Question 53·Medium·Command of Evidence
Desert marigolds (Baileya multiradiata) frequently grow in association with arbuscular mycorrhizal (AM) fungi, which colonize plant roots. Ecologist Layla Ramirez hypothesized that AM fungi help desert marigold seedlings tolerate drought by improving the plants’ water uptake.
To test this idea, Ramirez grew two groups of genetically similar seedlings in identical pots of sterilized sand. One group received a live AM fungal inoculation, and the other received a heat-killed (inactive) inoculation. After four weeks of growth under ample water, she withheld water from all pots while monitoring plant responses.
Ramirez concluded that AM fungi enhance drought tolerance in desert marigold seedlings.
Which result of the experiment, if observed, would most directly support Ramirez’s conclusion?
For questions asking which result best supports a scientific conclusion, first underline the key claim (here, that live AM fungi enhance drought tolerance by improving water uptake). Then, scan each answer choice and quickly eliminate options that (1) describe conditions before the relevant event (like before drought), (2) show no difference between groups, or (3) describe changes that are only indirectly related to the conclusion (like structure instead of performance). Choose the option that most clearly shows the experimental group doing better on the specific outcome named in the conclusion than the control group, under the critical conditions (here, during drought).
Hints
Restate what needs to be proven
Ramirez concluded that AM fungi help seedlings tolerate drought. Look for a result that shows seedlings with live fungi do better under drought conditions than those without effective fungi.
Focus on timing: before vs. during drought
Ask yourself: Which options describe what happens during the drought period, not just when water was plentiful? Evidence of drought tolerance must come from the no-water phase.
Compare the two groups directly
For each option, check whether it clearly compares the live AM fungi group to the heat-killed inoculation group and shows a benefit for the live-fungus group that relates to surviving or retaining water under drought.
Step-by-step Explanation
Clarify the scientist’s conclusion
Ramirez concluded that AM fungi enhance drought tolerance in desert marigold seedlings. That means we need evidence that seedlings with live AM fungi do better during drought than seedlings without effective AM fungi (the heat-killed group). The key phrases to focus on are “enhance drought tolerance” and “improving the plants’ water uptake.”
Identify what counts as direct support
Direct support would be an observed difference in drought performance between the two groups during the drought period, such as:
- Keeping more water in their tissues (better water status)
- Surviving longer under no-water conditions
Anything measured only when water is plentiful or that shows no advantage for the live-fungus group will not directly support the conclusion about drought tolerance.
Evaluate the answer choices against the conclusion
Go through each option and ask: Does this show that live AM fungi improve the seedlings’ performance specifically during drought, compared with the heat-killed group?
- If the result is about conditions before drought (when water was ample), it does not directly show drought tolerance.
- If both groups are the same, it does not support the idea that the fungi help.
- If the advantage, if any, goes to the heat-killed group, it would actually go against the conclusion.
Only one option clearly shows that seedlings with live AM fungi retain more water and survive longer during the drought than the control group.
Choose the result that directly matches “better drought tolerance”
The result that most directly supports Ramirez’s conclusion is the one stating that seedlings inoculated with live AM fungi maintained higher leaf water content and survived an average of three days longer during the drought period than seedlings given the heat-killed inoculation. This shows exactly what “enhanced drought tolerance” means: better water status and longer survival during drought for the live-fungus group compared with the control group.