2.6 Tropisms and nastic movements

Types of Tropisms

A tropism is a growth movement in response to a directional environmental stimulus. The plant doesn't just react; it actually grows toward or away from the stimulus. Because tropisms involve growth, they're relatively slow and generally irreversible.

Different types of tropisms help plants orient themselves to get the resources they need.

Phototropism in Plants

Phototropism is growth in response to light. Shoots typically grow toward light (positive phototropism), while some roots grow away from it (negative phototropism).

• Sunflower stems bend to follow the sun's movement across the sky
• Houseplants on a windowsill gradually lean toward the glass

This matters because it lets plants position their leaves for maximum light capture, which directly improves photosynthesis.

Gravitropism in Roots and Shoots

Gravitropism is growth in response to gravity. Roots show positive gravitropism (growing downward, with gravity), while shoots show negative gravitropism (growing upward, against gravity).

• If you tip a potted seedling on its side, the shoot will curve upward and the root will curve downward within a day or two
• Tree trunks maintain their vertical orientation even on slopes

This ensures roots reach soil, water, and nutrients while shoots reach light.

Thigmotropism in Vines and Tendrils

Thigmotropism is growth in response to touch or physical contact. When a tendril contacts a solid object, it coils around it.

• Pea tendrils wrap around a trellis after making contact
• Ivy vines cling to walls and tree trunks

Climbing plants use thigmotropism to grow vertically without investing energy in thick, self-supporting stems, giving them access to light they'd otherwise miss.

Chemotropism in Pollen Tubes and Roots

Chemotropism is growth in response to a chemical gradient. The plant organ grows toward (or away from) a higher concentration of a specific chemical.

• During pollination, pollen tubes grow through the style toward the ovary, guided by chemical attractants released by the ovule
• Roots can grow preferentially toward patches of soil with higher nutrient concentrations

Chemotropism in pollen tubes is essential for fertilization and seed production.

Hydrotropism in Roots

Hydrotropism is root growth in response to a water gradient. Roots grow toward areas of higher moisture.

• Desert plant roots grow toward deep soil layers where water is more available, sometimes overriding gravitropism to do so

This is especially important in environments where water is unevenly distributed.

Mechanisms of Tropisms

Tropisms follow a general sequence: the plant perceives a directional stimulus, transduces that signal internally, and then produces differential growth on one side of the organ, causing it to bend.

Role of Auxins

Auxin (primarily indole-3-acetic acid, or IAA) is the key hormone driving most tropisms. It's produced in shoot tips and young leaves, then transported downward through the stem.

When a directional stimulus hits, auxin gets redistributed unevenly across the organ. The side with more auxin grows differently than the side with less, and that difference in growth rate causes bending.

Perception of Stimuli

Plants have specific receptors for different stimuli:

• Light: Phototropins and phytochromes are light-receptor proteins that detect the direction and quality of light for phototropism
• Gravity: Starch-filled organelles called amyloplasts (also called statoliths) settle to the bottom of root cap cells. Their position tells the plant which way is down

Signal Transduction Pathways

Once the stimulus is detected, the plant relays that information through internal signaling:

1. The receptor activates a signaling cascade involving changes in ion flow and calcium levels
2. These signals alter the distribution of PIN proteins, which are auxin efflux carriers embedded in cell membranes
3. PIN proteins redirect auxin transport, creating an uneven concentration across the organ

Differential Growth Responses

The uneven auxin distribution produces bending:

• Phototropism: Auxin accumulates on the shaded side of the stem. Higher auxin there promotes cell elongation, so the shaded side grows faster and the stem bends toward the light.
• Gravitropism in roots: Auxin accumulates on the lower side. In roots, high auxin concentrations inhibit elongation, so the upper side grows faster and the root bends downward. (Note: auxin's effect differs between roots and shoots. In shoots, more auxin promotes elongation; in roots, more auxin inhibits it.)

Types of Nastic Movements

Nastic movements are non-directional, reversible plant movements triggered by a stimulus. Unlike tropisms, the direction of the response doesn't depend on where the stimulus comes from. They're also typically much faster than tropisms because they rely on changes in water pressure rather than growth.

Nyctinastic Movements

Nyctinastic movements (sleep movements) occur in response to the day-night cycle.

• Many legumes fold their leaves downward at night and reopen them during the day
• Tulip flowers close at dusk and open again in the morning

These movements are tied to the plant's internal circadian clock and continue even if the plant is kept in constant darkness for a few days.

