Plant Control Systems and Defenses

Plant Movement

Although they are anchored to soil, plants may exhibit spontaneous or induced movement.
Plants move and react to a variety of different stimuli in diverse ways. Plants may exhibit spontaneous movement without external stimulus, such as spiraling tendrils searching for a place to fasten to and continue growth. They may also exhibit induced movement in response to external stimuli. The three classes of movement are tropic, nastic, and tactic.

A tropism is a positive or negative response of a plant to a stimulus. Tropic movements are growth movements toward or away from a unidirectional stimulus, such as the movement in response to gravity. A nastic movement is a quick movement by plants in reaction to temperature, humidity, or touch; these movements are reversible and repeatable. Tactic movement is a plant's response to touch.

Nastic movements are responses to nondirectional stimuli. The frequency of nastic reactions increases proportionally to the intensity of the stimuli present. A Venus flytrap plant, for example, senses the presence of an insect touching its trigger bristles. The insect may enter the plant's blades at any point, but touching any of the trigger bristles causes a rapid reaction. The blades snap shut, trapping the insect, which is then digested by the plant. Another less violent but equally quick response comes from the mimosa plant. The mimosa is so sensitive that it responds immediately to direct touch, shaking, heat, or chill by closing its leaflets in seconds. Nutation movement, a form of nastic movement, is the bending movements by plants in which the plant inclines in a particular direction. These movements are executed internally by plant organs during their development. Nutation occurs in spiraling patterns without any physical contact with other objects. A plant's spiraling tendrils searching for a place to attach for support is an example of nutation movement.

Some plants experience tactic movement, a response to touch that may be phototactic, chemotactic, or thermotactic responses. Phototactic movements are responses to single-directional light. Algae swim freely toward light (positive phototactic reaction) or away from light (negative phototactic reaction). Chemotactic responses are reactions to chemical substances. An example of this is the movement of spermatozoids or male gametes moving toward female gametes in mosses, ferns, horsetails, and club mosses. This movement is the result of a chemical signal released by the egg cells.

One mechanism for movement is turgor movement. Turgor movement is a reversible positioning caused by changes in water content in a plant. This mechanism is related to the presence or absence of water within plant stems, branches, or leaves. Turgor movement is the result of increases and decreases in turgor pressure (internal fluid pressure) within a plant's cells. Turgor or hydrostatic pressure may be permanent or reversible. Ideally, a plant maintains a careful balance of water within its cells. Too much water can be as damaging as too little water. The amount of water in a vacuole (a fluid-filled organelle within a cell's cytoplasm) causes turgor pressure, which helps maintain the shape of each cell. The amount of water in the vacuole is determined by the osmotic flow of low-solute water through the xylem compared to the high-solute concentration of water in cell vacuoles. A healthy plant stem is turgid, providing strength and stability to the stem. Without sufficient water, turgor pressure decreases and plant stems wilt. When there is too much water, turgor pressure increases and may initiate cell rupture. An example of this is the barrel cactus. In normal conditions, a barrel cactus stores large amounts of water in its tissues. The cactus can become larger to hold more water when water is plentiful. This provides a mechanism to store water for later use during dry seasons. A barrel cactus may lose up to 82% of its water without dying. On the other hand, extensive water retention, which results in high turgor pressure, may cause epidermal layers on the plant's surface to split.

Movement may also be locomotive or growth/curvature movement. Locomotive movement is usually the movement of single-celled plants or multicelled plants that rotate or circulate in aquatic environments. Elodea and Hydrilla are plants in which the cytoplasm moves either clockwise or counterclockwise within a plant cell. Curvature movements are changes of direction, which can be vital movements for the survival of a plant. These might include movement toward or away from light, movement because of gravitational force, or movement to access needed water.

Plant movement may be reversible (such as turgor movement or movement toward a stimulus) or irreversible (such as the growth of new leaves and branches following the loss of an apical tip).

Turgor Pressure

Turgor pressure maintains a plant cell's shape. The vacuole in a turgid cell is filled with water, and the plant's cell shape is a normal rectangle. In a period of drought, when there is low turgor pressure, the vacuole shrinks, and cell walls begin to cave in. Externally, the loss of turgor pressure in cells results in a wilted plant.

Tropisms

Tropisms are plants' positive and negative responses to stimuli.
A tropism is a plant's reaction to stimuli, which may be either positive or negative. Plants bend or turn toward or away from stimuli. There are six basic types of tropism, each with a different stimulus and a different response mechanism in plants: phototropism, hydrotropism, geotropism, thigmotropism, thermotropism, and chemotropism.

Phototropism is plant movement based on light. It is primarily, but not exclusively, a plant's movement toward sunlight. Plants may have different reactions to light. Most commonly, plant leaves and flowers face toward light; roots grow away from light. Because light is essential for photosynthesis, it is necessary for a plant to make the most of the sunlight available. Thus, the broadest blade surface faces the sun, increasing the amount of photosynthesis that occurs. Leaves are positively phototropic, responding positively toward light. The plant hormone auxin is closely related to a plant's ability to bend toward light. As light strikes a plant at an angle, auxin moves to the shaded side of the stem or shoot. Auxin causes cells on the shaded sight to elongate, pressing the plant to lean toward the sun.

