(Original post by jonathan3909)
All Questions are related to the practicals in the spoilers
The effect of light on germination of seeds:
Q1 Experiments have shown that one wavelength of light seems to inhibit germination. Suggest a reason why light-sensitive seeds will germinate in white light (this contains all wavelengths).
Q2 Which form of phytochrome is required for germination by light-sensitive seeds such as Coleus, lettuce and feverfew?
Q3 Suggest a reason why light-sensitive seeds are almost always very tiny, and conversely why seeds needing darkness (such as peas) are usually larger.
Q4 Why do you think that seed packets recommend specific planting depths for seeds?
Q5 Describe how you could extend this experiment to test the hypothesis ‘The final flash of light experienced by a germinating seed is the decisive factor.’
The coleoptiles of grasses and cereals have been used extensively for studying phototropism – their simple shape makes it possible to observe the response to light and measure it. In this experiment you can do both.
Q1 Describe the response of the coleoptile to light which you observed in this experiment. Remember the reversing of the image down the microscope. Did the whole coleoptile respond?
Q2 Describe the effect of rotating the coleoptile through 180°. How can you account for any delay observed in the change of response on reversal?
Q3 Suggest a hypothesis to account for your results.
The effect of IAA on growth of seedlings
Auxins are a group of chemicals found in plants; they are involved in growth and development of plant tissues. Traditionally these chemicals have been called ‘plant hormones’. This is because they are produced in one area of the plant and are transported to, and have effects on, different parts of the plant. The term ‘auxin’ is derived from the Greek word auxein which means to grow. One of the most important auxins is indole-3-acetic acid (IAA). IAA is synthesised naturally by the plant from the amino acid tryptophan, and is made in rapidly growing tissues in the plant, especially the tip of the shoot. In this practical you will investigate the effect of different concentrations of IAA on the growth of roots and shoots.
Before completing the following experiment, predict what you think the effect of IAA will be on growth of shoots and the growth of roots.
Q1 Identify the dependent and independent variables, and the variables that should be controlled, in
Q2 Consider the following features of the experimental design. In each case, explain why this feature
• Several hundred seeds were germinated from which you made your selection.
• Each Petri dish contained ten seedlings.
• The Petri dishes were incubated in the dark.
• Measured the increase in shoot length and root length for several seedlings at each concentration.
• All factors kept constant except concentration of IAA.
Suggest another feature which ensures that valid results are produced.
1 Line six Petri dishes with filter paper or folded paper towel. You will need several layers.
2 Select ten similar-sized cress seedlings. These will have been germinated for you by placing seeds in an incubator at 28–30 °C for two days.
They will have been kept in the dark as light destroys IAA.
3 Measure and record the length of the root and shoot of each seedling. The easiest way of measuring is to place the seedling on graph paper.
You will be shown how to distinguish the root and shoot.
4 Measure 5 cm3 of distilled water, using a syringe or graduated pipette. Soak the filter paper. It should be wet but without excess liquid on the top.
5 Repeat steps 2 to 4 using a range of IAA solutions. Your teacher may ask you to make up the range of IAA solutions from a stock solution of 1000 ppm. Alternatively they may be supplied for you. The concentrations of IAA required are shown in Table 1. Prepare the dishes in order from the least
concentrated IAA to the most concentrated.
6 Place the Petri dishes in an incubator in the dark at about 25 °C for two days.
7 Measure and record the length of each root and shoot after two days.
8 Calculate the average increase in shoot length and root length for each concentration compared to the control, and the percentage increase in length of shoot and root compared to the control.
9 Draw an appropriate graph to represent your results.
10 Discuss the effect of IAA on the growth of shoots and roots with reference to your results. Do they
support your original prediction?
1 Select a germinating wheat grain with only the coleoptile visible, i.e. no leaves should have broken through the sheath. Place it inside a syringe.
2 Fit a graticule into the eye-piece of a microscope.
3 Position the microscope so that light from a window or lamp strikes it from one side. Arrange the microscope so that the stage is angled vertically.
4 Fix the coleoptile mount on to the microscope stage using rubber bands or Blu-tack™.
5 Focus the low-power objective on the coleoptile. Adjust the mount to line up the tip of the coleoptile with the mid-point of the graticule scale.
6 Note the time and start a stopclock. After 2 minutes, note the position of the tip of the coleoptile on the scale.Looking through a monocular microscope, the movement appears to be reversed; the movement seems to be away from the light.
7 Take further readings of the coleoptile position at 1- or 2-minute intervals. Do this for about 10 minutes, or until
a regular movement has been recorded.
8 Note the time and rotate the mounted coleoptile through 180° on its vertical axis. Reset the coleoptile tip on the centre of the scale. Repeat the measurements in steps 6 and 7.
9 Plot the positions of the coleoptile tip on graph paper.
10 If possible, leave the coleoptile in position for an hour or more, and observe its final position.
1 Line four Petri dish bases with filter paper.
2 Label the dishes on the base: 1 darkness, 2 white light, 3 red light, 4 far-red light (if available).
3 Count out 50 seeds into the lid of each Petri dish.
4 Measure 5 cm3 distilled water into each dish. Remove any air bubbles.
5 Spread the 50 seeds evenly over the surface of the wet filter paper.
6 Allow the seeds to soak for one hour in complete darkness. The seeds are not sensitive to light until after they have taken up water.
7 Leave dish 1 in complete darkness, making sure it receives no light after soaking. In the dark, expose dish 2 to 300 s of white light, and dish 3 to 300 s of red light. If far-red light is available, expose dish 4 to 300 s of far-red light. Return all the dishes to complete darkness – either a darkened area, or surround the dishes with aluminium foil. If an incubator is available, maintain all the dishes at the same optimum temperature for germination.
8 Examine the seeds after two or three days and record the percentage germination in each dish. Depending on the species used, you may have to leave the seeds for longer before recording percentage germination. Lettuce, for instance, is expected to germinate in seven days, while Coleus takes 21 days. A seed is considered to have germinated if the radicle projects 1 mm or more from the seed coat.
9 Present your results in a suitable table.
10 Write up your experiment, making sure your report includes:
• results presented in the most appropriate way
• a few lines describing and explaining your results; use evidence from your data and your biological knowledge
• a comment on the validity of your conclusions
• a comment on how you ensured that the results obtained were reasonably reliable.