Ecologist: Wild populations of the desert shrub *Larrea* produce volatile terpenes in their root zones. Although researchers once hypothesized that these subterranean terpenes primarily suppress the germination of competing plant species, recent field assays show that natural soil concentrations of the compounds are far too low to inhibit seed germination. However, *Larrea* specimens growing in soils with elevated terpene concentrations exhibit significantly less foliar tissue desiccation during prolonged droughts than do specimens in terpene-deficient soils. Given that terpene molecules are known to chemically bond with and thicken waxy cuticular layers on plant surfaces, the subterranean terpene vapor must rise into the canopy and directly reduce foliar water loss by reinforcing the leaves' cuticular wax.
Which of the following, if true, most strengthens the ecologist's argument?
- AChemical assays of *Larrea* root tissues demonstrate that root cells actively absorb subterranean terpenes and metabolize them into internal solutes that prevent root dehydration.
- When the aboveground foliage of *Larrea* shrubs is physically shielded from rising soil vapors while their roots remain in terpene-rich soil, the shrubs exhibit rates of leaf water loss identical to those of shrubs growing in terpene-deficient soil.Answer
- CSeveral unrelated desert plant species native to the same arid ecosystem produce cuticular waxes that have chemical compositions substantially different from those of *Larrea* shrubs.
- DAll arid-zone shrub species that produce subterranean chemical vapors rely exclusively on foliar cuticle modification rather than root-level adaptations to survive severe drought conditions.
- EField surveys indicate that soil concentrations of terpenes beneath *Larrea* shrubs are consistently lowest in geographic regions that receive frequent seasonal rainfall.