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Date	May 2018	Marks available	3	Reference code	18M.2.HL.TZ2.1
Level	Higher level	Paper	Paper 2	Time zone	Time zone 2
Command term	Discuss	Question number	1	Adapted from	N/A

Question

Arabidopsis is a small flowering plant in the mustard family (Brassicaceae) that is widely used in basic research. It has a short life cycle, flowers quickly producing a large number of seeds and is easy to cultivate. It forms a circle of leaves known as a rosette that lies close to the soil. Flowers form at the end of short stems.

A study was carried out of differences in development between Arabidopsis plants grown in long days (16 hours light, 8 hours dark) or short days (8 hours light, 16 hours dark). The sixth leaf (L6) to emerge in the rosette of each plant was used in all investigations.

New leaves are initiated by the meristem and go through four stages as they develop.
• Stage 1 (S1) – rapid cell division
• Stage 2 (S2) – cell division has ceased, cell expansion continues
• Stage 3 (S3) – decreasing cell expansion rate
• Stage 4 (S4) – leaf growth complete

The start of each stage of leaf development for plants grown in long days and short days is shown above the first graph.

Leaves were removed from Arabidopsis plants that had been grown in long day and short day conditions and the concentration of starch within them was measured. This was done both at the end of the day (D) and at the end of the night (N) in each of the four stages of development (S1, S2, S3, S4).

To account for the observed phenotypic and metabolic differences, researchers analysed mRNA transcript data. They found certain transcripts over-represented in Arabidopsis plants grown in long days (dark grey) compared with the amount expected due to chance.
Other types of transcripts were over-represented in Arabidopsis plants grown in short days (light grey).

Calculate the difference (in mm²) in the mean leaf area of L6 at the start of stage 4 between the leaves of plants grown in long days and short days.

[1]

a.i.

Distinguish between plants grown in long days and short days in the timing of the four stages of leaf development.

[2]

a.ii.

Distinguish between plants grown in long days and short days in the mean number of leaves per rosette during the experimental period.

[2]

Discuss the evidence provided in the bar chart for the hypothesis that plant leaves use up starch reserves for cell respiration during the night.

[2]

For each of the stages, identify whether the starch concentration at the end of the day is higher in the leaves grown in long day or short day conditions.

[1]

d.i.

Suggest reasons for the difference in end of day starch concentrations in stage 2 (S2) for the plants grown in long days and short days.

[2]

d.ii.

Using the data in the bar chart, discuss the evidence for Arabidopsis plants adapting to different daylight regimes by changing the pattern of gene expression.

[3]

Using all relevant data in this question, deduce with reasons whether Arabidopsis is a long day plant or a short day plant in terms of flowering.

[2]

Markscheme

«130 − 85» = 45 «mm²»

Allow answers in the range of 40 to 50 «mm²»

a.i.

a. S1/S2 is longer in short day plants
OR
the stages in long day plants are more variable in length. Accept vice versa.

b. leaves of plants grown in long day reach S2 / S3 /S4 stages sooner
OR
S1/S2/S3 completed earlier in plants grown in long days

c. leaves of plants grown in long day reach S1 later than plants grown in short days

a.ii.

a. rosette of plant grown in long day has fewer leaves. Accept vice versa.

b. rosette leaf number of plant grown in long day plateaus/stays constant while the number continues to increase for plants grown in short days. OWTTE.

a. lower starch levels at end of night in all stages

b. lower starch levels at end of night in both plants grown in short day and long days

c. no evidence that starch is being used for respiration
OR
starch may have been exported/stored in other tissues/example tissue «rather than used in respiration»

higher in plants grown in short days in S1 and higher in plants grown in long days for all other stages/S2, S3 and S4

Candidates must mention all stages for the mark.

d.i.

a. leaves in plants grown in long day receive longer period of light / more leaf surface area so more photosynthesis occurs resulting in more starch. Accept vice versa.

b. plants in short days using starch to produce more leaves/for growth/S2 a period of rapid increase in number of leaves. Accept vice versa.

d.ii.

a. «mRNA» transcripts differ in plants grown in long days and short days. Accept an example of such a transcript from the bar chart

b. indicates different genes are being expressed. Accept other valid reason.

c. plants adapt to different daylight regimes by altering gene expression

d. short day length causes struggle to get enough light to photosynthesize and more «mRNA» transcripts related to photosynthesis
OR
plants produce large leaves rapidly when grown in long days which may result in more transcripts for biotic stress

a. long day plant

b. flowering hormone metabolism gene over represented in long day exposure

c. fewer leaves produced «rapidly» by plant in long day as energy shifted to flower formation. Accept other valid reasons from the data

d. plants grown in short days produce more leaves over longer period before beginning to flower/need to use light more efficiently to photosynthesize

Allow ECF if student indicates short day plant.