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Coordination and Response: Question 5

Syllabus 14.5

Structured Extended 6 marks

A greenhouse manager arranges a row of young tomato seedlings in trays under a single fluorescent strip light fixed along one side of a growing tent, with no light reaching the seedlings from any other direction. After one week, every shoot tip has curved noticeably towards the strip light, and each stem is longer down its shaded side than down the side facing the light.

(a) Name the plant hormone responsible for this curvature, and state where in the shoot it is made. [2]

(b) Explain, in terms of the distribution of this hormone and its effect on cells, why the shaded side of each stem grows longer than the illuminated side, causing the shoot tip to bend towards the light. [4]

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Worked solution

Part (a): Naming the hormone and its site of production

The bending of the tomato shoots towards the strip light is an example of phototropism, a growth response controlled by the plant hormone auxin.

Auxin is made in the tip of the shoot.

Part (b): Explaining the curvature

Once made in the shoot tip, auxin diffuses down through the stem. When light reaches the shoot from only one side, as with the single strip light in the growing tent, auxin does not spread evenly through the stem:

  1. Auxin becomes unequally distributed, moving towards and accumulating in a higher concentration on the shaded side of the stem than on the side facing the light.
  2. Auxin stimulates cell elongation in the stem: the more auxin present, the more the cells elongate.
  3. Because the shaded side has more auxin, its cells elongate more than the cells on the illuminated side.
  4. This uneven growth, more elongation on the shaded side and less on the illuminated side, causes the shoot tip to curve towards the light.

Final answers

  • (a) Auxin, made in the tip of the shoot.
  • (b) Auxin accumulates on the shaded side of the stem and stimulates greater cell elongation there than on the illuminated side, so the shoot tip curves towards the light.