Many forest tree species form symbiotic relationships with mycorrhizal fungi, which supply the trees with nutrients and water in exchange for carbon. Ecologist Dr. Elena Ruiz hypothesized that during prolonged drought periods, this relationship shifts: rather than maintaining mutual benefit, certain mycorrhizal species become parasitic, consuming carbon from host trees without providing a corresponding increase in water uptake. To test this hypothesis, Ruiz monitored carbon allocation and water uptake in Douglas fir (*Pseudotsuga menziesii*) saplings inoculated with either fungus species A or fungus species B under simulated drought conditions.
Which finding, if true, would most directly support Ruiz's hypothesis?
- AUnder drought conditions, saplings inoculated with species B exhibited greater overall growth and needle development than did saplings inoculated with species A.
- BUnder normal moisture conditions, saplings inoculated with species A and species B showed similar rates of carbon transfer and water uptake.
- Under drought conditions, saplings inoculated with species A transferred a larger percentage of their carbon to the fungus than they did under normal conditions, but their overall water uptake did not increase.Answer
- DUnder drought conditions, saplings grown in soil without mycorrhizal fungi had lower water uptake than saplings inoculated with species B.
Answer
Under drought conditions, saplings inoculated with species A transferred a larger percentage of their carbon to the fungus than they did under normal conditions, but their overall water uptake did not increase.
The correct answer provides direct textual evidence that under drought conditions, a mycorrhizal fungus (species A) consumed more carbon from the host tree while failing to increase the tree's water uptake, validating Ruiz's hypothesis of a shift to parasitism.
Step-by-Step Solution
Key Concept
Command of Evidence: Textual
Estimated Time:1m 30s