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Biology F215

Plant Responses

Key Words

Key words Definition

Abiotic Non-living components of the environment.

Abscisic acid (ABA) Plant hormone that inhibits seed germination and growth, and causes
stomatal closure when water is limited.

Abscission The shedding of leaves from deciduous trees.

Apical dominance The growing apical bud at the…

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Lateral meristems Immature cells still capable of dividing, found in a cylinder near the outside of
roots and shoots. They are responsible for the roots and shoots getting wider.

Nastic movements Non-directional responses to stimuli such as touch, temperature or humidity.
Not dependant on the direction of the stimulus.


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What stimuli to plants respond to?

Plants will move in response to stimuli
o This stimuli can be either biotic (living) or abiotic (non-living)
The response to this stimuli can be
Tropic (directional)
Nastic (non-directional)

What are tropic responses?

Tropic responses or tropisms are directional plant growth responses
o These…

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An example of this is flowers opening and closing in
response to day length and temperature fluctuations
Thigmonastic: where the response is the same regardless of
the direction of the stimulus
The movement is the same once triggered, e.g.
closing petals, closing leaves
o An example of this type of…

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Promote growth of stems
Promote seed germination
Cytokinins are the third group of plant hormones
o Cytokinins
Promote cell division
Abscisic acid (ABA) is the fourth type of plant hormone
o Inhibits seed germination
o Causes stomatal closure during water stress
Ethene is the final type of plant hormone

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Auxin binds to its receptor and promotes the active transport of hydrogen ions into the cell wall by an
o The low pH provides optimum conditions for wall-loosening enzymes to work (expansins)
Expansins and low pH disrupts hydrogen bonds within the cellulose
These enzymes break the bonds within the…

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What is apical dominance?

The presence of the auxin-producing apex (shoot tip) normally inhibits the development of lateral buds
(side shoots) is a phenomenon known as apical dominance
o What are the advantages of this?
It allows plant to direct most of their assimilates to the apical bud to give…

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o This means normally cytokinins are concentrated
in the shoot tip ­ but when the tip is removed
the cytokinins spread more evenly throughout
the plant

What are gibberellins?

Gibberellins are a family of plant hormones that promote stem elongation and seed germination
o Plants that are genetically dwarf, such…

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This research confirmed that GA1 caused stem elongation

How is plant growth caused by gibberellins?

Cause growth at the internodes by stimulating cell elongation
by loosening cell walls
o They also increase cell division by stimulating the
production of a protein that controls the cell cycle
The internodes of dwarf…

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o Sugar cane stores sugar in the cells of the internodes
Gibberellin stimulates growth between nodes
Sugar yield is increased
o In the first growing season sugar beet produces a large storage root containing 15-20% sucrose
In the second year the energy stored in the root is used to produce…




These notes are well presented and give a good account of plant responses with a useful glossary of key terms. Use them as your notes, build on them or use them to create revision cards from. Everything is here - definitions, examples and annotated diagrams.

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