Biology Chapter 32 - Plant Responses

response

A response is the activity of a cell or organism as a result of a stimulus.

stimulus

A stimulus is any change in the environment that causes a reaction in an organism.

tropism

A tropism is a change in the growth of a plant in response to an external stimulus.

production of abscisic acid

Abscisic acid is produced in leaves, stems and root caps. It is often called the stress regulator of plants. Although it was thought that abscisic acid caused abscission (leaf fall) in autumn it is now known that ethene in combination with auxins is responsible.

IAA in root growth

At low concentrations, IAA caused roots to grow (at high concentrations it causes stems to grow) . IAA can be artificially applied to stimulate rooting. Synthetic commercial growth promoters are more effective.

IAA in cell elongation

Auxin loosens cell walls, allowing them to expand. Cell elongation is essential for normal growth and tropisms.

chemotropism

Chemotropism is the change in growth of a plant in response to chemicals. Roots grow towards minerals (calcium, magnesium and fertilizers such as nitrogen, phosphorus and potassium) and away from acids or heavy metals (lead or zinc) in the soil. Pollen tubes grow towards chemicals released by the ovule.

production of ethene

Ethene (formerly called ethylene) is the only growth regulator that is a gas. It is made in stem nodes, ripe fruits and decaying leaves.

IAA in apical dominance

If the apical bud is intact, auxin produced will pass down the stem and inhibit the growth of lateral buds. In conifers the inhibition decreases down the stem. The removal of the tip will facilitate more side branching.

tissue culturing (micro-propagation)

In tissue culturing, cuttings are used to grow new plants. A piece of plant tissue exposed to high levels of auxin will develop a mass of undifferentiated cells called a callus. The callus can be stimulated to form roots, shoots or an entire plant.

phototropism

Phototropism is the change in the growth of a plant in response to light, usually from a particular direction. Stems are positively phototrophic and many roots are negatively phototrophic.

rooting powder

Rooting powder often contains naphtylacetic acid (NAA) to stimulate rapid root formation in stem cuttings for use in horticulture.

production of auxin

There are a number of auxins, the most important being indoleacetic acis (IAA, or simply auxin). Auxin is produced in the meristematic tissue at the tips of shoots, as well as in young leaves and developing seeds. It moves down the stem by an unknown mechanism.

thigmotropism

Thigmotropism is a change in growth of a plant in response to touch. Tendrils exhibit positive thigmotropism.

structures required for response

~A chemical or hormonal system. ~An immune or defense system. ~Nervous and sensory system (animals only). ~Method of movement, including muscular and skeletal systems (animals only) and growth.

difficulty in establishing the exact role of plant regulators.

~Active in very small amounts. ~Effect depends on their concentration. ~Effect depends on the location they are active in, e.g. could cause opposite reaction in stem to root. ~Different regulators interact in different ways, e.g. some support each other while others interfere with each other.

functions of abscisic acid

~Causes plants to respond to harmful conditions. ~Causes stomata to close in dry conditions. ~Caused the production of bud scales to protect buds in winter. ~Inhibits seed germination, allowing seeds to remain dormant in the soil over winter.

environmental dangers to plants

~Consumption by herbivores. ~Fungal, bacterial or viral infection. ~Dehydration. ~Overheating.

stress proteins

~Damage micro-organisms by attacking their cell walls. ~Stimulate the formation of specialized plant cell walls to limit the spread of infection. ~Stimulate nearby plant cells to respond to the micro-organism.

anatomical protective features

~Epidermis (and occasionally a protective cuticle) or bark reduces water loss and infection chance. Certain plants have epidermal cells adapted to form thorns or stinging hairs. ~Guard cells shrivel with lack of water, closing the stomata and reducing transpiration.

internal factors for flowering plant responses / growth regulators.

~Growth regulators are produced in meristematic regions of plants, such as root tips or shoot tips and are chemicals that control the growth of a plant by acting as growth promoters or growth inhibitors.

chemical protective features

~Heat-shock proteins are formed above 40*C to surround and prevent the denaturing of other proteins. ~Stress proteins , including phytoalexins, are produced when infected.

IAA in phototropisms

~If the stem is exposed to light from an angle, IAA will diffuse down the shaded side. ~IAA causes these cells to elongate more then the cells on the bright side. ~As a result of uneven elongation, the stem bends towards the light, i.e. a phototropism.

external factors for flowering plant responses

~Light affects growth by providing energy for photosynthesis. ~Day length causes plants to flower and regulates seed and fruit formation, dormancy, leaf loss and germination of certain seeds. ~Plants grow against gravity. ~Temperature affect the rate of enzyme reactions and can regulate seasonal reproduction.

functions of ethene

~Ripening fruits (it is used to commercially ripen bananas). ~Causing fruit colour and flavour to form and fruit tissue to soften. ~Stimulating leaves to fall in autumn. ~Ageing of plants. ~Stimulating more ethene production ('one bad apple rots the barrel' is true!) .

functions of auxin

~Stimulating stem elongation. ~Stimulating root growth. ~Causing cells to forms different structures, e.g. the zone of differentiation in the root. ~Developing fruit. ~Inhibiting side branching in stems. ~Causing phototropisms. ~Causing geotrophism.

advantages of tropisms

~Tropisms allow plants to obtain more favorable conditions. ~Stems grow towards light so more photosynthesis can occur. ~Roots grow towards the source of gravity so they can penetrate deeper into the soil to increase anchorage and absorption.

positive tropism

A positive tropism occurs when the growth is towards the stimulus.

growth inhibitor

A growth inhibitor is a chemical that causes reduction in the growth of a plant, i.e. ethene and abscisic acid.

growth promoter

A growth promoter is a chemical that causes increased growth in plants, e.g. auxin.

negative tropism

A negative tropism is when the growth is away from the stimulus.

geotropism (gravitropism)

Geotropism is the change in growth in response to gravity. Roots are positively geotrophic and stems are negatively geotrophic.

hydrotropism

Hydrotropism is a change in the growth of a plant in response to water. Roots and pollen tubes are positively hydrotrophic.

IAA in fruit formation

IAA is made by developing seeds. It stimulates food to form that surrounds the seeds.