Tropic Movements & Plant Hormones - Tenth Grade Science
While plants lack muscular tissues, specialized nervous systems, and rapid electrical action potentials characteristic of animals, they are responsive organisms capable of sensing environmental stimuli and executing coordinated growth movements. When an indoor houseplant bends toward a sunlit window, when subterranean seedling roots grow downward into soil, or when the tendril of a climbing pea vine coils around an external support, the plant is responding to directional environmental cues. These directional growth responses are termed tropic movements, coordinated by chemical messengers known as plant hormones or phytohormones. Synthesized in minute quantities in actively dividing meristematic tissues and translocated to target cells, phytohormones regulate cell division, cell elongation, apical dominance, fruit ripening, and stress adaptation. In this chapter, you will investigate the differences between tropic and nastic plant movements, analyze the mechanisms of phototropism, geotropism, hydrotropism, and thigmotropism, and examine the physiological functions of five major classes of plant hormones: auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
Categorization of Plant Movements: Tropic versus Nastic
Plant movements are classified into two categories based on whether the direction of the movement is dictated by the direction of the environmental stimulus.
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| TROPIC VERSUS NASTIC MOVEMENTS COMPARISON |
+--------------------------+-----------------------+--------------------------------+
| Characteristic | Tropic Movements | Nastic Movements |
+--------------------------+-----------------------+--------------------------------+
| Stimulus Directionality | Directional; response | Non-directional; response is |
| | determined by vector | independent of the stimulus |
| | of incoming stimulus | spatial direction |
| Physiological Mechanism | Driven by asymmetric, | Driven by reversible, rapid |
| | irreversible cell | osmotic turgor pressure |
| | elongation (Growth) | changes in cells (Non-growth) |
| Response Latency & Speed | Slow; requires hours, | Rapid; occurs within seconds |
| | days, or weeks | to minutes |
| Reversibility | Irreversible growth | Reversible elastic recovery |
| Primary Examples | - Stem phototropism | - Thigmonasty in Mimosa pudica |
| | - Root geotropism | - Nyctinasty (sleep movements) |
| | - Pollen chemotropism | - Venus flytrap closing |
+--------------------------+-----------------------+--------------------------------+
Tropic Movements (Tropisms)
A tropism (from Greek tropos, turn) is a directional growth movement of a plant organ in response to an external environmental stimulus. The movement can be positive (growth toward the stimulus) or negative (growth away from the stimulus). Because tropisms rely on cell division and differential cell elongation, they are slow, permanent, and irreversible.
Major classes of tropisms include: Phototropism: Directional growth in response to unilateral light (shoots are positively phototropic; roots are negatively phototropic). Geotropism (Gravitropism): Directional growth in response to gravity (roots are positively geotropic; shoots are negatively geotropic). Hydrotropism: Directional growth in response to moisture gradients (roots grow toward water). Chemotropism: Directional growth in response to chemical stimuli (the growth of a pollen tube down the style toward the ovule). Thigmotropism: Directional growth in response to physical mechanical contact (the coiling of vine tendrils around supports).
Nastic Movements (Nasties)
A nastic movement is a non-directional response of a plant organ to an external stimulus, where the direction of the movement is determined by the internal anatomy of the plant rather than the spatial vector of the stimulus.
Most nastic movements are non-growth, reversible responses driven by rapid shifts in osmotic turgor pressure within specialized motor cells. A classic example is thigmonasty (seismonasty) exhibited by Mimosa pudica (the sensitive "touch-me-not" plant): when a leaflet is touched, mechanical stimulation triggers a rapid wave of electrical depolarization that sweeps to the pulvinus, a swollen articulation base at the petiole. Cells in the lower half of the pulvinus actively extrude potassium ions, causing water to exit via exosmosis. The lower half loses turgor pressure while the upper half remains turgid, causing the leaf to fold and droop within seconds. Once the stimulus ceases, potassium and water re-enter the pulvinar cells, restoring turgor and reopening the leaf.
Mechanisms of Tropic Responses
Tropic growth movements are mediated by the asymmetric distribution of the growth hormone auxin.
