Plant Biology
Environment: Tropisms

• Plants have adaptive mechanisms which change their growth in ways that help them adjust to and exploit their environment. For example, shoots grow up into the light and their leaves exploit that light for photosynthesis. Three types of environmental stimuli can cause a change in the direction of plant growth. The stimuli and the growth responses resulting from them are: gravity and gravitropism, light and phototropism, touch and thigmotropism. We will investigate two of these tropisms using fast plants, Brassica rapa.

  Gravity and light effects

  What color of light causes most change in direction of growth?

  We might expect that a plant would grow more strongly towards a bright light than a weak light. So to answer this question conclusively it would be necessary to arrange that the different colors had equal energy or intensity. This is impossible with simple equipment, so we adopt a somehat indirect approach. Seedlings will be exposed to light on one or two sides. First we can look at the effect of blue light from one side; then we can see if the presence of red or green light at the other side has any effect on growth towards the blue light. At the same time we can observe the effects of the different colors on the overall growth of the seedling and its development of chlorophyll. For comparison we will have two controls, seedlings in complete darkness and seedlings with all round (white) light.

  Procedure

   

  1. Working in groups of two, set up RCB seeds to germinate in five film canisterswith or without filters as follows: clear, dark (no filter), blue filter only, red and blue, green and blue.

      

  2. For each canister, wet a square of felt and place it on the lid.

      

  3. Put two RCB seeds in the center of the square.

      

  4. Cover with the base of the canister and mark the lid at the side with the blue filter (if present).

      

  5. Put an adhesive label with your group name on the bottom of the canister (which is facing up).

      

  6. Put the canisters under the light source for two days.

      

  7. At the next lab open the canisters carefully, without shaking up the contents.

      

  8. Observe the direction of growth relative to the colored filter(s). Measure the sideways displacement of the apical bud in mm, the length of the hypocotyl in mm and observe the extent of chlorophyll development.

      

  Filter Direction	lat disp	hypocotyl	Chloro*
  none (dark)	n/a
  none (clear)	n/a
  blue
  blue & red
  blue & green

  * for dark and clear say whether cotyledons are green or pale. For others say whether +, < or > (equal, less or more green) than dark or clear seedlings.

  Please make a second copy of your data on class record sheets.

  Gravity and light effects

  During the lab we will set up two demonstrations. The first involves 7 to 10 day old RCB plants which are just forming flower buds, so that their stems are extending rapidly. Later in the lab observe the plants which have been placed on their sides in the light or dark and consider:

  Do shoots grow upwards in response to gravity or light and explain your answer.

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  The second demonstration is with RCB seeds which have been growing for two days on filter paper in petri dishes placed on their sides and in the light. During the lab we will rotate the petri dishes through 90^o. They will be left to grow for two days.

  What are the differences between roots and shoots in their responses?

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  Can you relate this to your knowledge of the regions where roots and shoots are capable of elongating?

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