Glossary: Responses in Plants

This chapter's terms in context

Chapter 20 covers four terms describing how plants detect and respond to stimuli in their environment: tropism, phototropism, geotropism and auxin. A tropism is a directional growth response of a plant towards or away from a stimulus, and it is this growth-based nature that fundamentally distinguishes a plant's tropic response from an animal's much faster, usually movement-based response to the same kind of stimulus; a plant cannot run away from or move quickly towards a stimulus, so it instead grows differently on one side of the affected organ than the other, causing the whole organ to bend over time. A tropism is described as positive when growth is towards the stimulus and negative when growth is away from it.

Phototropism is the tropic response of a plant to light, and in most plants, a shoot shows positive phototropism, growing towards a light source, while a root typically shows negative phototropism (or no strong response), growing away from or unaffected by the direction of light. Geotropism (also called gravitropism) is the tropic response of a plant to gravity, and here the pattern is reversed compared with phototropism: a root shows positive geotropism, growing downward in the direction gravity pulls, while a shoot shows negative geotropism, growing upward against the direction of gravity. A very common exam error is stating a general rule such as 'roots always grow towards a stimulus', without recognising that a root behaves in exactly opposite ways to light and to gravity, negative to light, positive to gravity, so a correct answer must always specify which particular stimulus is being discussed.

Auxin is the plant hormone responsible for coordinating both of these tropic responses, and it works by causing unequal growth on the two sides of a shoot or root: in a shoot exposed to light from one side, auxin accumulates on the shaded side rather than the illuminated side, causing cells on the shaded side to elongate more than those on the illuminated side, which bends the shoot tip towards the light. In geotropism, gravity causes auxin to accumulate on the lower side of both a horizontally placed shoot and a horizontally placed root, but the two organs respond to the same higher auxin concentration in opposite ways, the shoot's lower side elongates more, bending the shoot upward, while the root's lower side elongates less, bending the root downward, because root cells and shoot cells respond differently to the same auxin concentration, with root cells being far more sensitive and actually inhibited by the concentration that promotes growth in a shoot.

This single mechanism, auxin causing unequal cell elongation between the two sides of an organ, with roots and shoots responding oppositely to the same concentration, is what most SPM structured questions on this chapter are actually testing, so a candidate who can explain this mechanism accurately for one tropism can usually transfer the same reasoning to the other with only the direction of the response changed.

Paper 3 questions on this chapter frequently ask a candidate to design or interpret an experiment using a seedling in a box with a one-sided light source, or a germinating seed mounted on a rotating clinostat to remove the directional effect of gravity, and full marks depend on stating auxin distribution as the underlying explanation for any observed bending, rather than describing the bending itself as if it were the explanation.

A frequently tested variation of this experiment places a seedling on its side (horizontally) rather than shining light from one side, specifically to isolate the effect of gravity from the effect of light, and a candidate is expected to predict that the shoot will still bend upward and the root downward even in complete darkness, since geotropism does not require light at all, unlike phototropism. Confusing these two experimental set-ups, one isolating light, the other isolating gravity, and describing a result caused by one stimulus using the vocabulary of the other is a specific and avoidable source of lost marks in this chapter.