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Role and use of synthetic phytohormones is taught in the note for Plant Hormones. Read it below and look for the section on this topic.

Plant Hormones

10 min read

Plant growth substances

Plant hormones (phytohormones) are chemicals made in one part of a plant that control growth and responses in another. The main groups are auxins, gibberellins, cytokinins, abscisic acid and ethene.

HormoneMade inMain effects
Auxin (IAA)Shoot and root tipsCell elongation, phototropism, apical dominance, fruit growth
GibberellinYoung leaves, seedsStem lengthening, seed germination, ends dormancy
CytokininRoot tipsCell division, delays leaf ageing, breaks apical dominance
Abscisic acidLeaves, roots, seedsCloses stomata, keeps seeds dormant, leaf fall
EtheneRipening fruit, ageing leavesRipening of fruits, leaf fall

Auxin and tropisms

A tropism is the growth of part of a plant in response to a stimulus. In phototropism, auxin moves to the shaded side of a shoot, where it makes the cells grow longer, so the shoot bends towards light. Roots respond to gravity: auxin collects on the lower side of a horizontal root, where it slows elongation, so the root bends down.

Auxin and phototropismThe plant hormone auxin is made at the shoot tip. When light comes from one side, auxin moves to the shaded side, where it makes the cells grow longer. The shaded side grows faster than the lit side and the shoot bends towards the light. This is phototropism. Roots respond to gravity and bend downward, which is gravitropism.lighttip makes auxinstraight up: even lightbends towards the lightmore auxin,longer cellsless auxin
More auxin on the shaded side makes cells there longer than on the lit side.

Synthetic phytohormones in farming

Synthetic substanceUse
2,4-D (like auxin)Selective weed killer for broad-leaved weeds in cereals
Rooting powder (NAA, IBA)Makes cuttings form roots
Gibberellic acidGrows larger seedless grapes; makes barley germinate evenly for brewing
Ethephon (releases ethene)Ripens bananas, mangoes and pineapples at the same time
Auxin spraysStop fruit falling early; make fruit set without pollination
Try itBending towards light

Auxin makes the shaded side grow faster than the lit side. Choose the auxin on the shaded side to see the difference in cell length.

Check yourself Which hormone promotes the ripening of fruits?
Check yourself Which hormone closes stomata?
Check yourself 2,4-D is used to...

Key terms

Phytohormone
A chemical messenger in plants.
Tropism
Growth in response to a directional stimulus.
Apical dominance
Inhibition of side shoots by the shoot tip.
Dormancy
A resting state of a seed or bud.

Summary

  • Auxins, gibberellins, cytokinins, abscisic acid and ethene are plant hormones.
  • Auxin causes cell elongation and phototropism.
  • Synthetic hormones are used as weed killers, rooting powders and ripeners.
  • Abscisic acid closes stomata and keeps seeds dormant.