Seismonastic Movements

Seismonastic movements are rapid responses to touch or vibration.

• Mimosa pudica (the sensitive plant) folds its leaflets within seconds of being touched, then reopens after several minutes

This likely functions as a defense against herbivores. The sudden movement may startle insects or make the plant look wilted and less appetizing.

Thermonastic Movements

Thermonastic movements are responses to temperature changes.

• Tulip and crocus flowers open when temperature rises and close when it drops

This can help protect reproductive structures from cold damage or control pollinator access.

Hydronastic Movements

Hydronastic movements are responses to changes in water availability.

• Stomata close during drought to reduce water loss through transpiration
• Some grasses roll their leaf blades inward when water-stressed, reducing the exposed surface area

Mechanisms of Nastic Movements

Nastic movements don't involve growth. Instead, they depend on rapid, reversible changes in water content within specialized cells.

Turgor Pressure Changes

The driving force behind nastic movements is a rapid shift in turgor pressure (the pressure of water pushing against the cell wall).

Here's how it works in Mimosa pudica:

1. Touch triggers an electrical signal that travels along the leaf
2. Motor cells in the pulvinus rapidly pump potassium ions ($K^+$) out of the cell
3. Water follows the ions out by osmosis
4. The cells lose turgor and shrink, causing the leaflet to fold
5. When the stimulus passes, ions and water flow back in, restoring turgor and reopening the leaflet

Specialized Motor Organs

Many plants with nastic movements have pulvini (singular: pulvinus), which are swollen regions at the base of leaves or leaflets. Pulvini contain the motor cells that undergo rapid turgor changes. The arrangement of these cells within the pulvinus determines which direction the leaf moves.

Role of Circadian Rhythms

Nyctinastic movements are regulated by the plant's circadian clock, an internal timing mechanism that runs on roughly a 24-hour cycle. This allows plants to anticipate dawn and dusk rather than simply reacting to light changes. The clock coordinates ion channel activity in pulvinar cells, driving the rhythmic opening and closing of leaves and flowers.

Ecological Significance

Optimizing Resource Acquisition

Phototropism directs shoots toward light, maximizing photosynthetic efficiency. Hydrotropism guides roots toward water. Together, tropisms help plants position their organs where resources are most available, which directly affects growth rate and fitness.

Enhancing Survival and Reproduction

• Gravitropism anchors roots in soil and points shoots skyward
• Thigmotropism lets climbing plants reach the canopy without building thick stems
• Chemotropism guides pollen tubes to ovules, which is critical for successful fertilization
• Seismonastic movements may deter herbivores from feeding

Adaptations to Environmental Factors

• Desert plants rely on hydrotropism to find deep water sources
• Nyctinastic leaf folding may reduce heat loss at night or limit water loss
• Thermonastic flower closing protects pollen and ovules from cold temperatures
• Stomatal closure during drought (a hydronastic response) prevents dangerous levels of water loss

Comparison of Tropisms vs. Nastic Movements

Directional vs. Non-Directional Responses

In phototropism, the shoot bends toward the light source. Move the light, and the direction of bending changes. In nyctinasty, leaves fold the same way regardless of where the light is coming from. The response is built into the structure of the pulvinus.

Reversibility of Responses

Tropisms involve actual cell growth (elongation), so they can't simply be "undone." A stem that has bent toward light would need new growth to straighten. Nastic movements reverse quickly once the stimulus is removed because they only involve water moving in and out of cells.

Stimulus Specificity

Each tropism responds to one specific stimulus type (light, gravity, touch, chemicals, or water). Nastic movements can also be stimulus-specific, but the key difference is that the direction of the stimulus doesn't matter for the response.

Applications and Research

Agriculture and Horticulture

• Understanding phototropism helps in designing greenhouse layouts and supplemental lighting to maximize crop growth
• Knowledge of thigmotropism informs the use of trellises and support structures for crops like tomatoes, beans, and grapes
• Gravitropism research guides practices for transplanting and training young trees

Biomimetics and Robotics

Plant movements have inspired engineers to develop "smart materials" that change shape in response to light, temperature, or moisture. Soft robotic systems modeled on plant tendrils are being explored for applications like environmental monitoring and minimally invasive surgery.

Space Biology Experiments

Plants grown on the International Space Station experience microgravity, which disrupts normal gravitropism. Studying how roots and shoots orient themselves without a clear gravity signal has deepened our understanding of how statoliths and auxin transport work together. These experiments also support the development of plant-based life support systems for long-duration space missions.