Gibberellins, cytokinins, ethylene and abscisic acid
Gibberellins lengthen stems, cytokinins make cells divide, ethylene ripens fruits, and abscisic acid closes stomata and causes dormancy.
Four more plant hormones
| Hormone | How it was found | Main effects |
|---|---|---|
| Gibberellins (e.g. gibberellic acid, GA₃) | Japanese scientist E. Kurosawa (1926) studied the fungus Gibberella fujikuroi. It made rice seedlings grow very tall and weak: the bakanae or 'foolish seedling' disease. | Stem lengthening; bolting (sudden stem growth before flowering) in cabbage; breaking seed dormancy; bigger fruits like grapes |
| Cytokinins (e.g. kinetin, zeatin) | Kinetin was first got from the DNA of herring fish sperm. Zeatin was found in maize. Coconut water is rich in cytokinins. | Cell division (along with auxin); delay ageing of leaves, keeping them green; growth of side buds; used in tissue culture |
| Ethylene | A gas given off by ripening fruits | Fruit ripening; fall of leaves, flowers and fruits; ageing (senescence); breaking dormancy |
| Abscisic acid (ABA) | Found in the 1960s in young cotton fruits that were about to fall, and in resting (dormant) buds | Closes stomata in drought; causes dormancy of seeds and buds; leaf fall; slows growth |
Growth promoters vs growth inhibitors
ABA and gibberellins work against each other: ABA keeps seeds dormant, gibberellins wake them up.
Uses in farming
- Gibberellins are sprayed on grape vines for bigger, longer bunches, and on sugarcane for longer stems with more sugar.
- Cytokinins are added to the jelly in tissue culture labs that grow banana plantlets.
- Ethylene is used in ripening rooms to ripen bananas and mangoes evenly.
- ABA helps crops survive a dry spell by closing stomata, so less water is lost.
Why is ABA called the stress hormone?
When the soil is dry, leaves make more abscisic acid. ABA makes the stomata close, so less water escapes by transpiration. It also keeps seeds and buds dormant (resting) through cold or dry seasons. Because it helps the plant survive hard times, ABA is called the stress hormone.
Fruit sellers ripen bananas and mangoes in closed rooms with ethylene gas; ripening with calcium carbide is unsafe and not allowed. A raw banana kept with a ripe one ripens faster, because the ripe fruit gives off ethylene.
2 marks: “Name the gaseous plant hormone. Give one of its effects.” Ethylene. It ripens fruits like banana and mango (or causes leaf and fruit fall).
2 marks: “What is bolting? Which hormone causes it?” Sudden lengthening of the stem just before flowering in rosette plants like cabbage. It is caused by gibberellins.
4 marks: “Write the effects of gibberellins on plants.” Stem lengthening; bolting in cabbage; breaking dormancy of seeds and buds; bigger fruits such as grapes; seedless fruits; making genetically dwarf plants grow tall.
Gibberellin makes it Grow tall, Cytokinin makes Cells divide, Ethylene makes fruit Edible (ripe), ABA puts it Asleep (dormant) and shuts stomata.