Plant Tropisms and Environmental Responses
A tropism is a directional growth response of a plant to a stimulus, such as light, gravity or touch, in which the plant grows toward or away from the source. In brief: growth toward a stimulus is a positive tropism and growth away from it is a negative tropism. Shoots usually show positive phototropism (they grow toward light) and negative gravitropism (they grow upward), while roots show positive gravitropism (they grow downward).
Plants cannot move from place to place, so they respond to their surroundings by changing how they grow. This guide explains how plants detect and respond to stimuli, the main tropisms, the role of the plant hormone auxin, how to set up a simple phototropism experiment and which mistakes appear most often.
How do plants respond to stimuli?
A stimulus is a change in the environment that a plant can detect. Plants sense light, gravity, water, touch, temperature and chemicals. They respond with changes in growth, not with muscles or nerves, so most responses are slow and take hours or days.
Responses fall into two groups. Growth responses, called tropisms, are slow and directional. Rapid movements that do not involve growth, called nastic movements, are not directed by the source of the stimulus.
What are tropisms?
Tropisms are named for the stimulus that causes them. The prefix indicates the stimulus, as in photo- for light and gravi- for gravity.
Phototropism
Phototropism is growth in response to light. Shoots grow toward light, which is positive phototropism, so leaves can capture more light for photosynthesis. Roots usually grow away from light, which is negative phototropism.
Gravitropism
Gravitropism, sometimes called geotropism, is growth in response to gravity. Roots grow downward, which is positive gravitropism and brings them into the soil. Shoots grow upward, which is negative gravitropism. A seed germinating in the dark knows which way is up for this reason.
Hydrotropism and thigmotropism
Hydrotropism is growth toward water. Roots can bend toward moist areas in soil. Thigmotropism is growth in response to touch, and it is seen when a pea tendril coils around a support or a vine wraps around a pole.
How does auxin control tropisms?
Plant hormones coordinate the responses, and the key hormone for shoot bending is auxin. Auxin is made in the tip of the shoot and moves down the stem. It makes cells in the stem elongate.
When light shines from one side, auxin moves toward the shaded side of the shoot. Cells on the shaded side receive more auxin and elongate more. The shoot therefore bends toward the light.
In roots the effect is different. Roots are much more sensitive to auxin, and a high concentration inhibits cell elongation. When a root lies sideways, auxin builds up on the lower side and slows growth there, while the upper side keeps growing. The root therefore curves downward. This explanation is widely taught and well supported, but the full molecular details are still being studied.
Main tropisms compared
| Tropism | Stimulus | Typical shoot response | Typical root response |
|---|---|---|---|
| Phototropism | Light | Grows toward light | Often grows away from light |
| Gravitropism | Gravity | Grows upward | Grows downward |
| Hydrotropism | Water | Little response | Grows toward moisture |
| Thigmotropism | Touch | Coils around supports | Bends around obstacles |
What other responses do plants show?
Not all responses are tropisms. Some happen quickly and do not depend on the direction of the stimulus.
- Touch-sensitive leaves: The leaflets of a mimosa fold within seconds when touched.
- Sleep movements: Some leaves and flowers open in the day and close at night.
- Photoperiodism: Flowering that depends on the length of day or night. Chrysanthemums flower when nights are long.
- Stomatal closing: In dry conditions, guard cells close the stomata to reduce water loss.
- Leaf fall: In autumn, many trees shed their leaves in response to shorter days and lower temperatures.
These responses help plants to survive harsh conditions, to avoid being eaten and to time flowering to suit pollinators.
Worked example: a phototropism experiment
A class wants to show that shoots grow toward light. They use young seedlings of cress or oat grown in small pots.
- Prepare three identical pots with seedlings that are the same age and about the same height.
- Pot A stands in a box with a window cut in one side, so light reaches the plants only from that side.
- Pot B stands in a closed box with no light at all.
- Pot C stands in a box with the same window, but it is turned slowly on a rotating stand or turned by hand each day, so light arrives from all sides on average.
- Leave the boxes for four days at the same temperature, then observe the direction of growth and measure the angle of the shoot from the vertical.
The expected result is that shoots in A bend clearly toward the window. Shoots in B grow straight but are pale and long, because plants in the dark stretch to find light. Shoots in C stay mostly upright. The comparison shows that one-sided light causes the bending. Measuring several shoots in each pot and finding the mean angle makes the conclusion more reliable than looking at a single plant.
Who discovered tropisms?
Charles Darwin and his son Francis studied how grass seedlings bend toward light and published their results in 1880. They found that covering the tip stopped the bending, and they suggested that a signal passed from the tip to the lower part.
Later work by Peter Boysen Jensen, Árpád Paál and Frits Went showed that the signal was a chemical substance that diffuses down from the tip. Went isolated it in the 1920s and named it auxin. Today’s research looks at the proteins that move auxin from cell to cell.
How are plant responses used in practice?
Farmers and gardeners use tropisms in many ways. Growers turn pots regularly so that house plants grow evenly, and they provide trellises that suit the climbing habit of beans and peas. Greenhouses are lit with specific wavelengths to control growth.
Understanding photoperiodism lets growers force flowering at the right time. Florists give chrysanthemums controlled periods of darkness to make them bloom for particular dates. Space biology also studies how plants grow without a clear direction of gravity.
Which mistakes are common?
Mistake 1: Saying that plants “want” to reach the light
Plants respond automatically to chemical signals. Wording such as “auxin makes cells on the shaded side grow longer” is more accurate.
Mistake 2: Thinking auxin accumulates on the lit side
Auxin moves toward the shaded side of the shoot. That side grows more, and the shoot bends toward the light.
Mistake 3: Assuming auxin has the same effect on roots as on shoots
High auxin levels promote elongation in shoots but inhibit it in roots. This difference explains why shoots and roots bend in opposite directions.
Mistake 4: Treating mimosa folding as a tropism
The mimosa response is a rapid nastic movement based on changes in water pressure in cells. It does not depend on the direction of the touch.