Plant Responses Flashcards by Roba Elsaigh

What is a stimulus?

Anything that causes a reaction in an organism or any of its parts.

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What is a response?

An activity of an organism or cell due to a stimulus.

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What is a tropism?

A plant growth response to a stimulus.
Can be positive or negative.
When the growth response is towards the stimulus it is positive.
When the growth response is away from the stimulus it is negative.

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What are the tropic responses?

Phototropism.
Geotropism.
Chemotropism.
Hydrotropism.
Thigmotropism.

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What is phototropism?

A change in plant growth in response to unidirectional light.
Stems grow towards light and are positively phototropic.
Roots grow away from light and are negatively phototropic.

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What is geotropism?

A change in plant growth in response to gravity.
Roots are positively geotropic as they grow down towards the pull of gravity in search of water and minerals and anchor the roots.
Stems are negatively geotropic as they grow up and away from the pull of gravity.

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What is chemotropism?

A change in plant growth in response to chemicals.
In flowering plants, following pollination, the pollen tube grows down towards the ovule which produces chemicals making it positively chemotropic.
Roots are negatively chemotropic to lead and zinc.

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What is hydrotropism?

A change in plant growth in response to water.

Roots are positively hydrotropic as they grow through the soil in search of water.

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What is thigmotropism?

A change in plant growth in response to touch.
Plants like ivy grow around any object they touch by forming tendrils, to provide a secure surface for herbaceous plants.

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What are the advantages of tropism?

Allow the plant to obtain more favourable growing conditions.
Stems are positively phototropic allowing for increased photosynthesis.
Roots are positively geotropic allowing for increased soil water and better anchorage of roots.

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Name two plant minerals and their roles.

Calcium- forms the middle lamella which cements the plant cells together.
Magnesium- is used for the production of chlorophyll.

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What is a meristem?

A region of the plant where cells are actively dividing by mitosis, to produce more cells for growth.

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Where is the apical meristem located?

In root tips and shoot tips.

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What are the functions of the apical meristem?

Responsible for growth in plant length.

Responsible for the formation of leaves and axillary buds.

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Where are the lateral meristems located?

Around the edge of plant stems or branches.

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What are the functions of lateral meristems?

Responsible for growth of plant width.

Formation of bark.

What are the external factors in plant growth?

Day length.
Temperature.
Light intensity.
Gravity.

What is the importance of light intensity in plant growth?

Light energy is needed for photosynthesis to sustain plant growth.
Needed for the formation of chlorophyll.

What is the importance of gravity in plant growth?

The pull of gravity causes the roots to grow down through the soil to obtain nutrients and water.
Allows better anchorage of roots.

What is the importance of temperature in plant growth?

Temperature affect the rate of enzyme activity which controls plant growth.

What is the importance of day length in plant growth?

Day length plays an important role in flowering and fruit and seed formation.

What is a plant growth regulator?

A chemical that controls the growth of a plant.

What are the examples of plant growth regulators?

Auxins.
Abscisic acid.
Ethene.

What are auxins?

Plant growth regulators or plant growth promoters.

They promote plant growth.

What is an example of a naturally occurring auxin?

Indole acetic acid- IAA.

Where are auxins usually found?

In the meristematic regions of root tips and shoot tips. | In developing seeds.

How are auxins transported?

In the phloem up and down the stem.

Where are auxins site of action?

Zone of elongation in the root tip, just behind the shoot tip.

What are the main functions of auxins?

To promote stem elongation and root growth. To inhibit side branching- apical dominance. To cause positive stem phototropism. To stimulate fruit development.

Why is it difficult to establish the exact role of plant growth regulators?

As they are active in small amounts. | Their effect depends on concentration and where they are active.

State two ways in which growth regulators in plants are similar to hormones in animals.

They are both chemical in nature and travel slowly. They both have prolonged effects. They are made at one site and function at another.

Explain the mechanism of response by a plant to a named external stimulus.

Stimulus- light. The stem is exposed to unidirectional light. More auxin diffuses down the shaded side of the stem. This causes unequal distribution of auxins as more is on the shaded side. Cells on the shaded side grow faster and this results in bending.

Suggest what could cause an unequal concentration of auxin in a shoot or root.

Exposure to unidirectional light results in more auxin diffusing down the shaded side.

With regard to auxins, state one example of an inhibitory function.

Apical dominance.

Describe how apical dominance occurs.

Auxin is produced inside the apical meristems and is transported down through the plant in the phloem. High auxin concentration inhibits auxiliary buds from developing into side branches, therefore stem grows in length only. If the apical bud is removed, no auxin is produced so the auxiliary bud is stimulated to produce side branches.

Give a biological reason for pruning.

Gardeners prune bushes to remove the apical buds and therefore allow side branches to grow to produce bushy plants.