abscission

Abscission (from the Latin ab meaning away and scindere meaning to cut) is the shedding of a body part. It most commonly refers to the process by which a plant intentionally drops one or more of its parts, such as a leaf, fruit, flower or seed, though the term is also used to describe the shedding of a claw by an animal^[which?], and is also the word used to describe the separation of daughter cells at the end of cytokinesis, a process that generally begins immediately following mitotic telophase.

Use

A plant will abscise a part either to discard a member that is no longer necessary, such as a leaf during autumn, or a flower following fertilisation, or for the purposes of reproduction. Most deciduous plants drop their leaves by abscission before winter, while evergreen plants continuously abscise their leaves. Another form of abscission is fruit drop, when a plant abscises fruit while still immature, in order to conserve resources needed to bring the remaining fruit to maturity. If a leaf is damaged a plant may also abscise it to conserve water or photosynthetic efficiency, depending on the 'costs' to the plant as a whole. The abscission layer is a greenish grayish color.

Types

In deciduous trees, an abscission zone, also called a separation zone, is formed at the base of the petiole. It is composed of a top layer which has cells with weak walls, and a bottom layer which expands in the autumn, breaking the weak walls of the cells in the top layer. This allows the leaf to be shed.

In woody plants, an abscission layer is formed composed of parenchyma cells bounded on both sides with cork. This layer is found at the base of the leaf petioles in woody angiosperms and gymnosperms and because of the disintegration of the parenchyma layer, the organ, such as a leaf or bark, is separated from the parent plant. Abscission is a natural process of plant growth induced by the plant, in contrast to decaying or falling off due to other causes.

The liberation of a fungal spore by the withering away of an adjoining layer is also called abscission.

Lack of chlorophyll from decreased sunlight

The reduction of chlorophyll production in leaves due to decreased sunlight in the fall explains why some leaves turn yellow. However, the yellow color can attract aphids, so some trees turn the leaves red instead by injecting a bright pigment.^[1] The loss of chlorophyll may also contribute to the abscission process.

Hormone involvement

The gaseous plant hormone ethylene can stimulate abscission. While researchers originally believed abscisic acid to be the hormone that stimulated abscission (for which the hormone was named), it was later proven that it does not play a primary role. http://www.plant-hormones.info/abscisicacid.htm

Auxin is a plant hormone which can prevent the formation of abscission layers and premature fruit drop. Auxin is also believed to play a part in the shedding of leaves and their autumn color change. This happens due to continuous release of auxin in a young leaf; however, as leaves gets old and auxin supply dwindles, an abscission layer forms and leaves shed. In woody plants preparing to shed their leaves, the abscission zone or layer cuts off the movement of auxin from the leaf blade to the leaf.^{[citation needed]}

See also

• Marcescence, the retention of normally shed plant parts

External links

• Absission layer microscopy

References

1. ^ Highfield, Roger (10:04PM BST 22 Sep 2008). "Why leaves fall off trees is discovered". Telegraph.co.uk (The Daily Telegraph). http://www.telegraph.co.uk/science/science-news/3352225/Why-leaves-fall-off-trees-is-discovered.html. Retrieved 01 Nov 2009. 

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