# Plant Responses - Plants respond to their **environment** in various ways to survive and thrive - Environmental **stimuli** can be: - **Abiotic** (non-living factors) - **Biotic** (living factors) - Examples of plant responses include: - Responses to **abiotic stress** - Responses to **herbivory** (e.g. chemical defences) - Response to **touch** - **Tropisms** (directional growth responses) ## Responses to Abiotic Stress - Abiotic stress for plants includes: - **Freezing** temperatures - **Drought** conditions - Increased **soil water salinity** - Presence of **heavy metals** (e.g. lead, copper, zinc, mercury) ### Drought stress responses - Plants respond to drought in ways that **reduce water loss** by transpiration: - **Closing stomata** to prevent water vapour escaping - **Shedding leaves** to reduce surface area for water loss

Think of closing stomata like shutting windows during a storm - it prevents valuable water from escaping the plant.

### Freezing temperature responses - Some plants respond to freezing temperatures by producing an **antifreeze chemical** in their cells - This chemical **decreases the formation of ice crystals** - Ice crystals can **destroy plant cells** if allowed to form within them - Ice crystals damage cell membranes and organelles ## Responses to Herbivory

**Herbivory** is when animals feed on plants, creating biotic stress for the plant.

### Chemical defences - Many plants have adaptations that help them **avoid being eaten** by herbivores - Some plants produce **defensive chemicals**, including: - **Alkaloids** - **Pheromones** | Chemical | Mode of Action | |----------|----------------| | **Alkaloids** (e.g. caffeine in coffee, nicotine in tobacco) | Bitter-tasting or toxic compounds that either deter herbivores from feeding or kill them if consumed | | **Pheromones** | Act as chemical signals to nearby plants of the same species, warning them of herbivore attack and triggering defensive responses

Also attract natural predators of herbivorous insects |

Alkaloids work like a plant's chemical warfare - making themselves taste terrible or poisonous to discourage being eaten.

## Responses to Touch - Some plants are **sensitive to touch** and exhibit rapid responses - Example: sensitive plant species that fold their leaves when touched ### Mechanism and advantages - Touch-sensitive leaves **fold rapidly** when mechanically disturbed - Possible **advantages** of this response: - May be an **adaptation to protect** from herbivorous insects - Could **reduce transpiration** when leaves are no longer photosynthesising - Leaflets often fold at night and reopen at dawn - The response occurs **very rapidly** - Most likely caused by **local bioelectrical signals** travelling through the plant tissue

The rapid folding response happens in seconds - much faster than hormone-based responses which take minutes to hours.

## Tropisms

A **tropism** is a directional growth response of part of a plant in response to a directional stimulus.

- Tropisms can be: - **Positive tropisms** - growth towards a stimulus - **Negative tropisms** - growth away from a stimulus ### Types of tropisms Plant responses include a variety of tropisms: - **Phototropism** - response to light - **Geotropism** (also known as gravitropism) - response to gravity - **Hydrotropism** - response to water - **Thigmotropism** - response to touch/contact - **Chemotropism** - response to chemicals ### Detailed tropism table | Tropism | In Response To | Type of Stimulus | Advantage of Tropism | |---------|----------------|------------------|----------------------| | **Phototropism** | Light | Abiotic | Ensures plant captures maximum light for photosynthesis by growing towards the light source | | **Geotropism** | Gravity | Abiotic | Shoots show **negative geotropism** (grow away from gravity's pull)

Roots show **positive geotropism** (grow towards gravity's pull)

Ensures shoots/roots from germinating seeds grow in correct directions, regardless of seed orientation | | **Hydrotropism** | Moisture | Abiotic | Root tips normally grow towards damper areas of soil, increasing water access for the plant | | **Thigmotropism** | Touch/physical contact | Abiotic or biotic | Important in climbing plants - allows them to climb as tendrils wrap around supports

Provides advantages like greater access to light in dense vegetation

Allows detection of living or non-living support structures to curl around | | **Chemotropism** | Chemicals | Abiotic or biotic | Some plants show tropic responses to certain chemicals

For example, pollen tubes grow down the flower's stigma towards the ovules due to chemotropism |

Remember tropisms with this: Photo = light, Geo = earth/gravity, Hydro = water, Thigmo = touch, Chemo = chemicals.

### Importance of tropisms - Tropisms are **essential for plant survival**: - Phototropism maximises energy capture - Geotropism ensures proper root and shoot orientation - Hydrotropism helps plants find water - Thigmotropism enables climbing and structural support - Chemotropism guides reproductive processes - Unlike animals that can move to favourable environments, **plants must grow** towards or away from stimuli - Tropisms allow **sessile (non-moving) plants** to respond appropriately to their environment