Phototropism

In the syllabus and the exam

Phototropism is tested as the specific tropic response of a plant to the direction of light, and it is almost always linked in examination questions to the hormone auxin as the underlying mechanism, so a complete answer rarely stops at simply stating that the shoot bends towards light. Candidates are expected to explain that auxin, produced at the shoot tip, is redistributed unevenly when light comes from one side, accumulating in greater concentration on the shaded side of the shoot, and that this higher auxin concentration causes the cells on that shaded side to elongate more than the cells on the illuminated side, bending the shoot towards the light. Paper 3-style questions sometimes describe a classic experimental setup, such as covering a shoot tip with foil or placing it in a one-sided light box, and ask candidates to predict and explain the outcome, or to identify what a decapitated shoot tip demonstrates about where auxin is produced. A common error is stating that light itself makes cells grow faster on the illuminated side, which reverses the correct explanation; it is the shaded side, with more auxin, that grows faster, not the lit side. Candidates should also be able to describe how this response benefits the plant in nature, explaining that bending towards light positions the leaves to capture more light for photosynthesis, which is a functional link examiners sometimes expect alongside the mechanical explanation involving auxin. This is also why houseplants kept on a windowsill are often rotated periodically, since without turning, continued phototropic bending towards the single light source would eventually produce a lopsided, unevenly growing plant.

Example

Explaining a classic observation, a candidate writes: 'A shoot placed near a one-sided light source bends towards the light because auxin accumulates on the shaded side, causing cells there to elongate faster than cells on the illuminated side.'

Related terms

TropismAuxinGeotropism

Source:SRC-DSKP-EN