Plant and animal responses: (b) the roles of plant hormones

The idea

Plant hormones are chemical messengers that coordinate growth and responses to the environment. Auxin, produced mainly in shoot tips, moves to regions of elongation and causes cell elongation; an unequal auxin distribution produces the differential growth seen in phototropism and geotropism. Gibberellin promotes stem elongation and can break seed dormancy, triggering germination. Abscisic acid acts as a stress hormone, promoting stomatal closure and maintaining seed dormancy. Ethene (ethylene) is a gaseous hormone that promotes fruit ripening and leaf abscission. Cytokinins promote cell division and delay leaf ageing. A multiple-choice question may ask which hormone matches a stated role, or which effect follows from a described change in hormone concentration, so link each named hormone to its specific function and site of action rather than a general idea of 'growth'.

What earns the marks

• Auxin promotes cell elongation and its unequal distribution causes differential growth in tropisms.
• Gibberellin promotes stem elongation and can break seed dormancy to promote germination.
• Abscisic acid promotes stomatal closure and maintains seed dormancy under stress.
• Ethene promotes fruit ripening and leaf abscission.
• Cytokinins promote cell division and delay leaf ageing.
• Selects the option that correctly matches the named hormone to its stated role, rejecting options that swap roles between hormones.

Common mistakes and how to put them right

• Swapping the roles of abscisic acid and gibberellin.

  Put it right: Abscisic acid maintains dormancy while gibberellin can break it.

• Treating auxin as a general growth promoter everywhere.

  Put it right: Its effect depends on concentration and site, producing elongation in shoots.

• Confusing ethene with a nutrient.

  Put it right: Ethene is a gaseous hormone promoting ripening and abscission.

• Assuming all plant hormones promote growth.

  Put it right: Some, such as abscisic acid, inhibit processes under stress.

Exam tips

• Match each hormone to one clear function before reading the options, then eliminate options that swap roles.
• Watch for wording about concentration and site, since auxin effects depend on both.
• Check whether the question asks for the hormone or its effect, and answer the one actually asked.