Geotropism

Where it happens

Geotropism happens in the growing regions of roots and shoots, and is most clearly demonstrated when a seedling is turned onto its side so that gravity acts across, rather than along, its length.

Two regions matter. Gravity is detected at the tip: the root cap in roots and the apical meristem in shoots, where auxin is produced.

The bending itself happens a short distance behind the tip, in the zone of elongation, where cells are still lengthening and can respond to auxin. A seedling whose root tip has been removed continues to grow but no longer bends towards gravity, which shows that detection and response occur in different places.

Inputs and outputs

• Input: the stimulus, gravity, acting on the tip of the root or shoot.
• Input: auxin, a plant hormone made in the apical meristem and transported downward through the tissue.
• Input: cells in the zone of elongation that can lengthen when their cell walls loosen and they take in water by osmosis.
• Output: an unequal distribution of auxin, with a higher concentration on the lower side of both organs.
• Output: unequal cell elongation, which produces a curvature.
• Output: a root growing downward (positive geotropism) and a shoot growing upward (negative geotropism).

The steps

1. A seedling is laid horizontally, so gravity now acts across the root and shoot instead of along them.
2. The tips detect the change: dense starch grains in the root cap and shoot tip settle to the lower side of the cells.
3. Auxin becomes redistributed so that more of it accumulates on the lower side of both the shoot and the root.
4. In the shoot, this higher concentration of auxin on the lower side promotes greater cell elongation there than on the upper side.
5. The shoot's lower side grows faster than its upper side, so the shoot curves upward, away from gravity, negative geotropism.
6. In the root, the same higher concentration of auxin on the lower side instead inhibits cell elongation, because root cells are far more sensitive to auxin than shoot cells.
7. The root's upper side grows faster than its lower side, so the root curves downward, towards gravity, positive geotropism.
8. Once each organ is again aligned with gravity, auxin is distributed evenly and straight growth resumes.

Why it matters and how it is controlled

Geotropism ensures that roots grow down into the soil, where they can anchor the plant and reach water and mineral ions, while shoots grow up towards light and air, where their leaves can photosynthesise. This happens correctly even for a seed that germinates upside down or sideways in the soil.

The response is controlled by auxin concentration and by the different sensitivity of the two organs. Shoot cells elongate more as auxin concentration rises across the range found in a plant.

Root cells respond to a much lower concentration and are inhibited by the concentration that stimulates shoots. The same hormone therefore produces opposite curvatures without any change in the hormone itself.

Geotropism interacts with phototropism. In a shoot growing at a window, light from one side and gravity from below both act on auxin distribution, and the final direction is a balance of the two.

In a root, gravity dominates because roots are only weakly sensitive to light.

How it is examined

You may be asked to explain why roots and shoots respond oppositely to the same auxin distribution, to predict the direction of growth of a horizontally placed seedling, or to compare geotropism with phototropism.

Diagram questions show a horizontal seedling with the auxin distribution shaded and ask you to draw or describe the curvature of the root and the shoot after two days. Experiment questions use a clinostat, a slowly rotating drum that cancels the one-sided effect of gravity; you are expected to explain why a seedling on a clinostat grows straight and to identify it as the control.

Essay questions often ask for a comparison table of geotropism and phototropism covering stimulus, organ, direction and hormone.

Common misconceptions

Worked exam-style question

Question. Three bean seedlings were grown in moist sawdust. Seedling A was left upright.

Seedling B was laid horizontally in the dark. Seedling C was laid horizontally in the dark and fixed to a clinostat rotating slowly about its long axis.

After three days the root and shoot of each seedling were observed. (a) Describe the appearance of the root and shoot of seedling B.

(b) Explain the observation for the root of seedling B. (c) Predict the appearance of seedling C and explain the purpose of the clinostat.

Model answer. (a) The root curves downward and the shoot curves upward. (b) Gravity causes auxin to accumulate on the lower side of the root.

Root cells are highly sensitive to auxin, so this higher concentration inhibits cell elongation on the lower side. The upper side elongates faster, so the root curves downward, showing positive geotropism.

(c) The root and shoot of seedling C grow straight, horizontally. The clinostat rotates the seedling so that gravity acts equally on all sides, auxin is evenly distributed, and no side elongates faster.

It acts as the control, showing that the curvature in B was caused by the one-sided action of gravity and not by the horizontal position or darkness.

Source:SRC-DSKP-EN

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