4.3A Co-ordination and control in plants

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11 Terms

1
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What are phytochromes?

Pigments found in the leaves of flowering plants.

2
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P660

Absorbs red light and rapidly converts to P730 in bright light.

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P730

Absorbs far-red light to rapidly convert to P660, and slowly converts to P660 in darkness.

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Short day plants

  • The photoperiod is below the critical value - the night exceeds the critical value.

  • During the night, more P730 is slowly being converted to P660 - flowering occurs.

  • P730 inhibits flowering.

  • When the dark period is not long enough, P730 can’t be converted to P660 to remove the inhibitory effect - no flowering occurs.

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Long day plants

  • The photoperiod is above the critical value - the daylight exceeds the critical value.

  • During daylight, more P660 is rapidly being converted to P730 and the critical level is reached - flowering occurs.

  • P730 promotes flowering.

  • When the dark period is long enough, P660 can’t be rapidly converted to P730 - no flowering occurs.

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What is the critical value for photoperiods?

12 hours

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How does a flash of white light affect the light regime in SDPs? [2]

The flash of white light allows P660 to rapidly be converted to P730 which causes delay in flowering.

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Suggest an advantage for plants growing naturally and commercial flower growers [2]

  • Ensures that plants flower and set seed at the appropriate time of year.

  • Plants can flower at particular times of the year not normally possible.

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Auxins

  • Produced in the tip and move down the stem - the concentration decreases.

  • Movement is polar.

  • Promotes growth by increased cell elongation.

  • Dependent on the elasticity of the cell wall.

  • Unidirectional light causes movement of auxin to the shaded side.

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Cytokinins

  • Produced in meristematic tissues in zone of division.

  • Promote growth by increased cell division in the apical meristems.

  • Only in the presence of auxins.

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Gibberellins

  • Produced in leaves.

  • Movement is non-polar.

  • Promote growth by cell elongation in the internodes.

  • Dependent on the presence of auxins.