abscisic acid

(noun)

a plant hormone that functions in many plant developmental processes, including bud dormancy, inhibition of seed germination, and plant stress tolerance.

Related Terms

Examples of abscisic acid in the following topics:

  • Abscisic Acid, Ethylene, and Nontraditional Hormones

    • All physiological aspects of plants are affected by plant hormones, including abscisic acid, ethylene, and nontraditional hormones.
    • In addition to the growth hormones auxins, cytokinins, gibberellins, there are two more major types of plant hormones, abscisic acid and ethylene, as well as several other less-studied compounds that control plant physiology.
    • The plant hormone abscisic acid (ABA) was first discovered as the agent that causes the abscission or dropping of cotton bolls.
    • Ethylene stimulates the conversion of starch and acids to sugars.
    • For example, chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest.

  • The Evolution of Seed Plants and Adaptations for Land

    • Furthermore, seeds remain in a state of dormancy induced by desiccation and the hormone abscisic acid until conditions for growth become favorable.

  • Auxins, Cytokinins, and Gibberellins

    • Five major plant hormones are traditionally described: auxins, cytokinins, gibberellins, ethylene, and abscisic acid.
    • While many synthetic auxins are used as herbicides, indole acetic acid (IAA) is the only naturally-occurring auxin that shows physiological activity.
    • Abscisic acid is a strong antagonist of GA action.
    • In grapes, application of gibberellic acid increases the size of fruit and loosens clustering.

  • Polyprotic Acid Titrations

    • Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule.
    • Common examples of monoprotic acids in mineral acids include hydrochloric acid (HCl) and nitric acid (HNO3).
    • On the other hand, for organic acids the term mainly indicates the presence of one carboxylic acid group, and sometimes these acids are known as monocarboxylic acid.
    • Polyprotic acid are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule.
    • An example of a triprotic acid is orthophosphoric acid (H3PO4), usually just called phosphoric acid.

  • Diprotic and Polyprotic Acids

    • Diprotic and polyprotic acids contain multiple acidic protons that dissociate in distinct, sequential steps.
    • As their name suggests, polyprotic acids contain more than one acidic proton.
    • Two common examples are carbonic acid (H2CO3, which has two acidic protons and is therefore a diprotic acid) and phosphoric acid (H3PO4, which has three acidic protons and is therefore a triprotic acid).
    • With any polyprotic acid, the first amd most strongly acidic proton dissociates completely before the second-most acidic proton even begins to dissociate.
    • Identify the key features that distinguish polyprotic acids from monoprotic acids.

  • Acid Dissociation Constant (Ka)

    • The acid dissociation constant (Ka) is the measure of the strength of an acid in solution.
    • The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution.
    • Acid dissociation constants are most often associated with weak acids, or acids that do not completely dissociate in solution.
    • Acids with a pKa value of less than about -2 are said to be strong acids.
    • Acetic acid is a weak acid with an acid dissociation constant $K_a=1.8\times 10^{-5}$ .

  • Strong Acids

    • The strength of an acid refers to the ease with which the acid loses a proton.
    • where HA is a protonated acid, H+ is the free acidic proton, and A- is the conjugate base.
    • Strong acids yield weak conjugate bases.
    • For sulfuric acid, which is diprotic, the "strong acid" designation refers only to the dissociation of the first proton:
    • p-Toluenesulfonic acid is an example of an organic soluble strong acid, with a pKa of -2.8.

  • Weak Acids

    • The majority of acids are weak.
    • Examples of weak acids include acetic acid (CH3COOH), which is found in vinegar, and oxalic acid (H2C2O4), which is found in some vegetables.
    • Acids with a Ka less than 1.8×10−16 are weaker acids than water.
    • The Ka of acetic acid is $1.8\times 10^{-5}$.
    • Although it is only a weak acid, a concentrated enough solution of acetic acid can still be quite acidic.

  • The Brønsted-Lowry Definition of Acids and Bases

    • Originally, acids and bases were defined by Svante Arrhenius.
    • A wide range of compounds can be classified in the Brønsted-Lowry framework: mineral acids and derivatives such as sulfonates, carboxylic acids, amines, carbon acids, and many more.
    • The conjugate acid is the species that is formed when the Brønsted base accepts a proton from the Brønsted acid.
    • Here, acetic acid acts as a Brønsted-Lowry acid, donating a proton to water, which acts as the Brønsted-Lowry base.
    • Chemistry 12.1 What are Acids and Bases?

  • Oxoacids

    • Halogen oxoacids include hypochlorous acid (HOCl); chlorous acid(HOClO); chloric acid(HOClO2); oerchloric acid(HOClO3); oerbromic acid (HOBrO3)
    • Consider the simple oxyacids HOI (hypoiodous acid), HOBr (hypobromous acid), and HOCl (hypochlorous acid).
    • The strongest acid is perchloric acid on the left, and the weakest is hypochlorous acid on the far right.
    • Carboxylic acids are the most common type of organic acid.
    • Mellitic acid is an example of a hexacarboxylic acid.