Plants_Long1

A group of Grade 12 learners wanted to investigate the effect of light coming from one direction on the growth of shoots. They planted some wheat seeds in two seed trays and allowed it to germinate. When young shoots appeared above the soil level, the shoots were exposed to light from all directions for three days. After three days, the trays received different treatments as follows:

Tray A: The shoots were exposed to light from all directions

Tray B: The shoots were exposed to light from one direction only.

The diagrams below show the effects of these treatments. Study it and answer the questions that follow.

Question 1

Formulate a hypothesis for the investigation. (2)

Shoots will grow towards the light; OR

Shoots will grow away from the light; OR

Light has no influence on the shoot.

Explain why is was important to include tray A as part of the investigation. (2)

It is the control - To verify the results of the experiment / To allow for one variable only.

State ONE conclusion that may be drawn from this investigation. (2)

Shoots grow towards the source of light.

A third tray (C) was set up in a similar way as tray A and tray B. The tips of the shoots were covered with aluminium foil. The diagram below shows the appearance of the shoots at the start and after being exposed to light from one direction only.

What conclusion can you draw from the results obtained in tray C? (2)

The auxins that make the shoot to grow towards the light are in the tips of the shoots.

State ONE use in agriculture of: a) Auxins and b) Gibberellins. (2)

a) Auxins: Apical dominance

b) Gibberellins: Tall growth of a plant/stimulate seed germination.

A learner conducted an investigation to determine the effect of auxins and the effect of gravity on root growth in pea seedlings. He used the following procedure:

Question 2

He germinated pea seedlings for seven days.
He then took a sample of 15 seedlings and divided them into 3 groups (A to C) of 5 seedlings each.
In each group the 5 seedlings were placed horizontally on 3 different clinostats.
He removed the root tips of all 5 seedlings at the same length in group B.

In groups A and B the clinostats were left stationary (no rotation).
In group C the clinostat was allowed to rotate.
All 3 clinostats were placed in a dark cupboard.

A summary of the learner’s procedure is shown in the table below.

Name the TWO groups were used to obtain information about the effect of auxins on root growth. (2)

A and B

Name the TWO groups were used to obtain information about the effect of gravity on root growth. (2)

A and C

Explain why the apparatus was placed in a dark cupboard. (2)

To ensure that the results are attributed to gravity and not to light/to eliminate the effect of light.

Describe the expected results for groups B and C in this investigation. (2)

B: No growth occurs.

C: Roots frow in the direction they had been placed – no change occurs.

Explain the expected results for group A. (3)

- auxins will move to the lower side of the root / attracted by gravity.

- and a high concentration will inhibit growth on the lower side of the roots.

- while growth will occur faster on the upper side of the root.

- causing the root to bend downwards

- this shows positive geotropism

(Any 3)

State THREE ways in which the learner ensured a higher level of validity for this investigation. (3)

Used same type of plant/pea only.

Seedlings were the same age/germination period was 7 days.

All groups were exposed to the same environment / light intensity/placed in dark cupboard.

Same number of seedlings for each group.

Root tips were cut at the same length.

All seedlings were placed in the same position / horizontally.

Allowed same amount of time for 3 groups.

Appropriate controls were set up.

(Any 3)

The graph below shows the growth response of roots and shoots to auxins applied at different concentrations.

Question 3

Choose

Set 1

Set 2

At which concentration of auxins do shoots show the most growth? (2)

10 parts per million

From what concentration of auxins is growth inhibited in roots? (2)

From 0.01 parts per million onwards

Which part of the plant, the root or the shoot, is more sensitive to auxins? (1)

Roots

Does the optimum auxin concentration for roots have an effect on shoots? (1)

Suggest how the information in the graph can be used in the manufacture of herbicides. (4)

Weed killers must have an AUXIN concentration of 100 parts per million or greater than this value. Both shoot growth and root growth will be inhibited. At 1000 parts per million, no shoots or roots will grow.

After 2 days the direction of root growth was recorded.

Set 1

Set 2

Questions source: DBE (Authorisation 11798)

- Greg Gopaul -