What are tropisms?
Contributed by: backyardchemist5 and diels alder
Definition: Tropisms are a growth response involving a bending or curving of a plant response toward or away from an external stimulus. Bending toward the stimulus is a positive tropism, away is negative.
Phototropism
(diels alder) is caused by the uneven distribution of auxin in the plant stem. Auxin is produced in young expanding leaves and is then transported down the stem, cell to cell (not in the vasculature of the plant). Auxin produces elongation in young cells, causing the internodes to elongate as it passes to the roots. Light causes the transporters (Normally located on the lower side of the cell membranes) to move to the side of the cell furthest from the source of light. As auxins are transported away from the light, they cause uneven elongation in the stem. The 'dark' side of the stem ends up getting higher levels of auxin - and elongates more.
Geotropism
is what the plant uses to sense in what direction the force of gravity is pulling. Using this, a germinating seedling knows to grow the taproot downward, and the stalk upwards to the surface of the soil.
Gravitropism
(diels alder) in the stem is also controlled by an effect similar to Geotropism. A horizontal stem (illuminated or not) accumulates auxin in the lowest portion of the stem causing it to curve upwards as those cells expand. Gravitropism in roots is controlled by special starch containing plastids (chloroplasts are a type of plastid) called statoliths in special cells in the root tip. As said above, they settle in different parts of the cell depending on the direction of gravity. How the signal is transferred and the root curves is not yet known.
Thigmotropism
(diels alder) is the bending of plant parts in response to touch. This is seen in plants with leaves modified to tendrils which twine around an object when the tendril touches it (like pea plants). It is believed to be controlled by a group of stretch-activated membrane transport proteins.
Hydrotropism
is what plants use to send their root growth in the direction of water. During times when the soil is drier the plants can send their roots in the direction of water to receive the maximum amount of water possible.
(diels alder) Hydrotropism is now actually believed to be an artifact. Roots appear to grow into moist soil and avoid dry, but this has been shown to simply be the fact that roots need available water to grow. This does not fill the definition of tropism. The only documented cases that could be called hydrotropism, are roots on plants grown in humid air which show slight curvature in response to the source of humidity. It is now accepted that this does not occur in solid or liquid growth mediums.
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