Plant AdaptationsStructural Adaptations, Tropisms and Hormonal Control

Plants - Photosynthesis Plants are photosynthetic organisms

that make their own carbohydrates for energy.

They need carbon dioxide, light and water for photosynthesis.

6CO2 + 6H2O → C6H12O6 + 6O2

++6x Carbon Dioxide 6x Water Glucose 6x Oxygen

Plants - Respiration

They also need oxygen for respiration.

Or as a balanced chemical equation:

Glucose + Oxygen → Carbon Dioxide + Water + Energy

Glucose

+

6x Oxygen 6x Carbon Dioxide

+

6x Water

+

Energy

Requirements for Plants

They use different ions as nutrients (equivalent to vitamins and minerals in humans).

Plants have leaves that contain chloroplasts that absorb light energy for photosynthesis.

Stomata on the under-side of the leaves control gas exchange and water loss. (carbon dioxide moves in, water and oxygen move out of the leaf)

Temperature and Metabolic rate Temperature is important to plants as it

affects metabolic rate (the rate of chemical reactions in the plant essential to life processes).

Metabolic rate controls growth and development.

The higher the temperature the higher the metabolic rate- up to a limit.

Plant Adaptations We will cover plant adaptations to:

Light Temperature Water Gaseous Exchange Support Fire

We will also look at the special case of epiphytes

Plants and WaterAdaptations to water availability

Water Balance

Plants like other organisms also need to maintain a constant water balance.

For plants this is especially important as water constitutes 90-95% of the living tissue of plants.

Plants therefore have specialized mechanisms to both conserve water and minimize loss.

Leaf structures important for water balance

Stomata: Stomata’s(mostly on the underside of the leaf) allow gas exchange; since a lot of water vapour can be lost through the stomata they only open for photosynthesis in daylight; at night they close to reduce loss of water vapour.

Most leaves are covered by a water proof layer called the cuticle.

The Stomata

By opening and closing the stomata regulate the amount of water loss. Unfortunately 98% of water is lost here.

Open Stomata Closed Stomata

Transpiration Transpiration explains how water

moves up the plant against gravity in tubes made of dead xylem cells without the use of a pump.

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration.

As more water is lost more is drawn up through the plant to replace it.

This creates a continuous tube from the leaf, down the stem to the roots, and acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.

Adaptations to Water

For plants that are exposed to sufficient amounts of water the opening and closing of stomata is sufficient to control water balance.

Many plants however live where water exposure is low and the challenge is to conserve water and reduce water loss

Water

Some plants have hard, thick cuticle which reduces evaporation of water.

Some plants have a reduced number of stomata or hairs on the surface of their leaves which trap water and increase the humidity at the surface of the leaf.

Some plants leaves roll inwards and therefore the stomata are covered. When water does evaporate it increases the humidity around the leaves reducing future water loss.

Some plants such as cacti and succulents store water in their leaves and stems.

A rolled leaf

Cacti

Types of Plants

Mesophytes - Plants in areas with adequate water

Hydrophytes - Aquatic plants

Halophytes - Salt-tolerant plants

Xerophytes - Plants in areas where water is scarce

Mesophytes Mesophytes require an

environment that is neither too wet nor too dry.

Water lost from stomata is matched by water gain from the environment

Under stress (Like winter) these plants shed their leaves

Perennials survive unfavourable conditions by dying down and surviving underground. Annuals survive as dormant seeds.

Most plants fall into this category

Hydrophytes

Hydrophytes are plants that require a large supply of water.

They can grow wholly or partly submerged in water.

The stems and leaves have little to no cuticle (outer waxy layer of leaf) as they do not need to conserve water

Halophytes

Salt tolerant Store water in special tissue Tissue has lots of air spaces Some can excrete salt though special glands or by

dropping yellowish leaves where salt has been accumulated.

Many are succulents (Water retaining plants)

Succulent PlantMangrove

All Cacti are xerophytesAll Cacti are Xerophytes

Xerophytes

Grow in hot, dry environments therefore have adapted to conserve water and to prevent leaf temperature from rising too much

Often these adaptations are of the leaves

Cacti Marram Grass

Adaptation How it works Example

thick cuticle stops uncontrolled evaporation through leaf cells

 

small leaf surface area

less surface area for evaporation

conifer needles, cactus spines

low stomata density

smaller surface area for diffusion

 

sunken stomata maintains humid air around stomata

marram grass, cacti

stomatal hairs (trichores)

maintains humid air around stomata

marram grass, couch grass

rolled leaves maintains humid air around stomata

marram grass,

extensive roots maximise water uptake cacti

Xerophyte adaptations summary:

Other Adaptations

Temperature Temperature can affect the

growth potential of a plant and plants have several adaptations designed to control heat gain.

