OCR F215 Plant Responses

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Module 4: Responding to the Environment
5.4.1 Plant Responses
(a) explain why plants need to respond to their environment in terms of the need to avoid predation and
abiotic stress
Like animals, plants must also need to respond to external stimuli. This is important to:
Avoid predation .
Avoid abiotic (nonliving) stress .
Maximise photosynthesis .
Obtain more light , water and minerals .
Ensure germination in suitable conditions/pollination.
Seed set/ seed dispersal .
(b) define the term tropism
Tropism ­ a directional growth response in which the direction of the response is determined by the direction of
the external stimulus. Tropisms may be positive (a growth response towards the stimulus) or negative (a growth
response away from the stimulus).
They include
Phototropism (light) ­ shoots grow towards light ­ they are positively phototrophic.
Geotropism (gravity) ­ roots grow towards the pull of gravity .
Chemotropism (chemicals) ­ on a flower, pollen tubes grow down the style, attracted by chemicals , towards
the ovary where fertilisation can take place.
Thigmotropism (touch) ­ shoots of climbing plants, such as ivy, wind around other plants or solid structures
and gain support .
(c) explain how plant responses to environmental changes are coordinated by hormones, with reference to
responding to changes in light direction
Hormones , also referred to as plant growth regulators , coordinate plant responses to environmental stimuli. Like
animal hormones, plant hormones are chemical messengers that can be transported away from their site of
manufacture, by active transport , diffusion and mass flow in the phloem sap or in xylem vessels , to act at target
cells or tissues of the plant. They bind to receptors on the plasma membrane . Specific hormones have specific
shapes , which can only bind to specific receptors with complementary shapes on the membranes of particular
cells. This specific binding makes sure that the complementary shapes on the membranes of particular cells.
Hormone Effects
Auxins, e.g. IAA Promote cell elongation, inhibit growth of
(indole3acetic sideshoots, inhibit leaf abscission (leaf
acid) fall).
Cytokins Promote cell division.
Gibberellins Promote seed germination and growth of
Abscisic acid Inhibit seed germination and growth of
Ethene Promotes fruit ripening.
The cell wall around a plant cell limits the cell's ability to divide and expand . Therefore, growth in plants happens
where there are groups of immature cells that are still capable of dividing ­ these places are called meristems.
Meristem Location Effect
Apical Meristem Tips or apices (singular: apex) of roots and Length of roots and shoots.
Lateral Bud Meristem Buds. Growth of sideshoots.

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Module 4: Responding to the Environment
(c) evaluate the experimental evidence for the role of auxins in the control of apical dominance and
gibberellin in the control of stem elongation
Apical Dominance:
Apical dominance ­ when a growing apical bud at the tip of the shoot inhibits growth of lateral buds further
down the shoot. So if you break the shoot tip (the source of auxin) off a plant, the plant starts to grow side
branches from lateral buds that were previously dormant.…read more

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Module 4: Responding to the Environment
Further studies have shown that gibberellins cause growth in the internodes by stimulating cell elongation (by
loosening cell walls) and (by stimulating production of a protein that controls the cell cycle). Internodes of dwarf
peas have fewer cells and shorter cells than those of tall plants, and mitosis in the intercalary meristems of
deepwater rice plants increases with gibberellin treatment.…read more


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