Question 20N.2.HL.TZ0.1d.ii
| Date | November 2020 | Marks available | [Maximum mark: 2] | Reference code | 20N.2.HL.TZ0.1d.ii |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Suggest | Question number | d.ii | Adapted from | N/A |
White clover (Trifolium repens) is native to Eurasia but is now a common plant found worldwide in lawns, next to roads, in pastures and similar habitats.
[Source: © International Baccalaureate Organization 2020.]
Some T. repens plants are able to produce the toxin hydrogen cyanide (HCN) by cyanogenesis. A study at 128 sites (2509 plants) in Toronto (Canada) looked at the proportion of T. repens plants producing HCN. The sites were at regular intervals from the city centre towards rural areas.
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
To determine whether similar patterns in cyanogenesis were seen in other cities, studies were also carried out around New York City and Boston (USA) and Montreal (Canada).
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
The researchers considered two possible ecological causes for the observed gradients in cyanogenesis. First, the researchers assessed the extent of herbivory of T. repens plants in Toronto by measuring the percentage of leaf area that was eaten. The graph shows the results for both cyanogenic and non-cyanogenic plants.
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
The researchers then investigated a possible correlation between cyanogenesis and exposure to freezing conditions. It had been proposed that when a cyanogenic plant freezes, its cells burst, releasing HCN which is toxic to the plant. Snow can insulate the ground and plants from freezing temperatures. However, snow is more likely to melt in cities, which then exposes plants to freezing temperatures.
All four of the cities studied receive below freezing temperatures and winter snowfall. Researchers looked at the number of days below freezing (0 °C) that did not have snow cover in these cities.
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
Using all of the data so far, suggest whether exposure to freezing temperatures in the four cities is supported as a reason for the differences in HCN production in T. repens.
[2]
a. cities with more days without snow cover have positive correlation between distance from city center and HCN / vice versa for Montreal;
b. fewer plants with HCN within cities that have more days without snow cover/have more exposure to freezing temperatures / converse for Montreal which has fewer days without snow cover;
c. HCN is 0.2 (or less) HCN in cities that have more days without snow cover proportion whereas city with fewer days/Montreal it is 0.5/more than 0.4;
d. in Toronto cyanogenic and non-cyanogenic plants show little difference in herbivory;
e. support for hypothesis/exposure to freezing temperatures as reason.
Do not accept ‘Yes’ unqualified as an answer, but accept it if supported by reasoning.
Many candidates found this to be another difficult question. There was a lot of data to consider and as in (c) it was necessary to take into account whether differences were likely to have been significant. In the histogram showing the amount of cold experienced, the large error bars show that only differences between Montreal and the other three cities are great enough to be considered reliable. When combined with data for the proportion of plants with HCN in each city, it was clear that there was support for exposure to cold as a reason.
