Phototropism

Where it happens

Phototropism happens in the growing regions of shoots, most clearly seen when a plant receives light from only one direction, such as a seedling on a windowsill.

Two regions do the work. The shoot tip (the apical meristem) senses the direction of the light and produces auxin.

The zone of elongation a few millimetres below the tip is where the cells actually lengthen, so this is where the bend forms. If the tip is shaded with an opaque cap but the zone below is lit, no bending occurs; if the tip is lit and the zone below is covered, the shoot still bends.

This pair of results is the standard evidence that the tip detects light and the response happens lower down.

Inputs and outputs

• Input: light from one direction (the stimulus), detected by the shoot tip.
• Input: auxin, a growth hormone made in the tip and moved down the shoot.
• Input: water and food for the elongating cells, whose walls stretch as the vacuoles take up water.
• Output: unequal elongation, the shaded side grows longer than the lit side.
• Output: a shoot that curves towards the light source (positive phototropism).
• Output: leaves held facing the light, which raises the rate of photosynthesis.

The steps

1. Light shines on the shoot tip from one direction only.
2. The cells of the shoot tip detect the difference in light intensity between the lit side and the shaded side.
3. Auxin made in the tip is transported sideways across the tip, so more of it collects on the shaded side.
4. The auxin moves down the shoot and reaches the zone of elongation, still concentrated on the shaded side.
5. In shoot cells, auxin loosens the cell walls, so the cells on the shaded side take up water and elongate faster than those on the lit side.
6. The shaded side becomes longer than the lit side, and the shoot curves towards the light.
7. The leaves are now angled towards the light, increasing the light available for photosynthesis.

Why it matters and how it is controlled

By growing towards light, a shoot positions its leaves to capture more light for photosynthesis. This response is especially important for seedlings and plants growing in shaded or uneven light conditions, where positioning leaves correctly can determine how much food the plant can make.

Control rests on the concentration of auxin and on the tissue that receives it. Shoot cells elongate more when the auxin concentration rises, which is why the shaded side grows faster.

Root cells respond to the same hormone in the opposite way: a higher auxin concentration slows their elongation. That difference explains why a root lit from one side bends away from the light (negative phototropism) even though auxin still gathers on the shaded side.

The response is slow, taking hours rather than seconds, because it depends on growth rather than on muscle or nerves.

How it is examined

You may be asked to explain the role of auxin in phototropism, to describe the result of an experiment where a seedling is lit from one side, or to explain what happens if the shoot tip, where auxin is produced, is removed.

Questions often present a set of coleoptile experiments: a tip removed, a tip covered with an opaque cap, a tip replaced on an agar block, or a mica sheet inserted on one side to block auxin movement. You must predict whether each shoot bends, and justify the prediction by tracing where auxin can and cannot travel.

The link to geotropism is also common, one table comparing stimulus, direction of response and the effect of auxin on shoot versus root cells.

Common misconceptions

Worked exam-style question

Question. Four oat seedlings, P, Q, R and S, were grown in a dark box with a single window so that light entered from the left. P was left intact.

The tip of Q was removed. The tip of R was covered with a small foil cap.

In S, a thin sheet of mica was pushed into the right (shaded) side of the tip. After 24 hours, only P had bent towards the window.

(a) Name the response shown by P. (b) Explain why Q and R did not bend.

(c) Explain why S did not bend, even though its tip was lit. (d) State one advantage of this response to the plant.

Model answer. (a) Positive phototropism. (b) In Q, removing the tip removed the source of auxin, so no hormone reached the zone of elongation.

In R, the foil cap stopped the tip from detecting the direction of light, so auxin was not moved to one side and both sides elongated equally. (c) In S, the mica sheet blocked the sideways movement of auxin to the shaded side; the auxin stayed evenly spread, so the cells on both sides elongated at the same rate and the shoot grew straight.

(d) The leaves receive more light, raising the rate of photosynthesis.

Source:SRC-DSKP-EN

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