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Structural Adaptations, Tropisms and Hormonal Control
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
They also need oxygen for respiration. Glucose + Oxygen → Carbon Dioxide + Water + Energy Or as a balanced chemical equation: + + + EnergyGlucose 6x Oxygen 6x Carbon Dioxide 6x Water
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 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.
We will cover plant adaptations to: Light Temperature Water Gaseous Exchange Support Fire We will also look at the special case of epiphytes
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.
Most leaves are covered by a water proof layer called the cuticle.Stomata: Stomata’s(mostly on the underside ofthe leaf) allow gas exchange; since a lot of watervapour can be lost through the stomata theyonly open for photosynthesis in daylight; atnight they close to reduce loss of water vapour.
Open Stomata Closed Stomata By opening and closing the stomata regulate the amount of water loss. Unfortunately 98% of water is lost here.
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.
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
A rolled leaf 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.Cacti
Mesophytes - Plants in areas with adequate water Hydrophytes - Aquatic plants Halophytes - Salt-tolerant plants Xerophytes - Plants in areas where water is scarce
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 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
Mangrove Succulent Plant 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)
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 leavesCacti Marram Grass
Adaptation How it works Examplethick cuticle stops uncontrolled evaporation through leaf cellssmall leaf surface less surface area for conifer needles, cactusarea evaporation spineslow stomata density smaller surface area for diffusionsunken stomata maintains humid air around marram grass, cacti stomatastomatal hairs maintains humid air around marram grass, couch(trichores) stomata grassrolled leaves maintains humid air around marram grass, stomataextensive roots maximise water uptake cacti
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 Plants with leaves that dangle reduce shiny leaves reflect light and heat. their exposure to the sun.
EffectPhotosynthesis: 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.
Water plants have more difficulty than land plants in obtaining the light they require for photosynthesis. About 30% of light striking the surface of water is reflected. 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
Surface Depth Algae 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 absorbDeep water Algae 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.
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.
The role of a plant’s roots is to anchor the A kelp holdfast 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.
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.
Banksia seed pods 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.
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 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.
Tank Bromeliads 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
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 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).
We can group plant responses into four broad groups: Taxis Tropism Nastics Nutation
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)
growth movement in response to an external stimulus; the direction of the stimulus determines the direction of plant growth
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.
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.
hydrotropisms is defined as movement towards water. In this case roots show a positive tropism towards water sources.
Thigmotropism is a plants response and movement to physical contact. This phenomenon is clearly illustrated by the climbing tendrils of some plants, such as the sweetThigmotropism: the hop vine responding pea. The tendrils actuallyto contact with the support string. "feel" the solid object, which results in the coiling response
Venus fly trap closing to capture an Thigmotropism in response to touch ininsect Mimosa Pudica Nastic movements of a plant are rapid movements of plant organs.
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.
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.
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.
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.
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.
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.
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.
Gibberellins promote cell division and elongation in plant shoots. They also extend internodes and can raise flower heads.
Cytokinins stimulate cell division/replication. They tend to be concentrated in the starchy material in seeds (endosperm) and in young fruit.
Abscisic acids promote the closure of stomata during times of water stress. They also stimulate dormancy in seeds and buds during unfavourable conditions.
Ethylene ripens fruit by stimulating the conversion of starch to sugar. It also stimulates colour change and softening of fruit tissue. Before After
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 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.
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.
The length of dark is a trigger to flowering