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| MECHANISM OF SHOOT PHOTOTROPISM |
+-----------------------------------------------------------------------------------+
| |
| UNILATERAL SUNLIGHT |
| \ \ \ |
| \ \ \ [ Shoot Apex / Meristem ] |
| v v v / \ |
| [Light Side] [Shaded Side] |
| Low Auxin HIGH AUXIN |
| Normal ACCUMULATION |
| Growth | |
| | v |
| | Enhanced H+ Pumping |
| | Cell Wall Loosening |
| | Rapid Cell Elongation |
| | | |
| v v |
| ================================= |
| Asymmetric elongation curves the |
| shoot towards the light source! |
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Phototropism: Auxin Redistribution and Shoot Curvature
When a growing shoot tip is illuminated uniformly from all sides, auxin synthesized in the apical meristem diffuses downward evenly, causing cells across all sides of the stem to elongate uniformly, resulting in vertical growth.
However, when a plant shoot is exposed to unilateral (one-sided) light: Phototropin photoreceptors in the shoot tip detect the directional blue wavelengths of light. Auxin molecules migrate laterally away from the illuminated side toward the darker, shaded side of the shoot tip. The elevated concentration of auxin on the shaded side stimulates active proton-pumping ($\text{H}^+$) into the cell wall matrix, activating expansin proteins that loosen cellulose microfibrils (the acid-growth hypothesis). Cells on the shaded side absorb water and elongate faster than cells on the illuminated side. This differential elongation causes the shoot to bend toward the light source, demonstrating positive phototropism.
Geotropism (Gravitropism): Differential Sensitivity in Shoots and Roots
When a potted seedling is placed horizontally on its side in darkness, its shoot bends upward away from gravity (negative geotropism), while its root bends downward toward gravity (positive geotropism).
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| DIFFERENTIAL AUXIN SENSITIVITY IN GEOTROPISM |
+-----------------------------------+-----------------------------------------------+
| SHOOT BEHAVIOR (Negatively Geotropic) | ROOT BEHAVIOR (Positively Geotropic) |
+-----------------------------------+-----------------------------------------------+
| Auxin settles on lower side under | Auxin settles on lower side under |
| gravity. | gravity (guided by statolith starch grains). |
| | |
| In shoots, HIGH auxin concentration| In roots, HIGH auxin concentration INHIBITS |
| STIMULATES cell elongation. | cell elongation. |
| | |
| Lower side elongates faster than | Upper side (low auxin) elongates faster than |
| upper side. | lower side (high auxin). |
| | |
| Shoot curves UPWARD, away from g. | Root curves DOWNWARD, toward g. |
+-----------------------------------+-----------------------------------------------+
This contrasting response arises from the different sensitivities of shoot and root tissues to auxin concentration: In shoot tissues, relatively high concentrations of auxin stimulate cell elongation. When a shoot is placed horizontally, gravity causes auxin to accumulate on the lower side; the lower side elongates faster than the upper side, curving the shoot upward. In root tissues, cells are more sensitive to auxin; concentrations that stimulate shoot elongation inhibit root cell elongation. In horizontal roots, auxin accumulates on the lower side (directed by settling starch-filled amyloplast organelles called statoliths in root cap cells). The high auxin concentration inhibits elongation of the lower cells, while cells on the upper side continue to elongate normally, causing the root to curve downward into the soil.
Hydrotropism, Chemotropism, and Thigmotropism
Hydrotropism: Plant roots display positive hydrotropism, growing toward regions of higher soil moisture, an adaptation that helps plants find water in arid soils. If hydrotropism conflicts with geotropism, the hydrotropic response can override gravitropic orientation, directing roots toward water sources.
Chemotropism: Growth guided by chemical gradients. An example is the growth of the pollen tube during angiosperm fertilization: following pollination, the female stigma and style secrete sugary chemical attractants that guide the emerging pollen tube down through the style into the ovary toward the micropyle of the ovule.
Thigmotropism: Growth in response to physical contact. Climbing vines (such as peas, gourds, and passionflowers) develop slender, sensitive structural coils called tendrils. When a growing tendril contacts a solid object like a bamboo stake, contact inhibits cell elongation on the contacting side while stimulating cell elongation on the opposite side. This differential growth causes the tendril to coil tightly around the support, anchoring the climbing plant.
The Phytohormone Chemical Regulatory Network
Plant hormones (phytohormones) are small organic signaling molecules synthesized by plant cells that regulate physiological processes at low concentrations.
