Movements Resulting from External Stimuli

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Permanent movements resulting from external stimuli coming from one direction are commonly referred to as tropisms. Tropic movements can be divided into three phases:

1. In the initial perception phase, the organ receives a greater stimulus on one side.

2. Then, transduction occurs, during which one or more hormones become(s) unevenly distributed across the organ.

3. Finally, asymmetric growth occurs as a result of the uneven distribution of the hormone causing greater cell elongation on one side.

208 Chapter 11

Negative Phototropism
Figure 11.Q A cyclamen plant that received light from one direction for several weeks. Note how all visible plant parts are oriented on the side that received light.

Phototropism The main shoots of most plants growing in the open tend to develop vertically, although the branches often grow horizontally. If a box is placed over a plant growing vertically and a hole is cut to admit light from one side, the tip of the plant will begin to bend toward the light within a few hours. If the box is later removed, a compensating bend develops, causing the tip to grow vertically again. Such a growth movement toward light is called a positive phototropism (Fig. 11.9). A similar bending away from light is called a negative phototropism. The shoot tips of most plants are positively phototropic, while roots are either insensitive to light or negatively phototropic.

We have already noted that if the tip of a coleoptile is covered or removed, the structure will not bend toward light and that auxin is produced in the tip (see Fig. 11.1). We have also noted that auxin promotes the elongation of cells, at least in certain concentrations. For some time, it was believed that stem tips bent toward light because auxin was destroyed or inactivated on the exposed side, leaving more growth-promoting hormone on the side away from the light, causing the cells there to elongate more and produce a bend. Careful experiments have shown, however, that stem tips growing in the open have the same total amount of auxin present as stem tips from the same species receiving light from one side. Other experiments indicate that the auxin migrates away from the light, accumulating in greater amounts on the opposite side, promoting greater elongation of cells on the "dark" side. Apparently, an active transport system enables the auxin to migrate against a diffusion gradient.

Different intensities of light may bring about different phototropic responses. In Bermuda grass, for example, the stems tend to grow upright in the shade and parallel with the ground in the sun. In other plants, such as the European rock rose, which grows among rocks or on walls, the flowers are positively phototropic, but once they are fertilized, they become negatively phototropic. As the pedicels (stalks) elongate, the developing fruits are buried in cracks and crevices, where the seeds then may germinate. Phototropic responses are not confined to flowering plants. A number of mushrooms, for example, show marked positive responses to light, and certain fungi that grow on horse dung have striking phototropic movements, which are discussed in Chapter 19.

Gravitropism Growth responses to the stimulus of gravity are called gravitropisms. The primary roots of plants are positively gravitropic, while shoots forming the main axis of plants are negatively gravitropic (Fig. 11.10). It originally was postulated that plant organs perceive gravity through the movement of amyloplasts containing large starch grains located in special cells of the root cap (discussed in Chapter 6). The amyloplasts are also found in coleoptile tips and in the endodermis. When a potted plant is placed on its side, the starch-containing amylo-plasts will, within a few minutes, begin to float or tumble down until they come to rest on the side of the cells closest to the gravity stimulus. In roots, the cells on the side opposite the stimulus begin elongating within 10 seconds to an hour or two, bringing about a downward bend, while the opposite occurs in stems (Figs. 11.11 and 11.12).

Recent experiments have cast doubt about the role of amyloplasts in bringing about gravitropic responses. It has been demonstrated that the roots of plants completely lacking root-cap amyloplasts still perceive and respond to gravity. Other recent research suggests that proteins on the outside of the plasma membrane next to the cell wall may be essential to sensing gravity and that the whole protoplast of a cell, rather than amyloplasts within, is involved in gravitropic responses.

Some cell biologists have suggested that mitochondria and dictyosomes also respond to gravity, but precisely how they bring about the response has been the subject of much conjecture. Auxin and ABA, along with calcium ions and proteins, well may all be involved in modifying cell elongation so as to produce these gravitropic bendings.

The stimulus of gravity can be negated by rotating a plant placed in a horizontal position. A simple device called a clinostat uses a motor and a wheel to rotate a potted plant slowly about a horizontal axis (Fig. 11.13). As the plant rotates, both the stem and roots continue to grow horizontally instead of exhibiting characteristic gravitropic responses. Obviously, neither the starch grains in the statoliths (gravity

Growth 209

Coleus Plant Placed Side

Figure 11.10 This Coleus plant was placed on its side the hours of the pot being tipped over.

before the photograph was taken. The stems bent upward within 24

Figure 11.10 This Coleus plant was placed on its side the hours of the pot being tipped over.

Root Cap Gravitropism

Figure 11.11 A root cap of a tobacco plant. The force of gravity is at the bottom of the picture. Note that the amyloplasts (more or less spherical dark objects) are toward the bottom of each cell. There is conflicting evidence as to whether or not the amylo-plasts play a role in the perception of gravity by roots, x 2,000. (Light micrograph courtesy John Z. Kiss)

Figure 11.11 A root cap of a tobacco plant. The force of gravity is at the bottom of the picture. Note that the amyloplasts (more or less spherical dark objects) are toward the bottom of each cell. There is conflicting evidence as to whether or not the amylo-plasts play a role in the perception of gravity by roots, x 2,000. (Light micrograph courtesy John Z. Kiss)

before the photograph was taken. The stems bent upward within 24

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Responses

  • clifford
    What causes phototropism in green plants?
    8 years ago
  • pansy
    How are the roots and stem of a plant on its side?
    7 years ago
  • Demet
    How does light bring tropic movement to a root or stem of a flowering plant?
    7 years ago
  • innes
    What is the stimuli and response of a plant placed horizontally when taken out?
    7 years ago
  • kalvin
    What is the permanent growth movement of a part of a plant in response to external stimuli?
    3 years ago
  • Caramella
    Which side will be greater in plant to the stimulus of gravity?
    3 years ago
  • Thorsten
    Are tropic movements permament?
    3 years ago
  • Asmarina
    How does light brings about trophic movrment in a stem of flowering plants?
    3 years ago
  • ulla
    How light brings about tropic movement in flowering plants?
    2 years ago
  • Prima
    What is a movement of a plant part to an external stimuli of the weather?
    2 years ago
  • Delmo
    How does light brings about tropic movement in the stem of a flowering plants?
    2 years ago
  • Katrin
    Which component of plants relieve stimulas for flowering?
    2 years ago
  • Dana Winslow
    How light brings tropic movement in the stem and root of a plant?
    2 years ago
  • becky
    How dose light bring about tropic movement in stem and root of plant?
    2 years ago
  • Michael
    What is a response or movement of a plant part to an external stimuli of the weather?
    1 year ago
  • Geraldine
    How light brings about tropic movement in stem of a flowering plant?
    11 days ago
  • Benjamin
    Which external stimulus is responsible for phototropic movement shown by roots?
    10 days ago

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