Plant Coordination & Tropisms

IGCSE Edexcel Biology
2.82–2.85 Tropisms, auxin and commercial uses of plant hormones
Key Concepts: Plants respond to directional stimuli by tropisms — directional growth responses. Auxin is a plant hormone produced in shoot tips that controls cell elongation. Unequal distribution of auxin causes shoots and roots to bend towards or away from stimuli.

Section A — Tropisms

1. Define the term tropism. [2]
2. Complete the table to describe each tropism. [6]
TropismStimulusResponse of shoot
Phototropism
Gravitropism (geotropism)
Hydrotropism
3. State whether each response is positive or negative for a shoot tip and a root tip in gravitropism. [2]

Section B — Auxin and Phototropism

4. State where auxin is produced in a plant. [1]
5. Explain, step by step, how auxin causes a shoot tip to bend towards light (phototropism). [4]
6. A student places a seedling horizontally in the dark and leaves it for 24 hours. Predict and explain how the shoot and root will have grown. [4]
7. Explain why high concentrations of auxin promote growth in shoots but inhibit growth in roots. [2]

Section C — Commercial Uses of Plant Hormones

8. Describe how auxin is used as a selective weedkiller. Explain why it kills broad-leaved weeds but not grass. [3]
9. Describe one other commercial use of plant hormones (other than weedkiller). [2]

Total marks: 26

Mark Scheme

1. A tropism is a directional growth response [1] of a plant to a directional (external) stimulus [1] [2]
2. Phototropism: stimulus = light; shoot grows towards light; Gravitropism: stimulus = gravity; shoot grows away from gravity (upward); Hydrotropism: stimulus = water; roots grow towards water [6 — 2 per row]
3. Shoot tip: negative gravitropism (grows away from gravity / upward) [1]; root tip: positive gravitropism (grows towards gravity / downward) [1] [2]
4. In the shoot tip (apical meristem) [1]
5. Auxin is produced at the shoot tip [1]; light causes auxin to move to the shaded side of the stem [1]; higher concentration of auxin on the shaded side causes those cells to elongate more [1]; the shoot bends towards the light due to unequal elongation [1] [4]
6. Shoot: auxin accumulates on the lower side due to gravity [1]; cells on the lower side elongate more, so the shoot bends upward (negative gravitropism) [1]; Root: same auxin distribution, but high auxin concentration inhibits growth in roots [1]; cells on the upper side elongate more, so the root bends downward (positive gravitropism) [1] [4]
7. Shoots and roots have different sensitivities to auxin [1]; the concentration that promotes elongation in shoots is high enough to inhibit elongation in roots [1] [2]
8. Synthetic auxin is applied at high concentration [1]; it causes broad-leaved weeds to grow uncontrollably and die [1]; grasses have narrow leaves and absorb less auxin / are less sensitive to auxin at this concentration, so they survive [1] [3]
9. Any one correct use with description: Rooting powder/gel — auxin applied to cut ends of stems promotes root growth in cuttings [2]; Fruit ripening — ethene used to ripen fruit during transport / control ripening time [2]; Seedless fruit — gibberellins used to produce seedless grapes [2] [2]