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| THE FIVE MAJOR PHYTOHORMONE CLASSES |
+-------------------+-------------------------------+-------------------------------+
| Hormone Class | Primary Sites of Biosynthesis | Dominant Physiological Effects|
+-------------------+-------------------------------+-------------------------------+
| Auxins (e.g., IAA)| Shoot apical meristems, | Apical dominance, cell |
| | young developing leaves, seeds| elongation, phototropism, |
| | | root initiation in cuttings |
|-------------------+-------------------------------+-------------------------------+
| Gibberellins | Developing seeds, young | Internodal stem elongation, |
| (e.g., GA3) | leaves, root tips | breaking seed dormancy, |
| | | bolting, fruit enlargement |
|-------------------+-------------------------------+-------------------------------+
| Cytokinins | Root tips (actively dividing | Promotes cytokinesis (cell |
| (e.g., Zeatin) | meristematic tissues), seeds | division), delays senescence, |
| | | breaks apical dominance |
|-------------------+-------------------------------+-------------------------------+
| Abscisic Acid | Mature leaves, chloroplasts, | Plant stress hormone, induces |
| (ABA) | roots experiencing drought | stomatal closure, promotes |
| | | seed and bud dormancy |
|-------------------+-------------------------------+-------------------------------+
| Ethylene (C2H4) | Ripening fruits, senescing | Only gaseous hormone; drives |
| | leaves and flowers, nodes | fruit ripening, stimulates |
| | | leaf and flower abscission |
+-------------------+-------------------------------+-------------------------------+
Auxins (Indole-3-Acetic Acid, IAA)
Auxins are synthesized predominantly at the shoot apex in dividing apical meristems: Cell Elongation: Stimulates proton pumping into cell walls, activating expansins to promote cell expansion. Apical Dominance: High auxin concentrations at the terminal shoot apex inhibit the outgrowth of lateral (axillary) buds. Pruning or decapitating the terminal bud removes the auxin source, releasing axillary buds from inhibition and promoting bushy lateral branching. Root Formation: While high auxin concentrations inhibit primary root elongation, low concentrations promote the initiation of adventitious roots in stem cuttings, used in commercial plant propagation. Parthenocarpy: Auxin application induces the development of seedless fruits (such as tomatoes) without prior pollination or fertilization. Synthetic Auxins: Synthetic analogues like 2,4-dichlorophenoxyacetic acid (2,4-D) are used as selective weedkillers, killing broadleaf dicotyledonous weeds without harming narrow-leaved monocotyledonous cereal crops.
Gibberellins ($\text{GA}_3$)
Gibberellins were discovered by Japanese scientists studying "foolish seedling" disease (bakanae) in rice plants infected by the fungus Gibberella fujikuroi: Stem Elongation: Stimulates cell division and cell elongation in internodes. Applying gibberellins to dwarf pea or corn mutants overcomes genetic dwarfism, causing the plants to grow to normal heights. Bolting: Induces bolting (rapid elongation of internodes prior to flowering) in rosette plants like cabbage and lettuce. Breaking Seed Dormancy: Stimulates the synthesis of hydrolytic enzymes (such as $\alpha$-amylase) in the aleurone layer of germinating cereal grains, mobilizing stored endosperm starch into maltose and glucose to nourish the embryo. Commercial Uses: Sprayed on seedless grapes to elongate stalks and increase fruit size, and used in the brewing industry to accelerate malting in barley.
Cytokinins
Cytokinins (such as zeatin and kinetin) are purine derivatives named for their ability to promote cytokinesis (cell division): Cell Division: Acts synergistically with auxin to promote cell division in active meristems, tissue culture callus, and developing seeds. Delay of Leaf Senescence (Richmond-Lang Effect): Delays chlorophyll degradation and protein breakdown in detached leaves by mobilizing nutrients, keeping cut flowers and green vegetables fresh longer. Overcoming Apical Dominance: Promotes the growth of lateral axillary buds, counteracting the inhibitory effects of apical auxin.
Abscisic Acid (ABA): The Plant Stress Hormone
Abscisic acid is a sesquiterpenoid hormone that acts primarily as a growth inhibitor and stress-response signal: Stomatal Closure: Synthesized rapidly in roots and leaves during water-deficit stress. ABA binds to guard cell receptors, triggering an efflux of potassium ions and causing guard cells to lose turgor and close stomata, conserving water. Inducing Dormancy: Promotes seed and winter bud dormancy, inhibiting premature germination during dry or freezing conditions until favorable environmental conditions return.
