Plants adapt to their environment through tropisms, which are directional responses to external stimuli, and nastic movements, which occur independently of the direction of the stimulus.
Basic Knowledge
Tropism: A growth response that depends on the direction of a stimulus.
①Phototropism: Stems grow toward light (positive phototropism), whereas roots tend to avoid light (negative phototropism). This is caused by the lateral redistribution of auxin.
②Gravitropism: Stems grow upward against gravity (negative gravitropism), whereas roots grow in the direction of gravity (positive gravitropism). Amyloplasts (starch grains) in statolith cells of the root cap act as gravity sensors. Unequal distributions of auxin and ABA are involved.
③Thigmotropism: The phenomenon in which the tendrils of climbing plants coil around objects they touch. Cells on the side opposite the contact elongate.
④Hydrotropism: Roots grow toward water.
⑤Chemotropism: Roots grow toward or away from chemical substances.
Nastic movement: A movement that occurs independently of the direction of a stimulus.
①Photonasty: Dandelion flowers open during the day and close at night (changes in cell turgor pressure).
②Seismonasty (thigmonasty): Mimosa leaves close when touched (rapid water release from pulvinus cells → decreased turgor pressure).
③Thermonasty: Tulips open as the temperature rises.
Polar auxin transport: PIN-like transporters are distributed with polarity in the cell membrane and transport auxin in one direction (tip → base). Polar transport is the basis of the directionality of tropisms.
Explain why a plant stem shows negative gravitropism rather than positive gravitropism (it grows upward), based on the unequal distribution of auxin and the sensitivity of cells.
Answer: In a stem laid sideways, gravity causes more auxin to accumulate on the lower side. Stem cells have a broad optimal auxin concentration range, so the high auxin concentration on the lower side promotes elongation. The upper side elongates less because its auxin concentration is low, and the difference in elongation, lower side > upper side, causes the stem to bend upward (opposite to the direction of gravity).
- Phototropism = lateral auxin distribution → increased elongation on the shaded side → bending toward light
- Gravitropism = stems upward (negative); roots downward (positive)
- Root-cap statolith cells = amyloplasts act as gravity sensors
- Thigmotropism = climbing plants grasping objects
- Nastic movement = independent of stimulus direction (photonasty and seismonasty)
- Mimosa = leaves close because of decreased turgor pressure in the pulvinus
- PIN transporters = responsible for polar auxin transport
Points to Note
① Tropisms are irreversible changes caused by growth, whereas nastic movements are often reversible movements caused by changes in turgor pressure. ② Roots are more sensitive to auxin than stems, so a concentration that promotes growth in stems may inhibit growth in roots. ③ The seismonasty of Mimosa is caused by the transmission of action potentials and changes in pulvinus turgor pressure, which differs from nerve transmission in animals.
Practice
- State the difference between the definitions of tropism and nastic movement.
- When plant roots show positive gravitropism, state the part that senses gravity and the cell organelle used for sensing it.
- Explain how Mimosa leaves close when touched from the viewpoints of the pulvinus, turgor pressure, and water movement.