Auxin

In the syllabus and the exam

Auxin is the single hormone underlying almost every tropism question in this topic, so candidates are expected to understand it as the shared mechanism connecting phototropism, geotropism, and general plant growth responses, rather than as an isolated fact to memorise on its own. Candidates must know that auxin is produced mainly at the tip of a shoot and diffuses backward through the tissue, and that its distribution can become uneven when a stimulus such as light or gravity acts unequally on one side of the plant, which is what ultimately causes bending. A key point tested is that auxin's effect depends on concentration and on the tissue involved: moderate concentrations promote elongation in shoot cells, while similar or higher concentrations can inhibit elongation in root cells, which is why the same hormone produces opposite bending directions in roots and shoots. Exam questions sometimes describe classic experiments, such as removing a shoot tip or inserting an impermeable barrier on one side of a shoot, and ask candidates to predict the outcome based on where auxin can and cannot reach. A common error is describing auxin as acting instantly like a nerve impulse; its effect works through gradual, unequal cell elongation over time, not through rapid signalling. Candidates should also know that synthetic versions of auxin are used commercially, for example as rooting hormones to encourage cuttings to develop roots or, at very high concentrations, as selective weedkillers, which shows how understanding a hormone's mechanism has practical applications beyond the classroom experiment.

Example

Summarising its role, a candidate writes: 'Auxin is a plant hormone produced mainly at the shoot tip that controls growth responses such as phototropism and geotropism by promoting cell elongation, usually on the side of the shoot with a higher auxin concentration.'

Related terms

PhototropismGeotropismTropism

Source:SRC-DSKP-EN