Ethylene: The Gaseous Ripening Hormone
Ethylene ($\text{C}_2\text{H}_4$) is the only gaseous phytohormone, synthesized during fruit maturation and tissue senescence: Fruit Ripening: Accelerates the ripening of climacteric fruits (such as bananas, apples, tomatoes, and mangoes), triggering the enzymatic breakdown of chlorophyll, conversion of starches into sugars, and softening of cell walls via pectinase. Commercial fruit distributors ship green, unripened fruit in refrigerated holds and expose them to ethylene gas in destination warehouses to induce uniform ripening. Abscission and Senescence: Promotes the formation of the abscission zone at the base of petioles and flower pedicels, promoting the shedding of senescent leaves, flowers, and mature fruits.
Chapter Exercises
Exercise 1: A student grows a bean seedling inside a lightproof box equipped with a single small pinhole on one side that admits a narrow beam of sunlight. Over two weeks, the shoot tip bends toward the pinhole, while the root curves downward into the soil. Explain the biochemical and cellular mechanisms driving the shoot's curvature, and explain why the root does not bend toward the pinhole.
Exercise 2: Contrast the physiological effects of auxin and cytokinin on apical dominance and lateral bud development, explaining why garden arborists prune the terminal buds of hedges to produce dense, bushy foliage.
Exercise 3: Explain the role of abscisic acid as a stress hormone during severe soil drought, describing the molecular mechanism by which it regulates guard cell turgor to prevent desiccation.
Solutions and Explanations
Solution 1: The bending of the bean shoot toward the pinhole is an example of positive phototropism, driven by the asymmetric redistribution of the hormone auxin. When light strikes the shoot apex from the unilateral pinhole, phototropin receptors detect the directional light, prompting auxin to migrate laterally from the illuminated side to the shaded side of the shoot. On the shaded side, the higher auxin concentration activates proton pumps, acidifying the cell walls and activating expansin proteins that loosen cellulose microfibrils. Driven by internal turgor, cells on the shaded side elongate faster than cells on the illuminated side, causing the shoot to bend toward the pinhole. The subterranean root does not bend toward the pinhole for two reasons: first, the root is buried in opaque soil and is shielded from light; second, roots are inherently negatively phototropic and positively geotropic. Even if exposed to light, concentrations of auxin that stimulate shoot elongation inhibit root cell elongation, so any lateral auxin accumulation would cause the root to bend away from light rather than toward it.
Solution 2: Auxin and cytokinin exert antagonistic regulatory effects on apical dominance and lateral bud development. Auxin is synthesized at the terminal shoot apex and transported downward through the stem; high concentrations of apical auxin suppress the outgrowth of lateral (axillary) buds, maintaining vertical growth of the main stem while keeping lateral branches dormant (apical dominance). Conversely, cytokinins are synthesized predominantly in root tips and transported upward through the xylem; cytokinins promote cell division and stimulate the outgrowth of lateral buds, counteracting apical dominance. Garden arborists prune the terminal buds of hedges (decapitation) to remove the primary site of auxin synthesis. Removing apical auxin eliminates the inhibition on axillary buds, allowing root-derived cytokinins to stimulate lateral bud development. The axillary buds grow into vigorous lateral branches, producing dense, bushy foliage.
Solution 3: Abscisic acid (ABA) functions as a plant stress hormone during severe soil drought by coordinating physiological water conservation. When soil moisture drops, root cells experience osmotic dehydration and synthesize abscisic acid, which is transported upward through the xylem to the leaves, supplemented by ABA synthesized locally in leaf mesophyll cells. In the leaves, ABA binds to specific protein receptors on the plasma membrane of guard cells, activating signaling cascades that increase intracellular free calcium ions ($\text{Ca}^{2+}$). The calcium surge depolarizes the plasma membrane and opens outward-rectifying potassium channels ($\text{K}^+$), while closing inward-rectifying channels. Potassium ions, malate anions, and chloride ions exit the guard cells into the surrounding apoplast, raising the intracellular water potential of the guard cells. Water leaves the guard cells via exosmosis, causing them to lose turgor pressure. As the guard cells become flaccid, their thick inner cell walls collapse together, closing the stomatal pore. Closing stomata halts transpirational water vapor loss, conserving water and protecting the plant from desiccation.