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control of growth in plants
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control of growth in plants revisiion notes for year 13 alevel students doing pearson edexcel exams
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Unit 5Topic 8 Note 8
Control of growth in Plants
Plant growth and development is influenced by
External factors- Light, detected by Phytochrome
Internal factors- Plant growth substances
The effect of light on the growth of plants
Tropism- The growth movements of plants in response to external stimuli.(eg: light).
• Tropic growth movements are directed by an external stimulus.
• If the growth is towards the stimulus it is called a Positive tropism, if it is away from
the stimulus it is called a negative tropism.
Eg: Shoots growing towards light- Positive Phototropism
Shoots are Positive Phototropic.
Charles Darwin and his son Francis first investigated the response of plant organs to the
direction of light.
They used Oat coleoptiles (the sheath
covering the Plumule) as they are very
sensitive to light and in the dark, they
grow first as stems. Also they could be
grown easily in large numbers and
they showed the same responses as
shoots.
The chemical was named AUXIN. It
was identified as Indoleacetic acid
(IAA).
Greater conc. of Auxin stimulated cell
elongation in the region just behind
the apex. (the zone of elongation)
Unilateral light causes the tip of the shoot to bend towards the light.
, Unit 5
Topic 8 Note 8
Plant growth substances
Five groups are recognized.
• Auxins Growth promoters (involved in the processes of cell
• Gibberellins division, cell elongation, differentiation and initiation of
• Cytokinins organs
• Abscisic acid Growth Inhibitors ( associated with processes such as
• Ethene ripening and leaf fall.
The plant growth substances can interact with each other either Synergistically (effects are
enhanced by two or more acting together) or Antagonistically (effects counteract each
other)
Auxins
Auxins are involved in:
• Elongation growth in coleoptiles and many stems by softening the cell walls.
• Initiation of roots
• Dormancy in lateral buds and fruit formation
Numbers of naturally occurring Auxins are found. Most important is Indoleacetic acid.
Some synthetic Auxins have important applications on a commercial scale in Horticulture
and agriculture.
IAA is synthesized in the regions of active cell division and enlargement, notably apical
meristems (at the tip) of shoots and leaves.
Transport of IAA away from the apex to older organs is in one direction ( polar).
It is by an active process, with the Auxins passing from cell to cell.
Transport from sites of synthesis in leaves appears to be non polar. It occurs in the phloem,
by a passive process.
It allows the Auxin to move up or down the plant at speeds considerably faster than that
achieved by the polar transport.
• Elongation growth in coleoptiles and many stems by softening the cell walls.
Auxin promotes growth by increasing the plasticity of the cell wall. As the cell wall
softens, the cell becomes less turgid and so takes up more water. Therefore it results
expansion of the cell. In the apical region due to the cellulose microfibrils in the cell wall,
cell expansion tends to be in a longitudinal direction, leading to elongation.
The softening of the cell wall is linked to an acidification process. Hydrogen ions are
pumped out of the cell by an active process, resulting in a decrease in pH activating
enzymes which promote breakdown of bonds within the cell wall structure.
The process of cell wall expansion is irreversible and auxin may promote the synthesis of
new material, ensuring growth.
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