Coordination and response in plants

Biology · Coordination and response in plants. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Plants respond to their surroundings by growing. A tropism is a growth response to a stimulus that comes from one direction. In phototropism a plant part grows towards or away from light. In gravitropism it grows towards or away from gravity. Shoots grow towards light and away from gravity. Roots grow towards gravity.

Shoot growth is controlled by auxin. Auxin is made in the shoot tip and spreads down the shoot. It makes cells grow longer (cell elongation). Light from one side, or gravity, makes auxin collect unequally. More auxin gathers on the shaded side or the lower side of a shoot. The cells there elongate more, so the shoot bends towards the light or upwards.

Rules to remember

Phototropism: growth towards or away from light. Gravitropism: growth towards or away from gravity.
Shoots: positive phototropism, negative gravitropism. Roots: positive gravitropism.
Auxin is made in the shoot tip and spreads through the plant from the tip.
Auxin stimulates cell elongation in shoots.
Light from one side: more auxin on the shaded side, so that side grows faster and the shoot bends towards the light.
Horizontal shoot: more auxin on the lower side, so that side grows faster and the shoot bends upwards.
No tip, or tip covered: no bending towards light. A clinostat turns the plant slowly so the stimulus acts equally on all sides.

Common mistake

The shoot does not bend because the lit side grows more. The shaded side has more auxin and grows faster, which pushes the tip over towards the light.

Worked example

A pot plant on a windowsill has bent towards the window. The pot is turned round so the shoot points into the room. A week later the shoot bends towards the window again. Explain.

Light now reaches the shoot from the opposite side, so the shaded side has changed.
Auxin made in the tip collects on the new shaded side, the side facing the room.
Cells on that side elongate more than cells on the lit side.
The unequal growth bends the shoot back towards the window.

Answer: Auxin gathers on whichever side is shaded and makes it grow faster, so the shoot always bends towards the light. This is positive phototropism.

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