Question 87·Hard·Command of Evidence
Botanists hypothesized that drought stress increases the concentration of a valuable medicinal alkaloid in the leaves of Catharanthus roseus (Madagascar periwinkle). The alkaloid is synthesized via a pathway the plant can up-regulate under certain environmental conditions.
Which finding, if true, would most directly support the botanists’ hypothesis?
For SAT questions asking which finding “most directly supports” a hypothesis, first restate the hypothesis as a simple cause-and-effect statement (X causes Y to increase/decrease). Then look for an answer that describes a controlled comparison: two groups that differ mainly in X, with a clear measured difference in Y that matches the prediction. Be wary of choices that are merely observational, focus on a different variable, or describe mechanisms without actually showing the predicted outcome; these are usually weaker evidence than a direct experimental result.
Hints
Clarify what the scientists predicted
Put the hypothesis into your own words: what do the botanists think happens to alkaloid levels when the plants don’t get enough water?
Think about the strongest kind of evidence
For supporting a cause-and-effect claim, what kind of study is stronger: an uncontrolled field observation or a controlled experiment where only one factor (here, water) is changed?
Match each option to the key variable
As you read each answer, ask: Does this specifically test or compare different water levels in C. roseus and measure alkaloid concentration, or is it about something else (like soil nutrients or humidity)?
Watch out for indirect or unrelated findings
Some choices might sound scientific but focus on other conditions or only on biochemical mechanisms; look for the one that actually shows the predicted change in alkaloid levels under drought.
Step-by-step Explanation
Restate the hypothesis in simple terms
The botanists’ hypothesis is: when C. roseus experiences drought stress (not enough water), the concentration of the medicinal alkaloid in its leaves goes up.
So we are looking for evidence that directly compares plants under drought to plants with normal water and shows higher alkaloid levels in the drought group.
Decide what “most directly support” means
"Most directly support" usually means:
- A controlled study or experiment
- Only the key variable (here, water availability) is changed
- A clear result that matches the prediction (drought → higher alkaloid concentration)
Evidence that is only indirect, about a different variable, or just observational is weaker.
Check each option against what we need
Go through the choices and ask:
- Does it compare drought-stressed vs well-watered plants?
- Are other factors controlled (like genetics, environment)?
- Does it actually measure alkaloid concentrations and show the predicted increase under drought?
Only an option that clearly answers "yes" to these questions will directly support the hypothesis.
Identify the choice that best fits
Choice D says that in a controlled greenhouse study, genetically identical C. roseus plants with limited watering had, on average, 60% more leaf alkaloid than plants with ample water. This is a direct experimental test: same genetics, same environment, only water level is changed, and the drought-stressed plants show higher alkaloid levels. Therefore, the correct answer is:
In a controlled greenhouse study, genetically identical C. roseus plants subjected to two weeks of limited watering contained, on average, 60% more leaf alkaloid than plants given ample water.