Although plants appear not to move, they have evolved adaptations to allow movement in response to various environmental stimuli; such mechanisms are called tropisms.
There are several kinds of tropism, each of which is named for the stimulus that causes the response. For example, gravitropism is a growth response to gravity, and phototropism is a growth response to unidirectional light.
Tropisms are caused by differential growth, meaning that one side of the responding organ grows faster than the other side of the organ. This differential growth curves the organ toward or away from the stimulus. Growth of an organ toward an environmental stimulus is called a positive tropism; for example, stems growing toward light are positively phototropic.
Conversely, curvature of an organ away from a stimulus is called a negative tropism. Roots, which usually grow away from light, are negatively phototropic. Tropisms begin within thirty minutes after a plant is exposed to the stimulus and are usually completed within approximately five hours.
Phototropism is a growth response of plants to light coming from one direction. Positive phototropism of stems results from cells on the shaded side of a stem growing faster than cells along the illuminated side; as a result, the stem curves toward the light. The rapid elongation of cells along the shaded side of a stem is controlled by a plant hormone called auxin that is synthesized at the stem’s apex.
Unidirectional light causes the auxin to move to the shaded side of stems. The increased amount of auxin on the shaded side of stems causes cells there to elongate more rapidly than cells on the lighted side of the stem. This, in turn, causes curvature toward the light.
Only blue light having awavelength of less than 500 nanometers can induce phototropism. The photoreceptors in this system are called cryprochromes and may alter the transport of auxin across cellular membranes, thereby facilitating its transport to the shaded side of the stem.
Phototropismis important for two main reasons: It increases the probability of stems and leaves intercepting light for photosynthesis and of roots obtaining water and dissolved minerals that they need.
Gravity-perceiving cells, called columella cells, are located in the center of the root cap. Each columella cell contains fifteen to twenty-five amyloplasts (starch-filled plastids) which, under the influence of gravity, sediment to the lower side of columella cells.
This gravity-dependent sedimentation of amyloplasts is the means where by roots sense gravity, possibly by generating electrical currents across the root tip. These gravity induced changes are then transmitted to the root’s elongating zone, located 3 to 6 millimeters behind the root cap. The differential growth that causes curvature occurs in the elongating zone.
When roots are oriented horizontally, growth along the lower side of the elongating zone is inhibited, thereby causing the root to curve downward. Among the first events that produce this differential growth is the accumulation of calcium ions along the lower side of the root tip.
Calcium ions move to the lower side of the cap and elongating zone of horizontally oriented roots. This movement may be aided by electrical currents in the root. The accumulation of calcium along the lower side of the root causes the auxin to accumulate there as well.
Because auxin inhibits cellular elongation in roots, the lower side of the root grows slower than the upper side of the root, and the root curves downward. When the root becomes vertical, the lateral asymmetries of calcium and auxin disappear, and the root grows straight down.
Gravity-sensing cells in stems are located throughout the length of the stem. As in roots, the auxin and calciumions in stemcells direct the negative gravitropism (in this case, upward curvature) of shoots. As auxin accumulates along the lower side, calcium ions gather along the upper side of horizontally oriented stems.
The accumulation of auxin along the stem’s lower side stimulates cellular elongation there. Gravitropism increases the probability of two important results: Roots will be more likely to encounter water and minerals, and stems and leaves will be better able to intercept light for photosynthesis.
This type of growth is called circumnutation, and it increases the tendril’s chances of touching an object to which it can cling. Contact with an object is perceived by specialized epidermal cells on the tendril.
When the tendril touches an object, these epidermal cells control the differential growth of the tendril. This differential growth can result in the tendril completely circling the object within five to ten minutes. Thigmotropism is often long-lasting.
For example, stroking one side of a tendril of garden pea for only a few minutes can induce a curling response that lasts for several days. Thigmotropism is probably controlled by auxins and ethylene, as these regulate thigmotropic-like curvature of tendrils even in the absence of touch.
Growing tendrils touched in the dark do not respond until they are illuminated. This light-induced expression of thigmotropism may indicate a requirement for adenosine triphosphate (ATP), as ATP will substitute for light in inducing thigmotropism of dark-stimulated tendrils.
Tendrils can store the sensory information received in the dark, but light is required for the coiling growth response to occur. Thigmotropism by tendrils allows plants to “climb” objects and thereby increases their chances of intercepting light for photosynthesis.
Hydrotropism and Heliotropism
|Hydrotropism and Heliotropism|
Heliotropism, or “solar tracking,” is the process by which plants’ organs track the relative position of the sun across the sky, much like a radio telescope tracks stars or satellites.
Different plants have different types of heliotropism. The “compass” plants (Lactuca serriola and Silphium laciniatum) that grow in deserts orient their leaves parallel to the sun’s rays, there by decreasing leaf temperature and minimizing desiccation.
Plants that grow in wetter regions often orient their leaves perpendicular to the sun’s rays, thereby increasing the amount of light intercepted by the leaf. Heliotropism occurs in many plants, including cotton, alfalfa, and beans. Sunflowers get their name from the fact that the flowers follow the sun across the sky.
On cloudy days, leaves of many heliotropic plants become oriented horizontally in a resting position. If the sun appears from behind the clouds late in the day, leaves rapidly reorient them selves they can move up to 60 degrees in an hour, which is four times more rapid than the movement of the sun across the sky. Heliotropism is controlled by many factors, including auxins.
Growth, Survival, and Beyond
Plants, like animals, are finely tuned to their environment; their growth and development are influenced strongly by that environment.
Tropisms are rapid, while other responses such as flowering are long-term and are associated with changes of season. Regardless of their duration, most responses of plants to environmental stimuli are the result of growth and are controlled, at least in part, by hormones.
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More important, tropisms help a plant to survive in its particular habitat, making use of separate systems for detecting and responding to environmental stimuli. Biologists are studying these systems in hopes of being able to mimic these detection and “guidance” systems.
Scientists at the National Aeronautics and Space Administration (NASA) study how plants perceive and respond to gravity in hopes that this knowledge will help in the understanding of how to grow plants in deep space.
NASA scientists also hope that understanding the gravity detection and guidance systems in plants will help people design more effective rockets which, like plants, must detect and respond to gravity to be effective.