Coordination & Control in Plants

• What do plants respond to?

  They have a range of stimuli to which they can respond.
• What are three examples of stimuli + what three parts of the plant may respond to them?

  Gravity, water and light are key examples and the parts of the plant involved in responses include roots, stems and flowers.
• What does the response of a plant rely on?

  Hormonal coordination.
• How do species of plants differ (in terms of flowering) + what is this indicative of? (2)

  - Different species of plant flower at different times, for example flowers found on the forest floor may flower in early spring before the leaf canopy closes.

  
  

  - This is indicative of the importance of light triggering the development of flowers.
• What does the photoperiod result in for plants?

  Control of flowering is a result of the photoperiod - the relative length of days and nights.
• What is the difference between a SDP + a LDP?


  Plants that flower when day length is short and night length longer are referred to as short day plants (SDP), whilst those that flower when days are long and nights are short are referred to as long day plants (LDP).
• What is phytochrome responsible for, where is it found + what forms does it exist in?

  - The control of flowering in plants is a result of a chemical called phytochrome.

  
  

  - This is found in the leaves and exists in 2 interchangeable forms: P660 and P730.
• What happens when P660 absorbs light of wavelength 660nm?

  When P660 absorbs light of wavelength 660nm (red light) it rapidly converts into P730

  
• What happens when P730 absorbs light of wavelength 730nm?

  When P730 absorbs light of wavelength 730nm (far red light) it rapidly converts into P660

  
• What is the concentration of P660 + P730 in the daytime vs the night? (2)

  - Daylight has proportionally greater amounts of light with the wavelength 660nm so during the day P730 tends to accumulate.

  
  

  - P730 is also unstable and therefore is converted back to P660 slowly during darkness (i.e. at night).
• What determines whether flowering will occur + what is the physiologically active form of phytochrome?

  - It is the concentration of P730 in the leaves that determines whether flowering will occur

  
  

  - It is important that you always refer to P730 being the physiologically active form.
• What depends on the plant species in terms of P730?

  Depending on the plant species, P730 is either inhibitory or stimulatory.
• What does P730 do in different species of plants?


  In LDPs P730 stimulates flowering, whilst in SDPs P730 is inhibitory.
• What has to be in place for a SDP to flower + why?

  For a short day plant to flower there has to be critically long period of uninterrupted darkness - this allows enough time for the accumulated P730 to be converted back into P660 thereby removing the inhibitory effect of P730.
• What has to be in place for a LDP to flower + why?

  In a long day plant the night length has to be short enough to prevent too much P730 being converted into P660 - the concentration of P730 needs to be high enough to stimulate flowering.
• Which is most important: the day length or period of darkness, + why?

  It is therefore the period of darkness that is most important - not day length (so it might be better to think of SDPs as long night plants and LDPs as short night plants!!) as it determines how much P730 can be converted back to P660.
• What does light regime 1+2 show? (LDP)

  In light regimes 1 + 2 the length of the day is not long enough to build up P₇₃₀ to sufficiently high levels to initiate flowering.

  
• What does light regime 3 show? (LDP)

  In 3 flowering is initiated as the period of light is sufficient to build up P₇₃₀ to the critical level (+ the period of darkness is not long enough to break down the P₇₃₀ + keep it below the critical level).

  
• What does light regime 4 show? (LDP)

  In light regime 4 the period of darkness that interrupts the light period does not reduce the P₇₃₀ level enough to inhibit flowering (there is still enough light to allow the P₇₃₀ to reach the critical level).

  
• What does light regime 1 show? (SDP)

  In light regime 1 the period of darkness is not long enough to remove the inhibitory effects of high levels of P₇₃₀.

  
• What does light regime 2 show? (SDP)

  In light regime 2 the period of darkness is long enough to allow enough P₇₃₀ to be converted back to P₆₆₀ (thus removing the inhibitory effect of high levels of P₇₃₀).

