Volume 29, Number 2, November 2016

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’, onpages 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.

1Mistletoe and dodder are parasitic plants. Give two features that are common to all parasites.
[2 marks]

2How does a completely parasitic plant differ from a half-parasitic plant?[1 mark]

3Plant 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 furtherenvironmental factor that stimulates germination of Strigahermonthica. [1 mark]

4Once 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]

5Cells 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.

6Witchweed, Strigahermonthica¸ 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]

Read the section of the article headed ‘Genes to the rescue’ and answer the following questions.

7Some crop varieties show natural resistance against Striga. Describe how this natural resistance could arise. [4 marks]

8Figure 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’.

9The 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.

10Table 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]

(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]

11Figure 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’.

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]

(d) What conclusions can you make from the graph? [3 marks]

Outline answers to questions

1 The two features are:

a parasite lives on or in another organism — its host;
the parasite harms the host

2The 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.

6The 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

7A 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, non-resistant 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.

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

9Push — 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.

(ii) Larger gross profit.

11(a) The independent variable (district) is discontinuous.

(b) Student’s t-test; comparing the mean values of two samples.

(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