Coordination in plants

A tropism is a directional growth response by a plant organ to an external stimulus. The direction of the movement is related to the direction of the stimulus. Tropisms are named based on the type of stimulus that causes the movement.

Geotropism (gravitropism)

1. Geotropism refers to the growth movement of plant parts in response to the force of gravity.
2. Positive Geotropism (Gravitropic): This is when plant roots grow towards the pull of gravity. For example, roots grow downward.
3. Negative Geotropism (Gravitropic): This occurs when shoots or stems grow away from gravity. Shoots grow upward.

The effect of gravity on the growth of roots and root.

Phototropism

Phototropism is the growth response of plants to light.

1. Shoots exhibit positive phototropism (grow towards the light).
2. Some roots exhibit negative phototropism (grow away from light), though many roots are not sensitive to light.

Mechanism:

1. Light causes auxins to migrate towards the darker side of the plant.
2. On the darker side, auxin concentration is higher, which causes the cells to elongate and grow faster than those on the lighter side, making the shoot bend towards the light.
3. In the case of roots, light exposure typically causes them to grow away from the light (negative phototropism).

Effects of light on shoots

Hydrotropism

1. Hydrotropism refers to the growth response of plant organs to moisture or water.
2. Roots exhibit positive hydrotropism, growing towards the source of water.
3. Shoots may exhibit negative hydrotropism (grow away from water), or sometimes show no response to moisture.

Example:

Roots tend to grow towards areas with higher moisture content, ensuring they absorb enough water for the plant.

Root is positively hydrotropic

Thigmotropism (or haptotropism) is the response of plant organs to touch or physical contact.

Plants with tendrils (like Passiflora and Gloriosa) show thigmotropism when they curl around or cling to objects.

Mechanism:

1. When the tendrils of a climbing plant make contact with a solid object, the auxins in the contact area move away from the point of contact.
2. The part of the tendril in contact with the object has lower auxin concentration, while the outer part has higher auxin concentration.
3. The increased auxin concentration on the outer side promotes faster growth, causing the tendril to coil and wrap around the object.

Thigmotropism

Chemotropism is the growth movement of plant organs in response to a unilateral source of chemicals.

Example: The growth of pollen tubes towards the ovary during fertilization. The pollen tubes are attracted to chemicals secreted by the ovules, guiding the pollen tube to its destination.

Thermotropism is the growth movement of plant organs in response to heat.

Example: Sunflowers orient their flowers to face the sun, a process known as heliotropism, which is a type of thermotropism.

Overlap with Phototropism: Often, thermotropism and phototropism occur together because both involve environmental stimuli related to energy and light (heat from the sun and light).

Rheotropism is the growth movement of plant organs in response to a unilateral air current.

Example: Aquatic plants may bend in the direction of the water current to minimize damage or enhance nutrient absorption.

1. Phototropism: Helps plants optimize their exposure to sunlight for photosynthesis by orienting their leaves towards the light.
2. Haptotropism: Helps plants with weak stems, such as climbing plants, by enabling tendrils to coil around structures for support.
3. Geotropism: Allows roots to grow downward into the soil to anchor the plant and access water and nutrients.
4. Chemotropism: Essential for fertilization in flowering plants, as it guides pollen tubes to the ovules.
5. Hydrotropism: Helps roots grow towards water, ensuring the plant can take up necessary moisture.

Nastic responses are non-directional movements of plant organs that occur in response to diffuse stimuli (stimuli that do not have a defined direction).

These movements occur as a result of changes in turgor pressure within cells.

Nastic responses are non-directional movements of plant parts triggered by external stimuli. Unlike tropisms, the direction of the movement is not dependent on the direction of the stimulus.

1. Nyctinasty (Thermonasty)

This type of movement occurs in response to changes in temperature, often linked to the day and night cycle.

• Example: Some plants fold their leaves at night to conserve heat or reduce water loss.

2. Photonasty

Photonasty is triggered by changes in light intensity. It is commonly observed in petals and leaves of certain plants.

• Example: Flowers like morning glories open in daylight and close at night.

3. Seismonasty

This is a plant response to shock or mechanical vibration. It is a rapid and temporary movement.

• Example: The Mimosa pudica (sensitive plant) folds its leaves instantly when touched or shaken.

4. Hydronasty

Hydronasty involves plant movements that occur due to changes in atmospheric humidity.

• Example: Some plants close their flowers during high humidity to avoid excessive water absorption or loss.

5. Haptonasty

Haptonasty is triggered by physical contact with an object or organism.

• Example: In Mimosa pudica, the leaves fold when touched, as a form of defense.

6. Chemonasty

This is the movement of plant parts in response to chemical stimuli in the environment.

• Example: Some roots may alter their growth direction when exposed to certain chemicals in the soil.

Tactic movement, or taxis, refers to the movement of a whole organism in response to an external stimulus. The movement can either be positive, when the organism moves toward the stimulus, or negative, when it moves away from the stimulus.

Taxis is most commonly observed in microscopic or motile organisms, such as bacteria, protozoa, and algae, and is crucial for their survival, enabling them to seek favorable conditions or avoid harmful ones.

1. Phototaxis: Movement in response to light. Example: Euglena swims toward light for photosynthesis (positive phototaxis).
2. Chemotaxis: Movement in response to chemical substances. Example: Sperm cells move toward chemicals released by the egg (positive chemotaxis).
3. Aerotaxis: Response to oxygen concentration. Example: Bacteria move toward areas with optimal oxygen levels.
4. Rheotaxis: Movement in response to the direction of water currents. Example: Fish orient and swim upstream in flowing water.
5. Magnetotaxis: Response to magnetic fields, observed in certain bacteria. Example: Magnetotactic bacteria align with Earth's magnetic field to find suitable environments.
6. Osmotaxis: Movement in response to osmotic pressure differences in the environment. Example: Protozoa move to areas with more suitable solute concentration.
7. Thermotaxis: Movement in response to temperature changes. Example: Some cells move to regions with ideal temperature for enzyme activity.

Phytochromes are light-sensitive compounds with two forms:

1. Stem elongation: Stimulated by far-red light (Pfr), inhibited by red light (Pr).
2. Leaf expansion: Stimulated by red light (Pr), inhibited by far-red light (Pfr).
3. Root growth: Stimulated by far-red light (Pfr), inhibited by red light (Pr).
4. Seed germination: Stimulated by red light (Pr), inhibited by far-red light (Pfr).

Photoperiodism refers to a plant's flowering response to light periods (day/night lengths). Phytochrome helps in this process by triggering the formation of the flowering hormone florigen.