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Volume 29, Number 2, November 2016
Activity
Interactions between plants
Martin Rowland
This exercise is based on Caroline Wood’s article, ‘The bewitching world of parasitic plants’, on pages
38–41 of this issue.
The experience of examiners shows that students tend not to perform as well on questions based on
plants as they do on those based on animals. This, perhaps, reflects a general lack of interest in plant
biology, other than in the role of plants as the producers in most food webs. Yet, Caroline Wood’s
article shows some of the fascinating ways in which plants interact with each other and with other
organisms in their habitat. It also introduces economic aspects of crop culture, which lend themselves
to questions testing your ability to evaluate data (assessment objective 3, AO3, in AS and A-level
biology exams).
Read the first three paragraphs and Box 1 of Caroline Wood’s article and answer the following
questions.
1 Mistletoe and dodder are parasitic plants. Give two features that are common to all parasites.
[2 marks]
2 How does a completely parasitic plant differ from a half-parasitic plant? [1 mark]
3 Plant seeds are able to remain dormant in the soil. The process by which they develop to produce a
new plant is called germination.
(a) Give two environmental factors that stimulate germination of all plant seeds. [1 mark]
(b) Give one further environmental factor that stimulates germination of Striga hermonthica. [1 mark]
4 Once the radical of a parasitic plant contacts a suitable host, it forms a wedge-shaped organ called a
haustorium, which forces its way through the cortex of the host plant.
(a) Explain why the haustorium is described as an organ. [1 mark]
(b) Name the type of enzyme that will enable the haustorium to digest the cell walls of the host.
Explain your answer. [1 mark]
5 Cells at the centre of the haustorium differentiate into xylem vessels.
(a) Give three ways in which a xylem vessel is different from other plant cells. [3 marks]
(b) Differentiation involves ‘turning on’ particular genes within cells. What stimulates this process in
eukaryotic cells? [2 marks]
Read the section of the article headed ‘When parasites plunder our food’ and answer the following
question.
6 Witchweed, Striga hermonthica¸ is thought to infect 50 million hectares in sub-Saharan Africa.
Explain those features of the life cycle of this plant that contribute to its widespread effect? [3 marks]
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Read the section of the article headed ‘Genes to the rescue’ and answer the following questions.
7 Some crop varieties show natural resistance against Striga. Describe how this natural resistance
could arise. [4 marks]
8 Figure 3.1 in Box 3 of the article shows how mapping populations might be possible.
(a) What does Figure 3.1 suggest about the nature of the allele conferring resistance to parasitic
plants, such as Striga? Use evidence from the artwork to justify your answer. [2 marks]
(b) Number the F2 offspring in Figure 3.1 from left to right, 1 to 8. Explain the difference in the
appearance of the F2 offspring 6 and 8. [2 marks]
The final questions relate to the technique described in the section of the article headed ‘Push–pull’.
9 The term ‘push–pull’ was first used in 1987 to describe a strategy developed to manage insect pests
of cotton plants in Australia. Explain what the term means. [2 marks]
Maize is one of the principal food and cash crops for many people in Kenya, a country in sub-Saharan
Africa. Moths called stem-borers lay their eggs in the stems of maize plants. The immature stages of
the moths’ life cycle are spent in the stem of the maize plants, causing a reduction in the crop yield.
The use of chemical pesticides is the main method to control stem-borers.
10 Table 1 summarises data collected by scientists over 7 years in two districts of Kenya. The data
compare trials of push–pull with the traditional practice of the farmers growing maize.
Table 1
District
Mean total amount of money involved/US$ per hectare
Labour costs
Total costs
Gross income
Gross profit
Farmers’
practice
Push–
pull
Farmers’
practice
Push–
pull
Farmers’
practice
Push–
pull
Farmers’
practice
Push–
pull
A
128
223
374
493
628
1290
254
797
B
134
167
250
278
329
679
79
401
(a) Use Caroline Wood’s article to name two of the traditional practices used by farmers. [2 marks]
(b) Use the data in Table 1 to explain how the gross profit was calculated. [1 mark]
(c) At the start of the trials, many farmers were reluctant to use the push–pull strategy.
(i) Use data from the table to suggest a reason for this initial reluctance. [2 marks]
(ii) Use data from the table to suggest a reason why farmers eventually adopted the push–pull
strategy. [1 mark]
11 Figure 1 shows data from a second investigation conducted over a single growing season in four
districts of Kenya. Maize was grown alone in some plots (maize monocrop) and in a push–pull regime
in other plots.
The symbols above each of the bars represent ‘+ 1 standard deviation’.
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Figure 1
(a) Explain why the scientists presented their data as a bar chart. [1 mark]
(b) Name the statistical test that the scientists should use to test the data in each district. Justify your
answer. [2 marks]
(c) What information is provided by a standard deviation? [2 marks]
(d) What conclusions can you make from the graph? [3 marks]
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Outline answers to questions
1 The two features are:

a parasite lives on or in another organism — its host;

the parasite harms the host
2 The completely parasitic plant takes water from its host’s xylem and assimilates (e.g. sucrose) from
its host’s phloem. A half-parasitic plant takes water from its host’s xylem but is able to produce its own
assimilates.
3 (a) The two most likely answers are:

suitable temperature;

presence of water
(b) Strigolactones (see Box 1).
4 (a) It contains several different tissues.
(b) It is likely to be a cellulase because the main component of plant cell walls is cellulose.
5 (a) A good answer will include three of the following features of a xylem vessel:

It lacks a protoplast/lacks cytoplasm (NB ‘is dead’ would not accurately reflect this feature).

It is cylindrical with no end walls.

Its wall is pitted.

Its wall is lignified.
(b) Transcription factors attach to the promoter region of the relevant gene.
6 The three features you can find in the article are:

a single plant can produce 50 000 to 200 000 seeds, so increasing the likelihood of
contamination

the seeds are extremely small (about 0.3 mm long and 0.2 mm wide — see Box 1) so are
easily dispersed

infection occurs underground, so is not recognised before the host plant is harmed
7 A good answer will include the following points:

An allele conferring resistance occurs by gene mutation.

The plant with the allele for resistance will have a reproductive advantage over other, nonresistant plants.

The allele for resistance is inherited by the offspring of the resistant plant.

Over many generations, the frequency of the allele will increase/the allele for resistance will
become the most common.
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8 (a) The allele for resistance is recessive. The evidence is that only those F2 offspring with two
alleles for resistance are shown to be resistant/F2 offspring with only one allele for resistance are not
resistant.
(b) A good answer will include:

no crossing over occurred during meiosis leading to either of the gametes that fused to form
individual 6

crossing over occurred during meiosis leading to both gametes that fused to form individual 8
9 Push — growing, among the crop, plants that deter parasites.
Pull — growing, around the crop, plants that attract pests.
10 (a) Any two of:

use of chemical pesticides;

applying nitrate fertilisers;

hand-weeding;

relocating farms from time to time
(b) Gross profit minus total costs.
(c) (i) Push–pull is more labour intensive; push–pull costs more (and the farmers have little money).
(ii) Larger gross profit.
11 (a) The independent variable (district) is discontinuous.
(b) Student’s t-test; comparing the mean values of two samples.
(c) The spread of data about the mean.
(d) Your answer will probably include:

the yield (of maize grain) of monocrop is different in the four districts, possibly reflecting
different abiotic conditions;

the yield of maize grain is higher from the push–pull regime than from the monocrop in all four
districts;

the standard deviation bars show that the increases in yield are all (statistically) significant/are
not due to chance
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