  
• What does light regime 3 show? (SDP)

  In light regime 3 the short flash of light during the dark period is enough to inhibit flowering, as during the shirt flash of light, P₆₆₀ will be rapidly converted to P₇₃₀ therefore not allowing a sufficiently long continuous period of darkness for enough P₇₃₀ to be converted to P₆₆₀.

  
• Why must the critical period in SDPs be continuous?

  In SDP's the critical period of darkness must be continuous

  - this is because a short flash of light will rapidly convert

  P660 to P730, but the conversion of P730 to P660 is slow.
• What does knowledge about the photoperiod of different plant species allow growers to do?

  Allows growers to have plants flowering when consumer demand will be higher for example at Christmas or Valentine's Day.
• Give an example of the commercial use of the photoperiod with chrysanthemums. (4)

  - It is a SDP that usually flowers in Autumn as the days shorten.

  
  

  - It can be prevented from flowering by providing them with a long day lighting regime.

  
  

  - A few weeks before Christmas, flowering can be initiated by providing a short day lighting regime in the greenhouse.

  
  

  - This can be done through using screens that block light from entering the greenhouse.
• How is the onset of flowering in LDPs stimulated? (2)

  - The onset of flowering in LDP's is stimulated by increasing day length (reducing darkness) by using artificial lighting in a greenhouse, the onset of flowering in SDPs can be manipulated by using a screen to reduce light.

  
  

  - However, there can be a time interval between the stimulation of the flowering process and the appearance of flowers
• When do some LDPs appear?

  In some British species, of LDP flowers don't appear until July/August when the days are getting shorter.
• What are plant growth responses controlled by?

  Plant growth responses are controlled by a number of different hormones.
• What is a hormone?

  A hormone is an organic compound produced in 1 part of an organism and transported to other parts to cause a response.
• What are 3 classes of plant hormone?

  - Auxins

  - Gibberellins

  - Cytokinins
• Give the following information about auxin:

  => What are they involved in?

  => What direction does the transport of auxin occur in?

  => What does the short vs long distance movement of auxin rely on?

  => What are the three effects of auxin?

  - Auxins are involved in phototropism.

  
  

  - The transport of auxin occurs in 1 direction - away from the tip.

  
  

  - Short distance movement of auxin relies on diffusion, long distance movement is reliant on the phloem.

  
  

  - A major effect of auxin is to cause cell elongation but they are also responsible for the maintaining the structure of cell walls and if in high concentrations can inhibit growth.
• What is the function of gibberellins, what does this cause + what have they been used to promote? (3)

  - Gibberellins elongate the internodal distance (the distance between leaves).

  
  

  - The effect of this is to increase the overall length of the stem.

  
  

  - They have also been used to promote growth in dwarf varieties of plants.
• What do cytokinins promote + what do they prevent? (2)

  - Cytokinins promote cell division but only in the presence of auxin.

  
  

  - They have also been shown to prevent senescence - the process of aging and breakdown of chlorophyll in older leaves.
• What do auxin cause plants to do in terms of growth?

  Auxins cause plants to grow towards light - positive phototropism.

  
• What are the main stages of phototropism? (7)

  - Auxins produced in the cells of the apical meristems

  
  

  - They diffuse down the shoot to the zone of elongation

  
  

  - The auxin binds to receptors in the cell membranes of the newly formed cells in the zone of elongation (as newly formed cells the cell walls are also thinner and therefore more flexible)

  
  

  - The cells pump H+ ions into the cellulose cell walls

  
  

  - This acidification loosens the cross links in the cellulose microfibrils making the cell wall more flexible

  
  

  - As the cells absorb water by osmosis the cell walls stretch more readily due to the hydrostatic pressure exerted

  
  

  - The greater the concentration of auxin, the more flexible the walls become
• What happens if there is differential illumination? (2)

  - If there is differential illumination (e.g. light from one side only) auxin concentration will be higher on the shaded side.

  
  

  - Greater cell elongation on the shaded side will result in curvature of the shoot towards the light source.