Topic 14.5 · Core
Gravitropism and Phototropism
Plants can’t relocate when conditions change, but the growing regions of a shoot or root can change the direction in which they grow. A tropic response is exactly that — a directional growth response — and this chapter focuses on responses to two stimuli: gravity and light.
Gravitropism and phototropism, defined
Gravitropism is growth of a plant part towards or away from gravity. Phototropism is growth of a plant part towards or away from the direction of a light source. In both cases, growth towards the stimulus is described as positive, and growth away from it is described as negative. Roots commonly show positive gravitropism, growing towards gravity, while shoots commonly show negative gravitropism, growing away from it — this helps orient a plant correctly even when a germinating seed starts out lying at an unhelpful angle. Shoots commonly show positive phototropism, growing towards a light source.
A tropism is a growth response, not an instant, non-growth movement — the organ isn’t simply leaning temporarily, unequal growth on the two sides is what causes it to bend. It’s also worth resisting any “always towards” or “always away from” rule that applies regardless of which organ is responding: shoots and roots commonly respond in opposite directions to the very same stimulus, so the direction of the response depends on which organ you’re describing, not on the stimulus alone.
Investigating tropisms
Cambridge explicitly requires being able to investigate, not just describe, both phototropism and gravitropism in shoots and roots — so it’s worth being ready to design or interpret an experiment, not just recall the definitions.
A phototropism investigation typically uses similar seedlings, provides light from one direction, records the seedlings’ initial orientation, allows time for growth, and then measures or describes the curvature or growth direction relative to the light. A gravitropism investigation typically uses germinating seeds or seedlings, changes their orientation relative to gravity — placing them horizontally, for example — and then observes or measures shoot and root curvature over time, while excluding or controlling directional light if it could confound the result.
Whichever stimulus is being investigated, an appropriate comparison or control is essential. A slowly rotating clinostatcan remove a persistent one-sided gravitational stimulus and serve as that control for a gravitropism investigation — without one, there’s no way to show that the response was actually caused by the directional stimulus rather than by ordinary growth on its own. Reliability comes from using several specimens, quantitative measures of angle, curvature or length where possible, repeated observations, and consistent timings between them. Whatever the setup, the evidence has to connect a directional stimulus to a directional growth response — and shoots and roots should always be described and measured separately, since there is no reason to expect them to respond identically to the same stimulus.
When you’re asked to interpret an unfamiliar investigation, the same four questions apply every time: what stimulus is being changed, which plant organ is being measured, what direction the response takes, and which variables — species or age, starting size, water, temperature, duration, light conditions other than the one being tested — need to be controlled.
Where the mechanism comes from
Both phototropism and gravitropism in shoots are driven by a single chemical, auxin, working through unequal cell elongation rather than any nervous or muscular process. The full mechanism — including exactly how auxin becomes unequally distributed, and why that produces bending — is covered on its own page: auxin and the chemical control of shoot growth.