Leaves with a smaller surface area do not absorb as much heat.

4.44 °C 35.56 °C

shiny leaves reflect light and heat. Plants with leaves that dangle reduce their exposure to the sun.

Temperature Effects on Plant Growth:

Effect

Photosynthesis: Increases with temperature to a point.

Respiration: Rapidly increases with temperature.

Transpiration: Increases with temperature.

Flowering: May be partially triggered by temperature.

Sugar storage: Low temperatures reduce energy use and increase sugar storage.

Dormancy: Warmth, after a period of low temperature, will break dormancy and the plant will resume active growth.

Light Water plants have more difficulty than land plants

in obtaining the light they require for photosynthesis.

by 1 m about 60% of the light is absorbed.

by 10 m about 85% of the light is absorbed.

by 150 m about 99% of light has been absorbed

About 30% of light striking the surface of water is reflected.

Light As seen on the previous slide

water does not absorb all lengths of light equally. Blue and green light is better able to penetrate water and reach deeper.

Algae at surface depths (0-10m) will be predominately green as it can absorb the red and orange light that penetrates this region.

As we move deeper however the algae will turn brown and then red. The brown and red algae are better able to absorb blue light.

Surface Depth Algae

Deep water Algae

Gaseous Exchange Water plants have more difficulty

than land plants in exchanging the required gases.

These plants may have stomata on surfaces other than their leaves.

Mangroves have special aerial roots called pneumatophores (peg roots) that extend out of the water. These roots obtain oxygen for respiration through special pores located on the root.

Support The role of a plant’s roots is to

anchor the plant to the ground and also absorb water and nutrients from the soil.

Water plants may have weaker roots systems as they rely on the water for buoyancy and support.

Water plants in fast moving waters have holdfasts.

A kelp holdfast

Adaptations to Fire

Before

After

Fire

Wild fires started most often by lightening are a natural occurrence and plants have adapted strategies to survive fires.

Plants have developed two strategies which they can either use separately or in combination. Producing a large volumes of

seeds. Structures and mechanisms for

regeneration

Some native plants actually rely on frequent fires to flower and cause seeds to sprout. Banksias require frequent fires to produce seeds.

Fire – Seed Volume The first strategy is to produce

a large volume of seeds that only germinate after a fire.

Advantage: Seeds have access to increased minerals from the ash in the soil.

Disadvantage: If the time between fires is too long the seeds may not mature and the next generation may be lost.

Banksia seed pods

Fire – Epicormic Buds

Many trees have thick bark that protects the internal structure of the tree. Under this bark are epicormic buds that sprout quickly after fire.

Many plants have shouts or roots called lignotubers underground that are protected by soil or dead plant matter during a fire.

Some plants combine both epicormic buds and lignotubers to completely regenerate plants after a fire.

Epiphytes

Epiphytes Epiphytes are unique given that they grow on

other plants and have no contact with the soil. The advantage of growing on other trees is

that they have better access to light than they would if they were located on the ground.

Epiphytes So how to epiphytes obtain

water and nutrients? Epiphytes such as mosses

absorb and store water releasing it when water is scarce.

Bromeliads have leaves that are rolled and form funnel like structures that capture rain water and plant debris- a source of nutrients.

The tank bromeliad above can hold up to 8 Litres of water!

Bromeliads

Tank Bromeliads

Epiphytes are not parasites

Epiphytes are plants which, like a parasite, grows on a host, but unlike a parasite, takes no nutrients from the tree itself and relies on nutrients from the air, falling rain, and the compost that lies on tree branches.

Epiphytes do not directly cause damage to the host plant they are on.

Plants responding to the environment

Responses by Plants to the External Environment

Plants need to respond to stimuli in the environment. They do so through the use of plant hormones.

There are several types of responses that plants may display in response to certain stimuli.

These responses may be negative (away from the stimuli) or positive (towards the stimuli).

Responses by Plants to the External Environment

We can group plant responses into four broad groups:

TaxisTropismNasticsNutation

Taxis

movement of a whole organism in response to a stimuli; e.g. algae moving towards a light source (positive phototaxis) or the movement of algae away from chemicals (negative chemotaxis)

Tropism growth movement in response to an

external stimulus; the direction of the stimulus determines the direction of plant growth

Phototropism

When a shoot is illuminated from one side, an auxin is transported across to the shaded side.

