Phytohormones

The main phytohormones and their functions

How auxin causes phototropism

When light hits a shoot from one side, auxin produced at the shoot tip accumulates on the shaded side rather than the illuminated side. This higher concentration of auxin causes cells on the shaded side to elongate more than cells on the lit side, bending the shoot towards the light.

Hormones often work together

Plant responses are usually controlled by more than one hormone interacting, not a single hormone acting alone. For example, cytokinin and auxin together regulate cell division and differentiation, while gibberellin and abscisic acid have opposing effects on seed dormancy and germination.

How it is examined

Common questions ask you to match a hormone to its function, to explain an experiment showing unequal distribution of auxin causing bending, or to interpret a described plant behaviour (such as delayed ripening or seed dormancy) in terms of which hormone is responsible.

Worked exam-style question

Question. The tip of a young oat coleoptile (shoot) was cut off. It was replaced with a small agar block that had earlier been left in contact with a separate, intact coleoptile tip for several hours.

This agar block was placed off-centre, covering only one side of the cut coleoptile stump, which was then left in the dark. (a) Predict what happened to the coleoptile stump over the next few hours, and explain your prediction.

(b) Name the phytohormone responsible for this response, and state where it was originally produced. (c) State one variable that should be kept constant in this investigation, other than the size of the agar block, and explain why.

(d) Predict what would happen if a plain agar block, which had never touched a coleoptile tip, had been used instead.

Model answer. (a) The coleoptile stump would bend away from the side where the agar block was placed, because the phytohormone diffused out of the block into the cells directly beneath it, causing greater cell elongation on that side than on the untreated side. (b) Auxin; it was originally produced at the shoot tip of the intact coleoptile before being absorbed into the agar.

(c) Light should be excluded or kept constant (kept in the dark), because uneven light alone can cause an uneven distribution of auxin, which would make it impossible to tell whether the bending was caused by the agar block or by light. (d) With a plain, untreated agar block, no additional phytohormone would be supplied to the stump, so no bending would occur, since there would be no uneven hormone concentration to cause unequal cell elongation.

Practice question

Try this. A fruit seller places a batch of unripe bananas in a sealed container together with a few already-ripe apples. A second, identical batch of unripe bananas is left in an open container without any apples.

After two days, the bananas in the sealed container have ripened noticeably faster. Suggest an explanation for this observation, naming the phytohormone responsible.

Exam tip

Key terms

These glossary terms connect directly to this standard:

• Hormone, a chemical messenger produced in small amounts that coordinates growth or a physiological response.
• Auxin, a phytohormone made at the shoot tip that promotes cell elongation and causes phototropism.
• Phototropism, the growth response of a plant towards or away from light, caused by unequal auxin distribution.
• Germination, the process by which a seed begins to grow, promoted by gibberellin and inhibited by abscisic acid.

Source:SRC-DSKP-EN

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