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Difficulty: MediumCommand of Evidence: Textual

Botanist Dr. Clara Martinez and colleagues investigated the ecological role of nickel hyperaccumulation in the plant *Alyssum bertolonii*. While many researchers hypothesize that accumulating high levels of toxic metals in leaves primarily serves to deter herbivores, Martinez's team proposed that it also functions as a form of elemental allelopathy. Specifically, they suggested that when nickel-rich leaves shed and decompose, they increase nickel concentrations in the topsoil directly beneath the plant's canopy, thereby suppressing the growth of competing, non-accumulator plant species.

Which choice, if true, would most directly support the team's proposal?

  1. A
    In laboratory feeding trials, herbivorous caterpillars consistently avoided eating leaves from *A. bertolonii* plants that had accumulated high levels of nickel.
  2. B
    In regions with high soil nickel levels, competing non-accumulator species show signs of cellular stress even when growing several meters away from *A. bertolonii* plants.
  3. In controlled experiments, soil collected from beneath mature *A. bertolonii* plants inhibited the germination of competing grass species, but this effect was not observed in soil from the same area that had its nickel removed.Answer
  4. D
    An analysis of *A. bertolonii* leaves showed that nickel concentrations were uniform across all leaves, regardless of whether the leaves were still attached to the plant or had fallen to the ground.

Answer

The finding that soil collected from beneath mature *A. bertolonii* plants inhibited the germination of competing grass species, except when nickel was removed from the soil.
The correct option is correct because the greenhouse study directly demonstrates that soil from beneath mature *A. bertolonii* plants inhibits competing grass species, and that this inhibition is dependent on the presence of nickel. This directly supports the proposal that nickel hyperaccumulation in leaves leads to soil modification that suppresses competing plants.

Step-by-Step Solution

1
Identify the team's specific proposal.
The team proposes that *A. bertolonii* suppresses the growth of competing, non-accumulator plant species through increased nickel concentrations in the topsoil directly beneath its canopy, caused by leaf shedding and decomposition.
To evaluate which choice supports the proposal, we must isolate the proposed mechanism (nickel in soil beneath the canopy suppressing competitors).
2
Evaluate the choices to find evidence showing that the soil beneath the plant inhibits competing species due to nickel.
The experiment demonstrating that soil from beneath the plant inhibits competitor germination, and that this effect disappears when nickel is removed, directly connects soil nickel to competitor suppression.
This establishes both the effect (suppression of competitors in the soil beneath the canopy) and the cause (the presence of nickel in that soil).

Key Concept

Command of Evidence: Textual
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Command of Evidence: Textual

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