Hydrotropism is a plant's response to the presence of water. For example, grass on a golf course grows toward sun and water. Roots nearly always seek water, and root system growth shows positive responses to collected water.

Hydrotropism Demonstration

The roots in this terrarium reach toward the water source. This is a common demonstration showing hydrotropism, the response of plant roots to the presence of water, but can also be seen in the "real world" when the roots of a tree move toward a water drain line or a natural stream.
Geotropism is the effect of gravity on plant movement; it is also called gravitropism. Because gravity is a constant force, its effect on plants is both constant and continuous, unlike the influence of sun, water, or touch. Primary roots (the first root produced by a germinating seed), including taproots (single, anchoring roots that grow vertically downward), tend to grow straight down in response to gravity. Secondary roots (root systems extending off the main root) may also grow downward, but they also spread laterally. Root growth downward is a positive response to gravity, while a plant's growth upward is a negative response. Some plants are forced to grow horizontally, and their responses to that positioning include sending out new root structures or tubers. Geotropism of horizontally growing plants is influenced by the uneven distribution of the plant hormone auxin. Auxin tends to collect on the downward side of the plant stem, encouraging growth on the lower side.

Thigmotropism, a reaction to touch, occurs when plants come in contact with solid objects. A typical response of most vines is to grow straight up until they touch an object. At that point, the plant reacts by sending out tendrils that wrap around the object. This is evident in home gardens where tomatoes, sugar snap peas, and pole beans latch onto trellises as they grow. Moonflowers, clematis, and morning glories are plants that also have thigmotropic reactions and latch onto trellises, strings, structures, or other plants as they grow.

Thermotropism, responses to dramatic fluctuations in air temperature, may be either positive or negative, depending on the plant. An example of thermotropism is the curling of rhododendron leaves in response to cold. The roots of corn shoots have been shown to bend toward cooler temperatures, even if root growth defies normal geotropic pressure for downward growth patterns.

Chemotropism, the reaction to specific chemicals in a plant, affects the movements of plant organs. The most common example is that of pollen tubes moving toward the ovary, which occurs when calcium and borate are absorbed from the style, the long stalk that connects the stigma and the ovary. Other chemical responses include negative reactions to attacks by bacteria and fungi and positive reactions to high-nutritive mineral content in water. Pollen shows a chemical reaction when a pollen grain lands on the stigma (the part of the pistil where pollen germinates) of a flower, which stimulates the growth of the pollen tube toward the ovules in the ovary.

Plant Circadian Rhythms

Day and night influence a plant's circadian rhythms.
All animals and plants have a biological clock that triggers specific events in their life processes. This automatic timekeeper is important in giving plants the potential for survival against competition. The term circadian comes from the Latin circa, meaning "about," and diem, meaning "day." A circadian rhythm is a repeated process that occurs within a 24-hour period; these processes continue even without external cues. Circadian rhythms tell plants when to flower or emit scent to attract pollinators. Many angiosperms have built-in timing mechanisms that are controlled by circadian rhythms. These circadian rhythms are controlled by genetic pathways, where genetic transcripts have specific roles in regulating the cycle through a series of negative feedback loops.

A variety of plant processes occur during a 24-hour cycle: gene expression, changes in calcium levels, protein modification, movement of chloroplasts (cell organelles that contain chlorophyll for photosynthesis), opening and closing of stomata, stem and leaf growth, and flowering. For example, photosynthesis occurs in daylight, accompanied by the opening of stomata. At night, photosynthesis stops in many plant species and plant stomata close down when the leaves cannot use carbon dioxide from the air.

Plants also react to the day and night patterns of life. Many plants "wake" with the sunlight, opening up flowers and leaves, and close flowers and leaves for the night. Crocuses, tulips, hibiscus, and poppies close nightly because of changes in temperature. Cooler air and darkness make the lower petals grow faster, forcing upper petals to close. In some plants, leaves react to night by closing their leaflets and taking on a vertical posture. Clover bean and coral trees are two such plants that appear to "go to sleep" for the night. In fact, their actions protect their leaves and give the plants an opportunity to grow. Bean plants follow this pattern of day and night leaf postures, and even when placed in continuous artificial sunlight, they continue to exhibit the same circadian leaf postures. The change in day and night patterns may slightly alter a plant's normal circadian plan but only by a few hours either way.

Circadian Rhythms in Plants

Plants follow a circadian rhythm involving daily processes necessary to maintain vital life functions. For example, gene expression (1) is critical for plant development and responses to environmental changes. Cytosolic calcium levels (2) play structural roles in cell wall and cell membrane strength. Protein phosphorylation (3) is the attachment of phosphorus to proteins to assist plants in maintaining homeostasis. As hypocotyl length increases (4), a germinating seed's cotyledon and leaves are pushed above the soil surface. The cotyledon is an embryonic leaf of a seed-producing plant that develops into a flowering plant.