Question 20N.2.HL.TZ0.1d.i
| Date | November 2020 | Marks available | [Maximum mark: 1] | Reference code | 20N.2.HL.TZ0.1d.i |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Identify | Question number | d.i | 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.]
Identify with a reason the city where the plants were more insulated from freezing temperatures.
[1]
Montreal because it has the lowest number of days below 0 °C without snow cover;
Do not award the mark for Montreal if the reason is not given.
90 % of candidates identified Montreal as the city where plants were more insulated because it was where there were fewer freezing days without snow cover.