Cells on the shaded side elongate.

The shoot then is able to bend towards the light.

Geotropism

In geotropism: Roots show positive geotropism Stem/shoot show negative geotropism

There are two different theories for geotropism: redistribution of auxins to the lower side

of root. Causing growth downward. the pull of gravity is detected by cells

near the stem or root tip (apex). These cells contain starch grains that change their location in the cell if the plant is moved from a vertical to a horizontal position.

Hydrotropism

hydrotropisms is defined as movement towards water.

In this case roots show a positive tropism towards water sources.

Thigmotropism

Thigmotropism is a plant's response and movement to physical contact.

This phenomenon is clearly illustrated by the climbing tendrils of some plants, such as the sweet pea. The tendrils actually "feel" the solid object, which results in the coiling response

Thigmotropism: the hop vine responding to contact with the support string.

Tropisms:Summary

Nastic

Nastic movements of a plant are rapid movements of plant organs.

Thigmotropism in response to touch in Mimosa Pudica

Venus fly trap closing to capture an insect

Other Movements - Nutation

Nutation describes movements of plant structures that are in response to internal rather than external stimuli.

Slow, upward, helical growth movements of seedlings have been caught by time-lapse photography.

Seemingly random movements of climbing plant stems increase the chance of making contact with a supporting structure.

Controlling the direction of growth Auxins play an importance role in

phototrophism (plants bending towards the light).

They cause the shaded side of a stem or shoot to grow more (elongate) causing the whole stem or shoot to bend toward a light source. The higher the concentration of auxins the greater the elongation and curvature of the stem.

The Role of Auxins in Tropisms

Auxins are also thought to play a role in geotropism. Greater amounts of auxins have been found in the lower side of horizontal organs.

The evidence is not convincing however and a more likely explanation is the statolith hypothesis which states that cells near the stem and root tip detect gravity. They detect gravity using starch molecules within the cell that change location when the plant is moved from a vertical to a horizontal position. This position shift is thought to active enzymes.

Plant HormonesThe role of hormones

Regulating Plant Activity

Plants cannot move (they are sessile) when they are exposed to adverse conditions. For this reason they need to take advantage of favourable conditions and often events in their life cycle are controlled to coincide with favourable external conditions.

Events such as germination, growth, flowering, seed setting and budding are often signalled by changes in the environment around them.

Phytohormones

Therefore there exists in plants just as other organisms a system that responds to the external environment.

Plants have hormones, just as animals do though they are not as complex and numerous. These hormones are known collectively as phytohormones (phyto = plant).

Unlike in animals where hormones are produced by glands, any plant tissue is capable of producing hormones.

Plant Hormones

There are five groups of plant hormones that together control the growth and development of the plant. These hormones are produced in response to the environment external to the plant.

1. Auxins

The effect of auxins on a plant are widespread and they often work with other hormones.

Auxins influences the length of a plant cell, ripening of fruit, falling of leaves and growth of shoot tips. They inhibit the growth of lateral buds and promote root growth from cut stems.

Auxins increase the circumference of a stem or trunk.

2. Gibberellins Gibberellins

promote cell division and elongation in plant shoots.

They also extend internodes and can raise flower heads.

3. Cytokinins

Cytokinins stimulate cell division/replication. They tend to be concentrated in the starchy

material in seeds (endosperm) and in young fruit.

4. Abscisic acids

Abscisic acids promote the closure of stomata during times of water stress.

They also stimulate dormancy in seeds and buds during unfavourable conditions.

5. Ethylene

Ethylene ripens fruit by stimulating the conversion of starch to sugar.

It also stimulates colour change and softening of fruit tissue.

Before After

Phytochrome

Phytochrome is a light receptor sensitive to red light found in a plants leaves.

It is involved in seed germination, stem elongation, expansion of leaves, growth of lateral roots and leaf fall.

When exposed to light, phytochrome causes the above events to occur.

Photoperiodism Photoperiodism is the reaction of

plants to the length of daylight. Phytochrome plays a role in regulating

the cycles of flowering plants in response to the length of sunlight in a day.

The length of day light and darkness controls flowering. Different plants will flower in response to long days or short days.

Short Day or Long Day plants

Different plants react differently to the photoperiod; some plants are described as ‘short-day’ plants and others as ‘long-day’ plants.

The example to the left is of a short-day plant flowering. If a dark period is interrupted by a light flash, no flowering occurs.

Photoperiodism

The length of dark is a trigger to flowering