of a senescent plant structure or a part.

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Leaf Abscission

Leaf abscission is a normal process of shedding an old leaf at the base of the petiole, commonly seen during the autumn season. This process involves many physical and biochemical changes that are regulated by plant hormones. It is an inevitable process that follows senescence, and involves the formation of an abscission layer at the petiole’s base.

At the early life of a leaf, comparatively higher concentrations of the plant hormone auxin are produced and are steadily transported out of the leaf via the petiole. The process of both senescence and abscission are delayed for as long as the levels of auxin remain high in the leaf and an adequate quantity of hormone is transported across the petiole.

When the leaf matures, the level of the senescence-retarding hormones, particularly auxin, decreases. Additionally, cytokinins and gibberellins are secreted in different locations of the plant and transported to the leaves to help delay the destructive process.

The levels of auxin are reduced, resulting in an increase of catabolic reactions which surpass the anabolic processes. This rise in catabolic reactions is accompanied by an increase in the levels of abscisic acid and ethylene. In the formation of the abscission layer, ethylene is particularly important.

Abscission Layer

An evident increase in the respiration rates of cells of the abscission layer, which are in proximity to the stem, is seen in the initial stages of abscission. This is likely due to the need for extra energy, and is triggered by ethylene, causing one or more of the cell layers near the stem to grow in size. This process results in the formation of the abscission layer, which is composed of one or multiple layers of cells across the petiole’s base.

The production of enzymes in cells found in the abscission layer far from the stem increases with the growth in size of these cells. This leads to the digestion of cell wall matter, as the enzymes disintegrate the polysaccharides in the cell walls.

The cell expansion in the proximal area of the abscission layer generates pressure, causing the two layers to disintegrate. As a result, the leaf separates and falls from the plant.

Abscission, similar to senescence, is a natural, progressive process that takes place during the life cycle of most plants. It is usually associated with the seasonal changes that occur naturally, but these environmental changes can also increase abscission.

Severe environmental conditions can lead to both senescence and abscission, due to deficiencies in nutrients such as nitrogen or lack of water. These conditions can accelerate the process of senescence and, as senescence always precedes abscission, they can ultimately stimulate the secondary result of the aging process.

Abscission Function

Many plants around the globe are exposed to freezing temperatures. Most of the plants’ leaves cannot handle these frigid temperatures and consequently die in the winter. If the leaves are not prepared for the cold weather through senescence, the first frost can kill them before any of their matter can be preserved.

Without an abscission process to remove the dead leaves, the tissues which have gone through senescence could block the new growth of spring that appears in the upcoming season. Thus, the process of senescence and abscission provides a way for perennials to recycle a large portion of the leaf material as they prepare for both the cold and the upcoming season of growth.

The process of senescence can be triggered by competition for nutrients from other structures of plants. The pull of the nutrients towards other parts of the plants such as flowers, roots or even fruits can reduce the amount of nutrients available to the leaves.

The decreased supply of nutrients can decrease the synthetic rates and the total result could be a drop in the main leaf macromolecules such as the chlorophyll, proteins and the nucleic acids.

Although competition can be a factor in senescence and abscission, it is not the only factor. Studies have found that leaf senescence can still occur even in plants without flowers, and even when the flowers are removed shortly after they form.

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