Abscisic acid is a plant hormone that is responsible for dormancy and the inhibition of growth, among other processes in plant development. When it was first discovered and tested, abscisic acid was thought to play a role in abscission, or the shedding of the plant's leaves or fruit. This is part of how the hormone got its name, although most scientists no longer believe the acid has that function. This hormone may also be produced at certain times by a plant under stress, which may occur in situations such as a lack of water.
The first time that abscisic acid was identified and described was in 1963. The scientist Frederick Addicott, in the company of his associates, was looking for the hormone that caused the abscission of fruit in cotton plants. Two different compounds were isolated, and at the time they were called abscisin I and abscisin II. The latter eventually came to be known as abscisic acid, even though its role in abscission is now considered dubious at best. Two other groups of scientists, who were contemporaries of Addicott, also discovered these compounds around the same time.
One of the major roles of this acid is to suspend cell growth and division when it comes time for the plant to go into a dormant state, such as in autumn. At this time of year, the hormone is produced in the buds which will become fruit during the following year. It not only stops growth in various areas of the plant, but in some cases, it causes the formation of a hard outer covering on the bud, which protects it from harsh winter conditions. At the end of a period of dormancy, other hormones, which act in opposition to abscisic acid, are produced to cause renewed growth in the plant.
Both types of hormones are always present to some degree in the plant, but it is their relative levels that determine the state of a plant's growth. Abscisic acid can also come into play during the season of growth, as a coping mechanism for stress. Specifically, it causes the tiny pores on the leaves, known as stomata, to close when water is scarce. This slows down the rate at which water can escape out of the leaves, thereby preserving the life of the plant. Finally, this acid can also be produced by a plant in response to an injury, in which case it induces the synthesis of certain proteins which contribute to the healing of the injury.